Precision air conditioner / CRAC

1) Twin Compressor with single/Twin Ckt. ( for more reliability)

• Capacity of air conditioning plant  is worked out based on peak outside conditions prevailing in that area, heat load, lighting load ,occupation in room ,heat gain from surroundings.
• It is worthwhile to note that both the operating conditions and the capacity of a refrigerating system change as the load on the system changes and also depending upon heat gain from surroundings.
• Equilibrium maintained between vaporizing and condensing sections depending upon internal load and heat gain from surroundings affects rated capacity at designed operating conditions of air conditioner.
• When the load on the system is light, the space temperature will be lower than the average design space temperature, the evaporator (Δt) will be less than the design (Δt) and the suction temperature will be lower than the design suction temperature. Therefore, the system operating conditions will be somewhat lower than the average design operating conditions and the system capacity will be somewhat less than the average design capacity.
• Good practice requires that the system be designed to have a capacity equal to or slightly in excess of the average maximum sustained load. This is done so that the system will have sufficient capacity to maintain the temperature and humidity at the desired level during periods of peak loading. Obviously, as the cooling load decreases, there is a tendency for the system to become oversized in relation to the load.
• The degree of variation in the length of the on and of cycles well depend on the degree of load fluctuation.
• The design conditions occur may be, for example, during only 1% of the total time the equipment is in operation throughout the year. 

·        Multiple-System Capacity Control is one of the methods of controlling capacity.

• With Double independent refrigerant circuit, capacity control of 50% and 100% is possible with blower fan common for total capacity , is essential as our load is predominately sensible load.
• It is desirable to use of Double independent refrigerant circuit where the refrigeration load is variable. ( Less capital cost and easy maintenance and more reliability )
• For Heat load  10 TR to 20 TR  -- 7  TR  with n+1 Configuration
• More than 20 TR -- 14 TR   with n+1 configuration. For n >6  n+2 configuration
• For heat load calculation fresh air requirement, duct heat & return air heat gain not to be considered. ( Fresh air arrangement not required, infiltration of air during opening of door is sufficient to make up oxygen as it unnecessarily increase latent load .) (Shortest air circulation path / route., Arrange racks in hot aisle / cold aisle, Matching server air flow by aisle., Reduce mixing and short circuits., Provide isolation between hot and cold spaces. And free return air path., Best option is to install units in area to be conditioned) 

2) Electronic Expansion valve.

                 The conventional TXV is controlled by springs, bellows, and push rods. The spring force is a closing force on the TXV. The evaporator pressure, which acts under the thermostatic element's diaphragm, is also a closing force. An opening force is the remote bulb force, which acts on top of the thermostatic element's diaphragm.

There is also a liquid force from the liquid line, which acts on the face of the needle valve and has a tendency to open the valve. However, this force is cancelled out when using a balanced port TXV. Working together, these forces maintain a constant evaporator superheat in a refrigeration system. There are no electronic devices associated with a conventional TXV.

The electronic expansion valve (EEV) operates with a much more sophisticated design. EEVs control the flow of refrigerant entering a direct expansion evaporator. They do this in response to signals sent to them by an electronic controller. A small motor is used to open and close the valve port. The motor is called a step or stepper motor. Step motors do not rotate continuously. They are controlled by an electronic controller and rotate a fraction of a revolution for each signal sent to them by the electronic controller. The step motor is driven by a gear train, which positions a pin in a port in which refrigerant flows. Step motors can run at 200 steps per second and can return to their exact position very quickly. The controller remembers the number of step signals sent by the controller. This makes it possible for the controller to return the valve to any previous position at any time. This gives the valve very accurate control of refrigerant that flows through it. Most of these EEVs have 1,596 steps of control and each step is 0.0000783 inches.

Sensors

The electronic signals sent by the controller to the EEV are usually done by a thermistor connected to discharge airflow in the refrigerated case. A thermistor is nothing but a resistor that changes its resistance as its temperature changes. Other sensors are often located at the evaporator inlet and outlet to sense evaporator superheat. This protects the compressor from any liquid floodback under low superheat conditions. Pressure transducers can also be wired to the controller for pressure/temperature and superheat control. Pressure transducers generally have three wires. Two wires supply power and the third is an output signal. Generally, as system pressure increases, the voltage sent out by the signal wire will increase. The controller uses this voltage to calculate the temperature of the refrigerant with the use of a pressure/temperature table programmed into the controller.

The benefits derived from the installation of electronic expansion valves are as follows:

• Improved control of liquid refrigerant flow to evaporator. The evaporator is always optimally filled with refrigerant. Even with large load variations, which means an extremely wide range of partial-load operating conditions, exactly the right amount of refrigerant can be injected.

• Improved heat transfer, since more evaporator surface area used for boiling liquid ,less superheat.

• Raised evaporating temperature and higher suction pressure, reducing energy use by 2% to 3% per 1ºC in evaporating temperature.

• Reduced risk of liquid carry-over to compressor, reducing risk of compressor

damage.

• Avoids need for constant pressure drop across expansion valve.

• Allows condensing temperature (discharge/head pressure) to reduce at times of low ambient temperatures

          Thereby making air conditioning unit energy efficient. Hence it is recommended to use Electronic expansion valves.

3) Direct driven fan motor assembly for evaporator.

Backward curved freewheel fans Direct driven by electronically communicated motors are recommended as they save upto 30% as compared to forward curved centrifugal fans having dampers for cfm control.

Backward curved centrifugal fans characteristics are Energy efficient as no transmission loss, easy maintenance, less noise , high static pressure ,high flow ,power reduces as flow increases beyond point of highest efficiency. Where as forward curved fans are self loading type power rises continuously ,dip in pressure curve moreover damper control is not particularly energy efficient method of air flow control.

 4) SMPS power supply for control ckt.

The Precision air conditioning units are used in telephone exchange building having their own transformer substation, having reasonable voltage stability .Requirement of SMPS power unit for may differ manufacturer to manufacturer depending upon their microprocessor controller to make specification generic ,SMPS power supply unit may not insisted.

Provision of Precision air conditioner as replacement to existing package units in existing setup is not recommended.

Following are additional comments on precision air conditioning specification:

1) Precision AC Unit designed for COP 2.9.

• The following definitions are taken from ASHRAE Standard 90.1-1999 (2001).

• Coefficient of performance (COP) – cooling: the ratio of the rate of heat removal to the rate of energy input, in consistent units, for a complete refrigerating system or some specific portion of that system under designated operating conditions.

• Energy efficiency ratio (EER): the ratio of net cooling capacity in Btu/h to the total rate of electric input in watts under designated operating conditions.

• Integrated part-load value (IPLV): a single number figure of merit based on part-load EER, COP, or kW/ton, expressing part-load efficiency for air-conditioning and heat pump equipment on the basis of weighted operation at various load capacities for the equipment.

• As per ASHRAE 90.1-2004 $ 6.4.1 & ECBC 2007

      Unitary Air Conditioning Equipment

Equipment class	Min COP	Min IPLV	Test Standards
Air cooled chiller <530 kw (<150 tons)	2.9	3.16	API 550/590-1998
Air cooled chiller >530 kw (>150 tons)	3.05	3.32	API 550/590-1998
Centrifugal water cooled chillers  <530 kw (<150 tons)	5.8	6.09	API 550/590-1998
Centrifugal water cooled chillers  >530 kw  and < 1050 kw (>150 tons and <300 tons)	5.8	6.17	API 550/590-1998
Centrifugal water cooled chillers  >1050 kw (>300 tons)	6.3	6.61	API 550/590-1998

• BSNL specification requires COP 2.9, at designed conditions i.e. SST 9 to 10 ⁰C & SDT  53 ⁰C for ambient temperature of 43 ⁰C.

• Considering designed condition and 35 mm static pressure evaporator fan, it is not possible to achieve required COP.

2) Check for good installation of Air cooled Condenser.

• Proper operation and giving rated capacity at designed conditions / peak conditions of PAC is mainly depend upon efficient working of Air cooled condenser.

• In case of water cooled system, checking of cooling tower efficiency by web bulb approach was invariably done during AT.
• However in case of air cooled system there is  no check of efficiency of air cooled condenser which is solely depend on entering and leaving dry bulb temperature.

• Seasonal  test shall be conducted only in summer months April to June  and  October.

      Air entering condenser shall be at ambient temperature.

               Or

• During testing ambient temperature shall be more than 40 ⁰C.

Capacity calculation by Enthalpy method (Evaporator side ), Condenser capacity    and Capacity from compressor manufacturer capacity chart at working conditions shall give clear picture about ENERGY EFFICIENT INSTALLATION OF AC UNITS.

3) Electrical console for Package AC units.

• In coming FP MCB ,insulated busbar for distribution ,Motor Protection Circuit Breakers (MPCB) ,suitable size contactor and overload relays shall incorporated to avoid fire hazards , as these units are preferably installed in area to be conditioned.

• Provision of wet floor sensor to indicate water leakage problem.

• Provision of Electrical control panel at entrance air lock lobby with two incomings electrically & mechanically interlocked through shunt trip coil and AC cutoff in case of Fire.

4) Insulation of refrigerant piping / drain pipe  inside the conditioned space.

5) Provision of false floor height 600 mm ,avoid refrigerant / drain piping in path of supply air. 

5) Use of site suitable package unit variants.

Different models suitable for site requirement to be selected as follows

v    Low static pressure (Installation inside switch room is preferred). / High static pressure ( Ducted units)

v    Condenser suitable for 35⁰C & 43⁰C ambient temperature ( shall reduce initial cost ).

v    Upward and downward flow.

v    Condenser top and side throw.

Calculation of Carbon footprints

Augest 2012

Calculation of Carbon footprint:-

Following methodology helps you to calculate your carbon footprint resulting from the use of Electricity, Petrol, Diesel and LPG.

 Step 1- Data collection;

1. Electricity: Collect data on your annual electricity bills. One can find number of  units consumed in establishment from the monthly electricity bills issues by State Electricity Board/ Distribution/Collection companies.
2. Petrol/Diesel:  Add number of liters of petrol/diesel you used in DG set in a year.
3. LPG: Generally one LPG cylinder has around 14 kg of liquefied petroleum gas. Multiply number of cylinders used in a year by 14 and add the resulted value in the calculation.    

Step 2 – Calculation Methodology;

1. Electricity : Input value (in KWh/Yr) X 0.00085 (Emission Factor) =  Output value in (Kg of CO₂)
2. Petrol: Input Value(In Litres/Yr) X 2.296 (Emission Factor) =  Output value in (Kg of CO₂)
3. Diesel: Input Value(In Litres/Yr) X 2.653 (Emission Factor) =  Output value in (Kg of CO₂)
4. LPG: Input Value(In Kg/Yr) X 2.983 (Emission Factor) =  Output value in (Kg of CO₂)
5. Your Carbon Footprint :  Add (1+2+3+4) = Output value in (Kg of CO₂)

Divide final value (no 5) with 1000 so that you get total carbon footprint in ton of CO_2.

Final Carbon footprint should be in tons of CO₂ (tCO_2.).

Know more about the source of emission factors;

• Electricity = 0.00085 kg CO₂ per KWh, Source: CO₂ emission factor database, version 06, CEA (Government of India), http://www.cea.nic.in/reports/planning/cdm_co2/cdm_co2.htm 
• Motor gasoline/ Petrol = 2.296 kg CO₂ per liter, Source: Emission factors are taken from the file “Emission factors from across the sector -tool”,  extracted from http://www.ghgprotocol.org/calculation-tools/alltools
• Diesel= 2.653 kg CO₂ per litre, Source: Emission factors are taken from the file “Emission factors from across the sector -tool”,  extracted from http://www.ghgprotocol.org/calculation-tools/alltools 
• Liquefied petroleum gas (LPG) = 2.983 kg CO₂ per kilogram, Source: Emission factors are taken from the file “Emission factors from across the sector -tool” extracted from http://www.ghgprotocol.org/calculation-tools/alltools

Carbon footprint calculation For Year : (Telecom application )

= Electricity use + Diesel use

Electrical  use

= Emission factor X  Electricity consumption /Year

= 0.00085  X  ( Average MD  X  Load factor  X  365  X  Average EB Hour  per day )

  Note -- 

Average MD  is  average  of  actual MD recorded in year.

Load factor  will vary  As 50% load of  telephone exchange load is air conditioning load which is depend on ambient temperatures during a day.

It observed  that 

BTS Indoor                          0.85                     AC without FCU   4.8 KW

BTS Indoor                          0.7                       AC with FCU        4.8 KW

BTS Outdoor                       1                           Without AC          2.4 KW

For Telephone Exchange    less than  0.65

Administrative buildings     0.7

Average EB Hour is Average Electricity Board supply available in one day (24hour).

Diesel use

= Emission factor X  Diesel Consumption /Year

= 2.653 X  ( Diesel Consumption in ltr   X  365  X  Engine Run Hour per day )

Diesel Consumption = SFC  X  bhp   /  0.83 X 1000   Liter

SFC will vary depending upon loading of DG set For Full load  167 gms/hp-hr approx.

As per Recommendations

on Approach towards Green Telecommunications

April 12, 2011 

by   TRAI  ( Available on TRAI website )

Refer page 28

If the consumption of power of the Network element, in KW (including Air

Conditioning etc) is P , the Grid power is for ‘x’ hrs , the power from ‘z’

KVA DG is for ‘y’ hrs and the efficiency of the generator is ‘η’ then

C = 0.365 [0.84Px + (0.528 yz /η) ] in Tonnes

Similarly carbon footprints for each of the network elements are to be

calculated. The detailed calculation of footprints of various network

elements is at Annexure –I

page no 30

sample calculation for estimation carbon footprint

2.24 Calculation of carbon footprints towards the conclusion of the peak

season – of winter and summer – may be desirable. Hence the telecom

service providers can declare their Carbon footprints twice in a year.

2.25 All service providers should declare to TRAI, the carbon footprint of

their network operations in the format provided in Annexure -II.

This declaration should be undertaken after adopting the formulae

and procedures mentioned under para 2.20 and at Annexure -I. The

Declaration of the carbon footprints should be done twice in a year

i.e. half yearly report for the period ending September to be

submitted by 15th of November and the succeeding half yearly report

for the period ending March to be submitted by 15th of May each

year.

2.25 All service providers should declare to TRAI, the carbon footprint of

their network operations in the format provided in Annexure -II.

This declaration should be undertaken after adopting the formulae

and procedures mentioned under para 2.20 and at Annexure -I. The

Declaration of the carbon footprints should be done twice in a year

i.e. half yearly report for the period ending September to be

submitted by 15th of November and the succeeding half yearly report

for the period ending March to be submitted by 15th of May each year.

Certain assumptions has been for calculating  carbon footprint for Diesel use.

Also efficiency in case of DG set  is taken as 1 in Annx –I.

BASICS OF REFRIGERATION AND AIR CONDITIONING

BASICS OF REFRIGERATION AND AIR CONDITIONING

Er .Deepak Sathe

DEE

SDE(E)

BSNL,Mumbai
2006

BASICS OF REFRIGERATION :-

The word ‘refrigerate’ means to chill or freeze a substance i.e. to lower its temperature by removing heat.

Refrigeration is the process of removing heat from a substance and rejecting the heat so removed to the atmosphere which is at a higher temperature level.

Heat always flows from a body at a high temperature to another at a lower temperature. Hence when we want heat flow in reverse direction i.e. from lower to higher temperature level or in other words when we want to lower the temperature below the surrounding atmospheric temperature it requires external energy.

Refrigeration is accomplished by various methods:- 1) Vapour compression system   2) Vapour absorption system  etc…..

Mechanical vapour compression system is practical application of Charles’s law Pressure varies proportionate to temperature. The boiling point of a liquid ( or condensing  temperature of a gas)  can be varied at will by varying pressure of vapour in a enclosed space above the liquid level (i.e. in saturated condition)

                           

Refrigerants are heat carrying mediums which during their cycle in the refrigeration system absorb heat at ( Low pressure) a low temperature level and discard the heat so absorbed at a higher level i.e. ambient temperature (at High pressure).

                                                                                

 These refrigerants have boiling points much below ordinary room temperature. So they exist as gases and are held in liquid state by keeping them under pressure such as in refrigerant cylinder.

From pressure-temperature chart (at saturation) we get boiling point and their corresponding pressure.

For R 22
Boiling point ( 0 F)	Pressure   (PSIG)
-41	0  (At atmospheric pressure)
49.1	82.5
75	132.2
120	259.9
125	277.9
127.4	286.9
130	296.9

1) Evaporator: - The process of heat removal from the substance / air to be cooled is done in evaporator. The refrigerant is in liquid state as it absorbs maximum heat during vaporization. At the inlet of evaporator, the refrigerant is predominately in the liquid form with small amount vapour formed as a result of flashing at the expansion valve.

                         SST is 9⁰ F to 10⁰ F (49.1 ^OF) corresponding pressure is 82.5 PSIG neglecting pressure losses in evaporator.

                            As refrigerant passes through the evaporator more and more liquid is vaporized by the load, when it reaches the end of evaporator it is purely in the super heated vapour state.  

D-X system which we use in package air conditioner, is a system where the air to be cooled is directly passed over the evaporator coil inside which refrigerant is boiling.

2) Compressor:- For closed cycle of  vapour compression system ,the refrigerant coming out of the evaporator ( superheated low pressure vapour) must be compressed corresponding to the saturation temperature higher than ambient air temperature ( in case of air cooled system).

The compressor acts as pump to circulate refrigerant through closed  system.

                           

Types:- a)  Open or Hermetically closed.

b)  Reciprocating, Screw, Rotary, Centrifugal or Scroll

Compressor manufacturers publish rating charts/tables showing capacity for various suction temperatures and discharge temperatures. Capacity increases with increase of suction temperature whereas it decreases with increase of discharge temperature.

                                    Discharge pressure in absolute units

Compression Ratio=

                                   Suction pressure in absolute units

Pressure in Absolute units = PSIG + 14.7

                 Refrigeration effect produced                  BTU / hour

EER     =                                                    = 

                               Power consumed                                     Watt

                             EER is maximum in case of Scroll type compressor.

3) Condenser:- The function of condenser are

                           1)  To desuperheat high pressure gas.

                           2) To condense it from gas to liquid.

                           3) To Sub cool liquid.

                           SDT is 53⁰ C (127.4 ⁰F) corresponding pressure is 286.9 PSIG neglecting pressure losses in condenser.

                          Depending upon type of cooling medium condensers are 1) Air cooled 2) Water cooled 3) Evaporative (combination of both air & water).

                          In air cooled condenser refrigerant is cooled by ambient air, the ambient air temperature is more than the temperature of water. Hence the condensing temperature is higher in case of air cooled condenser than water cooled condenser. Heat transfer efficiency increases when the medium is liquid. This makes water cooled system more efficient than air cooled but the scarcity of water compels to use air cooled system.

4) Throttling device:- (Thermostatic expansion valve) The pressure of the liquid refrigerant  coming out of condenser / receiver  has to be reduced so that it can  vaporize at desired temperature in the evaporator (SST).

                           Sufficient refrigerant has to be fed into the evaporator to meet load. Externally equalized thermostatic expansion valve is normally used, feeler bulb is placed on suction line.

Superheating:-If the temperature of the refrigerant gas is more than its saturation temperature ,the gas is said to be  in superheated condition.

TEV is adjusted for superheat of 10⁰F means the suction line where feeler bulb is mounted there is no liquid but vapour in superheated state by 10⁰F above its saturation temperature.

Superheat  provided is the difference between the temperature of the suction line ( place where feeler bulb is mounted) and the saturated temperature corresponding to the evaporator pressure( i.e. suction pressure neglecting pressure drop in evaporator or add 2⁰F for suction pressure drop).

As the superheat increases compressor capacity drops due to reduction in density of gas, HP per ton increases. But this superheat is required to safe guard compressor against liquid flood back.

                  

Sub cooling:- If the temperature of the refrigerant gas is less than its saturation temperature ,the liquid is said to be  in sub-cooled condition.

The high pressure ,high temperature liquid coming out of condenser must be cooled down and reduce its pressure before entering evaporator, this is achieved by throttling device. The cooling occurs automatically when liquid passes through TEV.A portion of liquid refrigerant boils taking latent heat of vaporization from liquid itself.

For thermodynamics tables of R22

Temp. (0F)	Enthalpy of liquid (BTU/lb)	Enthalpy of vapour (BTU/lb)	Latent heat of vaporization(BTU/lb)= Enthalpy diff between vapour & liquid
119	42.45	112.5	70.05
40	21.42	108.14	86.72

The amount of heat to be removed to cool liquid from 110⁰ F TO 40⁰ F = 42.45 – 21.42 = 21.03

 latent heat of vaporization at 40⁰F = 86.72

i.e. 24.2% of latent heat of vaporization is utilized to cool liquid refrigerant to evaporator temperature level.

The refrigeration is accomplished mainly by absorption of its latent heat of vaporization by boiling liquid refrigerant in the evaporator.

The 24.2% of  latent heat of vaporization is used in cooling down liquid while passing through TEV ,balance 75.8% gives effective refrigeration effect ,known as Net Refrigeration Effect (NRE).

Thus NRE decreases as the difference between condensing and evaporating temperature widens.

Also with sub cooling ,as the temperature from which liquid is to be cooled is reduced, the NRE increases and mass flow rate per ton reduces.

Manufacturers rating chart  gives capacity / power for wide range of SST & SDT at liquid sub cooled at 15⁰F.

For every degree increase above 15⁰F of liquid sub cooling, there is increase in compressor capacity by 0.5%.without any change in power requirement, thus increases efficiency.

Over sizing air cooled condenser ( or  by use of liquid –suction heat exchanger ) so as to take care of de-superheating, condensing and liquid sub cooling, we can increase efficiency of Refrigeration system.

Small amount of vapour  present in liquid line occupy substantial volume ,which offer high resistance to flow in liquid line. This effects capacity of refrigerant system. Vapour can be present in the liquid line 1) If condensation is not complete. 2) Shortage of refrigerant. 3) Excessive pressure drop in liquid line causing formation of flash gas.

Pressure Drop – Liquid lift:- When liquid / gas flows through pipe ,resistance is offered by pipe walls, results in pressure drop .In refrigeration ,pressure drops are expressed in temperature units. If liquid is in saturated condition (not sub cooled) even small pressure drop can cause liquid flashing resulting in formation of flash gas. Similarly where an evaporator is located at higher level than the condenser, the pressure drop due to liquid lift (lift against gravity) is very high causing heavy liquid flash.

	R22
Kg/cm2 per mtr	0.035
Psi per ft	0.5

Selection of Design criteria:-

1) Saturated suction temperature (SST):- Compressor capacity varies directly with SST. Higher SST  better will be the capacity.

Hermetic / Semi hermetic compressor -   10⁰C   or 50⁰F

Open type compressor-     15⁰C    or   59⁰F

This is governed by 1) for ensuring suction gas cooling of motor windings.

2) Saturated discharge temperature (SDT):- Compressor capacity varies inversely

 with SDT.

Depend upon i) type of condenser air cooled or water cooled.

 ii) Highest outside condition.

 ( ETD) Entering Temperature diff = (SDT – entering temp of condenser i.e.max ambient temp)

                 Normally taken as    10⁰C to 17⁰ C   ( 20⁰F  to  30⁰F)

3) Sub cooling:-  Minimum 15⁰F ( sub cooling can save energy up 10%)

4) Super heat:-  Minimum 6⁰C ( for safety of compressor)

5) Static pressure:- 35 mm WG.( for high sensible units it is better to take 25 mm WG as  cfm for is more than the units for comfort cooling and installing units in area to be air-conditioned, as power consumption of continuous operated blower is @ 25% )

VRLA Battery

VRLA BATTERIES

2011

The battery is an important constituent of the Telecom Network for sustaining the trouble free and uninterrupted service to its users. Till 1994, the whole of the Indian Telecom Network relied wholly on Flooded type batteries to provide the necessary Primary power back-up.

Moreover the Conventional lead acid (Flooded type) batteries were being used in the department as primary standby source. 

Flooded batteries were bulky and required a separate battery room with exhaust fans to throw out the acid fumes emitted by these batteries. 

Flooded batteries can not be transported in charged condition, hence assembly and charging at site is essential.

These batteries also require periodical special charging process at comparatively higher voltage of 2.7V/cell (a total voltage of 65 V), also known as boost-charging, to agitate the electrolyte thoroughly to prevent stratification of electrolyte, as well as to reduce sulpahtion of plates. As the telecom equipment can not withstand such a high voltage, the battery under boost-charging and the charger have to be isolated from the exchange equipment.

Maintenance of these batteries needs more efforts and is more labour oriented.

Moreover, it will be more difficult for the flooded batteries to meet the pollution norms, issued by Ministry of Environment and Forest.

The electrode reactions in all lead acid batteries are basically identical. As the battery discharges, the lead oxide (+ve active material ) and spongy lead (-ve active material ) both react with sulphuric acid ( Immobilized electrolyte ) to form lead sulphate and water. During charging this process is reversed. At the end of charging oxygen gas evolved at positive electrode & hydrogen gas is evolved at the negative electrode, these gases make their way out and escape into environment, thereby losing water. In VRLA batteries, the oxygen generated at positive plate is transported in gaseous phase through the micro porous (AGM ) Absorbent Glass Mat separator to the surface of negative plate. Due to oxygen recombination process, VRLA battery do not lose water, therefore no topping up is required and formation of hydrogen gas is suppressed. Under normal float conditions, approximately 95 to 99 percent of hydrogen and oxygen is recombined.   

In 1994 the VRLA batteries were introduced in Indian Telecom Network for the first time. Since then, the Department has been procuring VRLA batteries for the purpose.

This is because of the fact that these batteries do not require the rigid routine maintenance like periodic topping up, recording SG of each cell and periodic discharge/charge cycles.

VRLA batteries require very little maintenance that is why they are confused as “maintenance free batteries”. These batteries emit extremely low amount of gases and do not require periodic toping up. Hence, they can be installed in the equipment room itself, thus saving the manpower and long connecting bus-bar or cables. Plates of these batteries have a higher current density, hence have a small size. As these batteries are based on starved electrolyte principal, can be installed in any orientation. All the above characteristics make VRLA batteries ideal stand by power source for Telecom applications.

But these batteries have their own problem and limitation, which require timely and intelligent handling. Some of the constraints are :

Self Discharge : As these batteries are supplied in charged condition. These cells/batteries get discharged of its own on its internal resistance and some localised action within the cells. This is termed as self discharge. Self discharge of these cells/batteries is about 14 percent every four weeks. If these cells/batteries are allowed to remain in this state for a long period of time, say, 3 months or more, the sulphation of the plates starts setting in. More the time it is allowed to remain in this state, more the sulphation gets hard and more it becomes difficult to remove it from cells/batteries.

It is essential that these cells/batteries are commissioned in the least possible time so that self discharge and subsequent sulphation layer is kept to minimum.

Temperature Effect : Batteries are very temperature sensitive. The kinetics of these batteries are temperature dependent. The chemical reaction increases/decreases in geometric progression which the increases or decrease in temperature. The Temperature in case of VRLA Batteries is significant because of limited volume of electrolyte which restricts heat conduction. In the light of this fact, the proper temperature is essential for the optimization of battery life. Internationally the batteries are design at 25 deg Celsius or 27 deg Celsius. The recombination of gases within a VRLA cell can only take place at a certain rate. If the rate is exceeded, gas pressure will built up beyond the safety valve level, and gases/water will be vented out and permanently lost.

The highest float voltage at which a cell still recombine all the gases driven off the plates is approximately 2.30Volts. If the cell temperature increased while holding the voltage constant, the cell would dry out and possibly go into thermal runaway. Thermal runaway leads to a melting down of the jar and under worst- case scenario, will lead to an explosion and fire.

a) Low Temperature effect : Battery capacity is diminished at low temperature. Also the charge acceptance decreases with the decrease in temperature. Therefore at low temperatures, a higher float voltage is required to maintain full charge and if charger is not adjusted properly, cell may be undercharged, leading to the problems described under low voltage.

b) High Temperature effect : The chemical reaction becomes faster and faster with rise in temperature above the design temperature (27 degree Celsius) and give rise to a chain reaction. High temperature causes loss of life because every 10 deg Celsius rise in operating temperature, the life is cut to half. High temperature also causes gassing, which means dry out and thermal runaway in VRLA cells.

Recommendation :

i) Batteries temperature shall be maintained to such a value that it does effect the life and performance of the battery i.e. the average temperature shall be maintained around the designed temperature at 35°C.

ii) Charging voltage shall be such that it does not contribute to rise in temperature i.e. 2.25V for float and 2.30 for charge.

iii) The charging current to the battery may be restricted to defined limit ( 0.1C recommended).

iv) A provision shall be made so that the charger voltage shall be regulated as per battery temperature to slow down the battery kinetics. The provision for battery temperature compensation has been made in the Power Plants. It may be ensured that Battery Temperature compensation unit in the power plant is fully functional & all the terminations are in proper place.

v) Though not mandatory, it is preferable that the battery may be installed in controlled environment for longer life and better performance.

Failure of VRLA batteries : Most common reasons for failure of VRLA batteries are :

Positive grid corrosion : Every time battery discharges, PbO2 of the +ve plate gets converted into PbSO4 there is a large increase in its volume, which puts pressure on the paste. Deeper the discharged, more is the increase in the volume of the paste of the positive plate and more is the pressure on the grid.

During charge the paste of the +ve gets converter back into PbO2 The paste gets contracted.

This expansion and contraction results in loosening of the active material in the grid, over a period of time. This results in loss of contact of the active material with the +ve plate and increase in the cell resistance.

This is a gradual process and under normal circumstances is a sign of ageing, which shall only take place when the battery has lived its stipulated life.

In addition to ageing there are other factors which accelerate the process and lead to premature ageing. Some of them are :

1. Larger number of charge/discharge cycles faster the ageing process.

2. Deep Discharge, deeper the discharge faster the ageing.

3. Fast rate of Charging.

4. Fast rate of Discharging.

Corrosion is un-recoverable and indicates the end of cell life.

Sulphation : Sulphation of the cells plates due to prolonged under charge, overcharge or prolonged storage may also increases the cell resistance.

This is a recoverable damage unless it is hard enough to remove and can be recovered by careful and controlled charging of the cell. In case the sulphation is soft (not very thick), the same may be recovered by charging the cell along with the battery.

The medium type of sulphation can also be removed by prolonged charging, which may be complete or part depending on hardness of the sulphation.

In both the above cases a few charge/discharge cycles are required for the optimum recovery.

This may be achieved by cycling the cell inside the battery or removing the cell from battery bank and reconditioning, special treatment of charging etc. at the factory by the manufacturer at his premises.

Some times sulphation becomes too hard to overcome. In such a case cell is supposed to be damaged beyond repair.

Dry out (loss electrolyte) : There is a misconception that these batteries are fully sealed and no escape of gasses take place. However, the gasses escape through the safety valve to maintain the pressure within cells in prescribed limits and therefore, most common mode of failure of the cell/battery is dry out. This may happen due the excessive escape of gasses from the cell, which leaves the cell with little or no electrolyte. This may happen due to :

a) Fast charging

b) Operation of the battery at a very high temperature

c) Leakage through cracks in the container or sealing.

Thermal Runaway : Batteries are very temperature sensitive. The chemical reaction increases/decreases in geometric progression with the increases or decrease in temperature. The Temperature in case of VRLA Batteries is significant because of limited volume of electrolyte which restricts heat conduction. In the light of this fact, the proper temperature is essential for the optimization of battery life. The recombination of gases within a VRLA cell can only take place at a certain rate. If the rate is exceeded, gas pressure will built up beyond the safety valve level, and gases/water will be vented out and permanently lost.

The highest float voltage at which a cell still recombine all the gases driven off the plates is approximately 2.30Volts. If the cell temperature increased while holding the voltage constant, the cell would dry out and possibly go into thermal runaway. Thermal runaway leads to a melting down of the jar and under worst- case scenario, will lead to an explosion and fire.

In the initial stage of VRLA battery development of thermal runaway was considered one of the failure mode, but in India it has not been observed till date and moreover at present it is not considered failure mode. Thermal runaway is calliopes of the battery due to very very high temperature developed inside the battery.

Recommendation :

i) The SMPS Power plants are fully compatible to prevent any excessive temperature in the battery by the features Battery path current limiting, Temperature compensation of the battery, High voltage cut-off and Precise voltage set in the Power Plant.

ii) The user shall ensure that all the above features of SMPS Power Plants are fully functional and properly set.

iii) The battery room shall be employed with the natural air convection (ventilated room) and exhaust fan.

iv) The user shall ensure that the ambient temperature of the battery room has been reduced by employing the natural air convection (ventilated room) with exhaust fan.

Factors leading to the premature failure and affecting the life & performance of the battery : Some of the common factors which affect the premature failure and affecting the life & performance of the battery are :

a) Battery discharge : The battery shall not be allowed to discharge beyond 80% of its rated capacity.

b) Improper Charging : Under ideal conditions i.e. moderate ambient temperature (10°C to 35°C), Charging not faster than C/10 rate, and compatible charger a telephone exchange battery, on true float may give about 15 years of its service life, The same battery in cyclic discharge application to 80% DOD (Depth of discharge) will give only 1400 cycles i.e. Four year approximately. The battery application in Indian Telecom Network is neither of the two extreme categories. So more the deep discharge cycles/year, lesser the expected life of the battery.

The faster the rate of discharge of a battery lower is its expected life for example discharging battery at C/1 rate of discharge will give just half the number of DOD cycles than the battery which is discharged at C/8 or C/10 rate of discharge.

i)The exchange/transmission & similar application batteries shall be so chosen that they are not allowed to discharge faster than C/6.

ii) Where faster discharge( C/0.5 to C/5) is an essential requirement, as in the case of UPS systems etc., the battery may be planned with the lower expected life as per the rate of discharge.

c) No Temperature control : Batteries are very temperature sensitive. The kinetics are temperature dependent. The chemical reaction increases/decreases in geometric progression with the increases or decrease in temperature. The proper temperature will optimize battery life and is especially critical for VRLA Battery. The recombination of gases within a VRLA cell can only take place at a certain rate. If the rate is exceeded, gas pressure will built up beyond the safety valve level, and gases/water will be vented out and permanently lost. At 27 deg Celsius, the highest float voltage at which a cell still recombine all the gases driven off the plates is approximately 2.30Volts. If the cell temperature increased while holding the voltage constant, the cell would dry out and possibly go into thermal runaway. Thermal runaway leads to a melting down of the jar and under worst- case scenario, will lead to an explosion and fire.

a) Effect of Low Temperature : Battery capacity is diminished at low temperature. Also the charge acceptance decreases with the decrease in temperature. Therefore at low temperatures, a higher float voltage is required to maintain full charge and if charger is not adjusted properly, cell may be undercharged, leading to the problems described under low voltage.

b) Effect of High Temperature : The chemical reaction becomes faster and faster with rise in temperature above the design temperature ( 27° Celsius) and give rise to a chain reaction. High temperature causes loss of life because every 10° Celsius rise in operating temperature, the life is cut to half. High temperature also causes gassing, which means dry out and thermal runaway in VRLA cells.

Recommendation :

i) Ensure that battery temperature compensation in the power plant is fully functional & all the terminations are in proper place.

ii) Do not resort to fast charging by setting higher float/charge voltages.

iii) Though not mandatory it is advisable to place the batteries in cooler environment for longer life and better performance.

d) Cell Matching

e) Storage of VRLA Batteries

f) Improper installation

g) Manufacturer Problems

h) Operational/Maintenance issues.

The VRLA batteries for slow discharge application (Discharge rate C/6 to C/120) GRs No. GR/BAT-01/03 MAR 2004 and GR/BAT-03/01 MAR 2006 with Amendments if any : Table-1
Rate of Discharge	Cell				Mono-Block
	Discharge Current	Capacity expressed as % of C/10 Discharge rate	Discharge time (min) Minutes	End cell voltage	Discharge Current	Capacity expressed as % of C/10 Discharge rate	Discharge time (min) Minutes	End cell voltage
C/3	0.333C	71.7	129	1.74V	0.333C	75.0	135	10.44V
C/4	0.25C	78.2	188	1.74V	0.25C	81.0	195	10.44V
C/5	0.2C	83.3	250	1.75V	0.2C	85.0	255	10.5V
C/6	0.167C	87.3	314	1.75V	0.167C	91.0	328	10.5V
C/8	0.125C	95.0	456	1.75V	0.125C	95.0	456	10.5V
C/10	0.1C	100-120	600-720	1.75V	0.1C	100-120	600-720	10.5V
C/20	0.05C	120.0	1440	1.75V	0.05C	120.0	1440	10.5V
C/72	0.014C	130.0	5616	1.75V	0.014C	130.0	5616	10.5V
C/120	0.0083C	150.0 (SPV Application only)	10800	1.75V	0.0083C	150.0 (SPV Applicationonly)	10800	10.5V

The VRLA batteries for high rate of discharge (UPS) application (Discharge rate C/0.5 to C/5) GR No. GR/BAT-02/02 MAR 2006 : Table-2
Rate of Discharge	Cell				Mono-Block
	Discharge Current	Capacity expressed as % of C/5 Discharge rate	Discharge time (min) Minutes	End cell voltage	Discharge Current	Capacity expressed as % of C/5 Discharge rate	Discharge time (min) Minutes		End mono- block voltage
C/0.5	2*C	45	13.5	1.70V	2*C	50	15	10.2V
C/1	1*C	60	36	1.70V	1*C	65	39	10.2V
C/2	0.5*C	81	97	1.70V	0.5*C	85	102	10.2V
C/3	0.33*C	86	155	1.74V	0.33*C	91	164	10.44V
C/4	0.25*C	95	228	1.75V	0.25*C	96	231	10.5V

                   The battery gives rated capacity only at the discharge rate for which it is designed. For example VRLA batteries for Telecom applications as per GRs No. GR/BAT-01/03 MAR 2004 and GR/BAT-03/01 MAR 2006 with amendments if any are designed at C/10 (0.1*C) rate of discharge. The VRLA batteries as per GR No. GR/BAT-02/02 MAR 2006 are designed at C/5 rate of discharge and will give rated capacity only when it is discharged at a discharge current equal to 0.2*C. At all other discharge rates faster than the 10 hours or 5 hour rate as the case may be the battery will deliver less than its rated capacity. Faster the rate of discharge lower the capacity the battery will deliver. The relation between rate of discharge and expected capacity is as given in tables 1& 2 above.

                   As per the table-1 given above, when the battery is discharged at a current equal to 0.33 times its rated capacity to the end cell voltage of 1.74V/cell, will be only 71.7% of the rated capacity and last for 129 minutes instead of 180 minutes as anticipated. In case the discharge current is equal to 0.2 time its rated capacity the battery will be 83.3% of its rated capacity and last for about 250 minutes instead of 300 minutes and so on.

The temperature at which the battery is to work : At temperature lower than 27° Celsius the capacity of the battery is reduced by 0.5% with every one degree decrease in temperature. This factor is of relevance at places where the ambient temperature goes very low (below 5 degree Celsius) and the rate of discharge is also low, less than C/20.

Charging of battery at rates faster than C/10 is not recommended : As the faster rate of charging is not recommended for VRLA batteries, it is essential that the power plant shall be programmed to ensure that current allowed to battery is restricted to 10% of its rated capacity. For this purpose the proposed battery capacity is essential for battery path current limit setting.

Sample Calculation 1 : Switching System :

(Slow Discharge Application (Battery back-up 6 hours & higher)

Data Required :

a) Load Present : 400A …. Say

    Ultimate : 700A

b) back-up required : 6 hours

c) Permissible DOD : 80%

Battery Capacity Calculations :

- As the battery is for slow discharge application, - The Battery is expected to deliver the effective capacity of 87.3% of C/6 rate at discharge rate of C/10 as indicated in the given table -1.

The required capacity of the battery, considering all the above factors can be calculated as given below :

At present : 400*(6/0.8)/0.873 = 3436.4 AH

Ultimate : 700*(6/0.8)/0.873 = 6013.7 AH say 6000 AH

The battery-bank can be formed by selecting available batteries. The solution in this case is : two 2000AH batteries for the present load and additional 2000AH battery can be added at the later stage.

Sample Calculation 2 : Computer Terminal :

(High discharge application (Battery back-up 0.5 to 5 hours)

Data Required :

a) Load Present : 50A …. Say

    Ultimate : 80A

b) Back-up required : 0.5 hours

c) Permissible DOD : 80%

Battery Capacity Calculations :

- As the battery is for high discharge application,  Battery is expected to deliver the effective capacity 45% at C/0.5 rate of discharge of C/5 as indicated in the given table - 2 on page 26.

The required capacity of the battery, considering all the above factors can be calculated as given below :

At present : 50*(0.5/0.8)/0.45 = 69.44AH

Ultimate : 80*(0.5/0.8)/0.45 = 111.11AH

The battery-bank can be formed by selecting available batteries. The solution in this case is : one 80AH battery for the present load and additional 80 AH battery.

Terminology

Absorption : The taking up or retention of one material or medium by another by chemical or molecular action.

Activated Stand Life : The period of time, at a specified temperature, that a cell/mono-block/battery can be stored in the charged condition before its capacity falls below a specified level.

Activation : The process of making a reserve cell/mono-block/battery function.

Ageing : Permanent loss of capacity due to either repeated use or the passage of time.

Ambient Temperature : The average temperature of the surroundings.

Ampere-Hour (AH) Rating : The rating assigned to the cell/mono-block shall be the capacity expressed in ampere-hours (after correction at 27° Celsius) and stated by manufacturer to be obtainable when the cell/mono-block is discharged at 5 hour rate (C/5) to a final end voltage of 1.75V/cell or 11.5V/mono-block.

Ampere-hour Efficiency : The percentage ratio of the output of the secondary cell or mono-block or battery, measured in ampere-hours, to the input required to restore the initial state of charge, under specified conditions.

Available Capacity : The total capacity, AH or WH, that will be obtained from a cell, mono-block or battery at defined discharge rate and other specified discharge rates or operating conditions.

Capacity : The total number of ampere-hours or watt hours that can be withdrawn from

a fully charged cell, mono-block or battery under specified conditions or discharge.

Capacity Fade : Gradual loss of capacity of a secondary battery with cycling.

Capacity Retention : The fraction of the full capacity available from a battery under specified conditions of discharge after it has been stored for a period of time.

Charge Acceptance : Willingness of a battery or cell or mono-block to accept charge. It is affected by cell/mono-block temperature, charge rates and state of charge.

Closed Circuit Voltage (CCV) : The difference in potential between the terminals of a cell/mono-block or battery when it is discharging.

Conditioning : Cycle charging and discharging of a battery to ensure that it is fully formed & fully charged. Sometimes indicated when a battery is first placed in service or returned to service after prolonged storage.

Constant Current Charging : A method of charging the battery using a current having little variation.

Constant Voltage Charging : A method of charging the battery by applying a fixed voltage, and allowing variations in the current. Also called constant potential charge.

Continuous Test : A test in which a cell/mono-block or battery is discharged to a prescribed end-point voltage without interruption.

Counter Electromotive Force : A voltage opposing the applied voltage. Also referred to as Back EMF.

Current Density : The current per unit active area of the surface of an electrode.

Cut-off Voltage : The cell/mono-block or battery voltage at which the discharge is terminated. It is also called end voltage.

Cycle : The discharge and subsequent or preceding charge of a secondary battery such that it is restored to its original conditions.

Cycle Life : The number of cycles under specified conditions which are available from a secondary battery before it fails to meet specified criteria of performance.

Deep Discharge : Withdrawal of at least 80% of the rated capacity of a cell, mono-block or battery.

Depth of Discharge (DOD) : The ratio of the quantity of electricity (usually in ampere-hours) removed from a cell or battery on discharge to its rated capacity.

Efficiency : The ratio of the output of a secondary cell or battery to the input required to restore it to the initial state of charge under specified conditions.

Electrolyte : The medium which provides the ion transport mechanism between the positive and negative electrodes of a cell/mono-block.

End Voltage : The prescribed voltage at which the discharge (or charge, if end-of-charge voltage) of a cell/mono-block or battery may be considered complete (also cut off voltage).

Energy Density : The ratio of the energy available from a cell/mono-block or battery to its volume (WH/V). Also used on a weight basis (WH/Kg).

Fast Charge : A rate of charging which returns full capacity to a rechargeable battery, usually within an hour.

Float Charge : A method of maintaining a cell/mono-block or battery in a charged condition by continuous, long-term constant-voltage charging, at a level sufficient to balance self-discharge.

Gas Recombination : Method of suppressing hydrogen generation by recombining it with oxygen on the negative electrode, as the cell approaches full charge.

Half-Cell : An electrode (either the anode or cathode) immersed in a suitable electrolyte.

Hourly Rate : A discharge rate, in amperes, of a cell/mono-block or battery which will deliver the specified hours of service to a given end voltage.

Internal Resistance : The opposition or resistance to the flow of an electric current within a cell or battery. It is the sum of the ionic and electronic resistances of the cell/mono-block components.

Life : For rechargeable batteries, the duration of satisfactory performance, measured in years float life) or in the number of charge/discharge cycles (cycle life).

Load : The term used to indicate the current drain.

Lot : All batteries of the same type, design and rating, manufactured by the same factory during the same period, using the same process and material, offered for inspection at a time shall constitute a lot.

Maintenance-Free Battery : A secondary battery which does not require periodic "topping up" to maintain electrolyte volume.

Memory Effect : A phenomenon in which a cell, operated in successive cycles to the same, but less than a full, depth of discharge experiences a depression of its discharge voltage and temporarily loses the rest of its capacity at normal voltage levels.

Open-Circuit Voltage (OCV) : The potential or voltage of a cell/mono-block or battery when it is at the surface of the electrode.

Overcharge : The forcing of current through a cell/mono-block after all the active material has been converted to the charged state. In other words, continued charging after 100% state of charge is achieved.

Over discharge : Discharge past the point where the full capacity of the cell/mono-block has been obtained.

Over voltage :The potential difference between the equilibrium potential of an electrode and that of the electrode under an imposed polarisation current.

Oxygen Recombination : The process by which oxygen generated at the +ve plate during charge is reacted at the -ve plate.

Parallel : Term used to describe the interconnection of cells or batteries in which all of the like terminals are connected together. Parallel connections increase the capacity of the resultant battery as follows :

Cp = n X Cu ; Where Cp is the resultant capacity, n is the number of cells or batteries connected in parallel & Cu is capacity of the each cell or battery.

Rated Capacity : The number of ampere-hours a cell/mono-block or battery can deliver under specific conditions (rate of discharge, end voltage, temperature): usually the manufacturer's rating.

Recombination : A term used in a sealed cell construction for the process whereby internal pressure is relieved by reaction of oxygen with the negative active material.

Reference Electrode : A specially chosen electrode which has a reproducible potential against which other electrode potentials may be referred.

Self-Discharge : The loss of useful capacity of a cell/mono-block or battery due to internal chemical action (local action).

Semi-Permeable Membrane : A porous film that will pass selected ions.

Separator : An ion permeable, electronically non-conductive, spacer or material which prevents electronic contact between electrodes of opposite polarity in the same cell.

Series : The interconnection of cells/mono-blocks or batteries in such a manner that the positive terminal of the first is connected to the negative terminal of the second, & so on. Series connections increase the voltage of the resultant battery as follows :

Vs = n X Vu Where Vs is the resultant voltage, n is the number of cells/mono-blocks or batteries connected in series & Vu is voltage of the each cell/mono-block or battery.

Service Life : The period of useful life of a cell/mono-block or battery before a predetermined end-point voltage is reached.

Shelf Life : The duration of storage under specified conditions at the end of which a cell/mono-block or battery still retains the ability to give the specified performance.

Short Circuit Current : The initial value of the current obtained from a cell/mono-block or battery in a circuit of negligible resistance.

Specific Gravity : The specific gravity of a solution is the ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature.

Standby Battery : A battery designed for emergency use in the event of a main power failure.

Starved Electrolyte Cell : A cell containing little or no free fluid electrolyte. This enables gases to reach electrode surfaces during charging and facilitates gas recombination.

State-of-Charge (SOC) : The available capacity in a cell/mono-block or battery expressed as a percentage of rated capacity.

Sulphation : Process occurring in lead batteries that have been stored & allowed to self-discharge for extended periods of time. Large crystals of lead sulphate grow and interfere with function of the active materials.

Thermal Runaway : A condition whereby a cell/mono-block or battery on charge or discharge will overheat and destroy itself through internal heat generation caused by high overcharge or over discharging current or other abusive condition.

Trickle Charge : A charge at a low rate, balancing losses through a local action and/or periodic discharge, to maintain a cell/mono-block or battery in a fully charged condition.

Vent : A normally sealed mechanism which allows for the controlled escape of gases from within a cell/mono-block.

Vented Cell/mono-block : A cell/mono-block design incorporating a vent mechanism to relieve excessive pressure and expel gases that are generated during the operation of the cell/mono-block.

Voltage Delay : Time delay for a cell/mono-block or battery to deliver the required operating voltage after it is placed under load.

Voltage Efficiency : The ratio of average voltage during discharge to average voltage during recharge under specified conditions of charge and discharge.

Watt Hour (WH) Capacity : The quantity of electrical energy measured in watt hours which may be delivered by a cell/mono-block or battery under specified conditions.

Watt Hour (WH) Efficiency : The ratio of the watt hours delivered on discharge of a battery to the watt hours needed to restore it to its original state under specified conditions of charge and discharge. The percentage WH efficiency is the product of AH efficiency & the ratio of average discharge and recharge voltage.

Wet Shelf Life : The period of time that a cell/mono-block or battery can stand in the charged or activated condition before deteriorating below a specified capacity.

Working Voltage : The typical voltage or range of voltage of a cell/mono-block or battery during discharge.

The ventilation requirements for battery rooms as recommended in various applicable standards

are given below.

Table 1: Ventilation Requirements

Applicable Standard	Battery room Ventilation Requirements
NFPA 76	The battery room exhaust fan capacity in Cubic Feet Minute (CFM) should be the room area (in square foot)
Tamil Nadu Factory Rules, 2002	The air exchange (fresh air for one air) for a general factory shall be six times the cubic capacity of the work room without dead pockets or undue draught
ASHRAE 62	1 CFM per charging ampere to be provided but not less than 6 air changes per hour
IS :12332	• 12 air changes per hour for battery room • Forced air supply & positive exhaust system • Use of flameproof electrical fittings • Air inlets to be located near the floor & outlet openings at the high point in the room Although this standard is for industrial environment, the recommendations can logically be extended to all battery rooms.
Document on Battery Rooms by EXIDE Technologies EN 50272-2 - Safety requirements for Secondary Batteries and Battery Installations	Volume of Air Changes per Hour:The following formula is to calculate the hourly exchange of air volume, Q in Cubic Meters / hour recommended for battery rooms. Q = 0.05 x n x I (cubic meter / hour) Where: n = number of cells I = Value for the current from table of EN 50272-2 Minimum Inlet & Outlet Area (A): With natural ventilation, the minimum inlet and outlet area is calculated as follows: A should be greater than or equal to 28 x Q (sq. cm)

Way of air circulation in battery rooms:

Table 2 : Recommended Layout Features of UPS Rooms

Design Aspects	Recommended Considerations
Installation	The electrical installation in battery rooms should be limited to: • Lighting • Charging facilities • Ventilation • Hoisting & lifting provisions
Smoke / Gas Detection	Smoke detectors may be installed in battery rooms. In rooms where vented type lead acid batteries are installed, Hydrogen detectors may be installed. Fan operation may be interlocked with Hydrogen detector actuation. If Hydrogen detectors are not installed, the fan shall run continuously.
Battery Room Ceiling	Preferably the room ceiling should be flat to ensure that pockets of trapped Hydrogen gas do not occur, particularly at the ceiling, to prevent the accumulation of an explosive mixture, as per NFPA 70 E- 83.
Ventilation	Refer the table 1 for details. A back up fan also may be considered.
Fire Protection	Carbon Dioxide portable fire extinguishers to be provided.
Lighting	Light fittings should be fixed to the wall or suspended at more than 50 cm from the ceiling, but not vertically above the batteries or charging units. Light fittings as well as any other equipment should be of closed type to prevent accumulation of gas. However, watertight fittings, with flameproof construction are recommended because of possible corrosive & flammable gas environment.

For VRLA batteries, optimal gas recombination is a function of operating temperature.

Heat generated during charging

Float mode the heat generation in watt-hrs

= 0.1 x 2.23 x Ah @ C10 x No. of cells in the battery bank / 100

Boost mode the heat generation in watt-hrs

= 0.2 x 2.3 x Ah @ C10 x No. of cells in the battery bank / 100

The VRLA batteries do not have their own mechanism to remove heat, hence requires skin cooling mechanism.

The UPS system is designed to operate continuously at full load without degradation of its reliability, operational characteristics or  service life in following environmental condition:

UPS Ambient temperature          0⁰C to 40⁰C

Battery Ambient temperature      27⁰C

Humidity                                     upto 95% RH non-condensing

The UPS has its own fan to remove heat from semiconductor devices and dissipate out side the enclosure into room, proper mechanical ventilation is required to remove heat to atmosphere.

Tabular comparison of Batteries based on different technologies                   (VRLA AGM, Tubular VRLA GEL and Flooded)

S. No.	Feature	VRLA (AGM)	Tubular GEL VRLA	Tubular Flooded
1.	Gassing/ fuming	No gassing/fuming, can be installed anywhere	No gassing/fuming, can be installed anywhere.	High gassing/fuming, separate battery room with exhaust system is essential.
2.	Topping up of electrolyte	No topping-up required normally	No topping-up required normally	Topping up required frequently
3.	Charging current level	High	Lower	Lowest
4.	Space requirement	Small cell size, Low space requirement.	Small cell size, Low space requirement.	Large cell size, Large space required.
5.	Stacking	Horizontal or vertical	Up to 1500 AH : Horizontal or vertical >1500 AH: Vertically, in tiers	Vertical stacking only. Tier stacking not practical for large size.
6.	Transportation in charged condition	Easy	Easy	Not possible. Transportation in uncharged (unfilled) condition recommended.
7.	Self-discharge during storage, at an average temperature of 35°C.	50% self-discharge in 6 months. Recovery easy.	50% self-discharge in one year. Recovery easy.	Self-discharge is very high. Long duration storage not recommended. Recovery difficult.
8.	Cyclic Life (to 80% DoD).	1400 cycles at an average temperature of 35°C in normal environmental condition	Better than 2100 cycles at an average temperature of 35°C in normal environmental condition	Theoretically maximum 2000 cycles at 27°C.
9.	Float life at 35°C.	Good	Good	Not known
10.	High temperature performance	Average, but temperature compensation provision made in the Power Plants	Good	Good
11.	Low temperature performance	Good	Good	Poor
12.	Stratification	Negligible, no boost charging required.	Negligible, no boost charging required.	Prominent, requires frequent boost charging for prevention.
S. No.	Feature	VRLA (AGM)	Tubular GEL VRLA	Tubular Flooded
13.	End cell voltage	1.75V/cell	1.75V/cell	1.85V/cell
14.	Capacity at very low rate of discharge	Good	Good	Average
15.	Deep discharge recovery	Average, after 4 to 5 charge/discharge cycles	Average, after 4 to 5 charge/discharge cycles	Poor, hard sulphation prevents recovery.
16.	Charge efficiency	Excellent, 6 to 8 hours for 90% recovery.	Slightly poor, 8 to 10 hours for 90% recovery	Poor, 12 to 14 hours for 90% recovery.
17.	Under-charged performance	Average	Good	Poor
18.	Overcharging	Poor, damages the battery. Over charge protection provision made in SMPS Power Plants.	Good	Good
19.	Performance under partial state of charge	Good	Good	Poor
20.	Charging Requirement	Constant voltage charging by SMPS Power plants	Constant voltage charging by SMPS Power plants	Periodical boost charging at 2.7V/cell essential
21.	Thermal runaway	Probable, yet rare	Not possible	Not found
22.	Risk of internal short-circuiting	Remote	Remote	High, due to active

Design and Construction:

Positive Plate: Flat pasted type with Lead-calcium High Tin alloy grid to resist corrosion & longer life.

Negative Plate: Flat pasted type with Lead-calcium alloy grid for maintenance free characteristics

Container: High impact Polypropylene co-polymer, ribbed jar design for better heat dissipation and strength. Flame-retardant polypropylene UL 94 V 0/28% LOI is optional

Separator: Low resistance, high porosity and highly absorbent type glass mat separator (AGM) Oxygen & hydrogen are recombined internally to form water.

Electrolyte: High purity Sulphuric acid to maximize shelf life. Electrolyte is immobilized (not allowed to flow).

Terminals: Lead plated Copper inserts for high conductivity

Safety Valve: Self resealing, pressure regulated and explosion proof

Container and cover sealing: Heat Sealing Method for better joint strength

Operation:

Type of charging: Constant potential, current limited to 10% of the rated capacity (0.2 C ₁₀ Amp)

Float Voltage: 2.25+/- 0.01 VPC at 27 0C

Boost Voltage: 2.30 +/- 0.01 VPC at 27 0C

AC Ripple: Ripple content shall not exceed 3% RMS

Performance:

Float life: 20 years designed life at 27^O C on full float with recommended charging methods

Cyclic life at 27^O C:

1200 cycles at 80% Depth of Discharge

2000 cycles at 50% Depth of Discharge

4000 cycles at 20% Depth of Discharge

Design life – is used by manufacturers as a measure of comparison. It is a theoretical figure.

It is used as a short-hand method of comparison, as in “5-year,” “10-year,” and “20-year”

battery, and is the basis for pro-rated warranties

Service life - is the more realistic time (in years) from the installation of the battery until its

capacity falls below 80% of its nominal rating. Service life implies a replacement interval

shorter than the design life. In practice, the end of useful life for VRLA batteries can be as

much as 50% below its design life. In extremely harsh operating environments life as low as

20% have been reported. When properly applied, monitored and cared for, VRLA batteries

frequently achieve 70-80% of design life.

Cycle Life

A battery that is seldom used will obviously last longer than a battery that is discharged and recharged every day. Every time you pull voltage out of a battery you discharge it. The amount of watts and the length of time you discharge determine the “depth of discharge” (DOD). The rate at which you pull watts out of a battery, the amount of recharge time between discharges, and the rate of recharge are also important. Battery designs generally assume 2-3 deep discharges per year (100% depth of discharge). As a general rule, a VRLA battery can provide hundreds of shallow discharges (e.g., <25% DOD). Actual field experience will include a wide variety of cycle conditions.

High Ambient Temperature

Battery manufacturers usually describe their warranted or design life in terms of years operating at a particular ambient temperature. In controlled environments reasonable predictions can be made. The rule of thumb for a stationary VRLA battery kept at a constant state of charge (float life) says there is a 50% reduction in life for every 8°C (14.4°F) increase in temperature above optimum 25°C (77 °F). VRLA batteries have gotten a lot of bad press due in part to their widespread use in confined and uncontrolled environments such as outdoor cabinets where wide extremes of temperature are common. The graph in Figure 1 gives a reasonable service life expectancy for continued operation at stable temperatures such as expected in a data center.

Rules of thumb come with many disclaimers. Figure 1 addresses only one variable (temperature), but VRLA batteries are affected by many variables. For example, even though room temperature may be optimum, batteries packed tightly together or stuffed into unventilated cabinets may actually experience a higher internal temperature leading to premature capacity loss.