What are the standards and the basic conditions for the planning of a battery installation? How can the proper Ah size be determined?
Standards
In order to ensure safe and reliable operating conditions, the lay-out and the operation of the battery installation should be carried out in accordance with code of practices and the specifications of the respective country, the manufacturerˇŻs recommendations and general site considerations. For samples of specifications refer to International Standards
Battery Capacity Sizing
When the battery application is considered, we make a distinction between a float service with
• discharges with constant current
• discharges with constant power.
In lead acid batteries and other electrochemical systems the available capacity is dependent on the magnitude of the discharge current (high current = lower capacity - low current = higher capacity).
In lead acid batteries this relation is described by the Peukert equation which has the form of
C = Ivˇ¤t
C= capacity (Ah)
I = current (A)
t = time (seconds, minutes, hours)
v = constant
To facilitate the proper battery size selection CCB battery .supplies discharge current and discharge power tables for a wide range (1.90 - 1.60 Vpc) of end-of-discharge voltages and back-up times of 1 to 1200 minutes.
The following two examples can be used as guide for the sizing of battery installations.
Battery capacity sizing for a discharge with constant current.
Example:
An electrical circuit installation with a user load of 20 A, an operating DC voltage of 48 V -10 % to 48 V +15 % has to be powered by a VRLA backup battery for at least 5 hours at 0 ˇăC (32 ˇăF).
The basic calculations are the following:
Maximum DC bus voltage Vmax = 48ˇ¤1.15 = 55.20 V
Minimum DC bus voltage Vmin = 48ˇ¤0.90 = 43.20 V
Number of 2V cells Vmax/Float Voltage 2.25 =55.20/2.25 Vpc = 24.5 approximately 24 cells
Minimum discharge voltage Vmin/ 24 cells = 43.20/24=1.80 Vpc
Nominal end of life at 80 % of rated back-up time
With above data of 20 A, 300 minutes and an operation at 20 ˇăC, one would find in the constant current discharge data tables, for 1.80 Vpc end-of-discharge voltage, the 12V110AH with 20.9 A - 300 minutes the most suitable one. In order to achieve the desired minimu string voltage of 43.20 V a total of 43.20/3ˇ¤1.80 Vpc = 4 monoblocs are required. However, the requirement of 0 ˇăC (32 ˇăF) operation and the related lower battery performance at this temperature requires a capacity correction as follows:
Back-up time at 20 ˇăC = 300 min ˇ¤ 20 A =100 Ah
Reduction of back-up time at 0 ˇăC = -0.6% per ˇăC at Tˇă lower than 20 ˇăC
Back-up time reduction at 0 ˇăC = -0.6 x 20ˇă = -12%
Required 20 ˇăC back-up time to = 300 min + 12 % = 336 min or
112Ah meet 0 ˇăC requirement
The 12V110AH battery has only 104.5 Ah capacity which is insufficient to satisfy the 0 ˇăC requirement. Therefore, one would select the next larger type, 12V120AHmonoblocs which have a 300 minute capacity of 24.2 A x 300 minutes = 121 Ah. Operated at 0 ˇăC this unit has 121 Ah - 12 % = 106 Ah, thus, meets the 300 minutes 20A @ 0 ˇăC = 100 Ah requirement.
End-of-service life (EOL) in a lead acid battery is defined to occur when less than 80 % of the rated capacity or back-up time is available. In the case that in the above installation a permanent minimum back-up time of 300 minutes is required over a defined period of service life, then an additional capacity reserve of +20 % has to be present from start.
Under such a condition the capacity required is:
20 A x 300 minutes =100 Ah+12 % (0 ˇăC service) + 20 % (full back-up)
100 Ah x 1.12 x 1.2 = 134 Ah. 26.8 A x 300 minutes
The most appropriate monobloc would then be:
A 12V134AH unit with 27.7 A for 300 minutes to 1.80 Vpc.
Battery capacity sizing for a discharge with constant power
Example:
A battery back-up UPS installation has to deliver 60 kVA to a computer system for 20 minutes at 20 ˇăC ambient temperature. The inverter and charger system has following characteristics:
Load S = 60 kVA Maximum input voltage = 432 Vmax
Power factor cos phi = 0.8 Minimum input voltage = 308 Vmin
Efficiency eta = 91% Charging voltage = 400 Vcharger
The basic calculations are the following:
Power drawn from the battery P= Sˇ¤cos phi/eta=60ˇ¤0.8/0.91=52.75kw
Number of 2 V cells (with 2.25 Vpc float voltage)
Cell # = Vcharger / 2.25 Vpc = 400/2.25 = 178 cells
Maximum number of cells Cell # = Vmax / 2.25 Vpc = 432/2.25 = 192 cells
End-of-discharge voltage per cell Vmin/# cells = 308 V/192
cells = 1.60 Vpc
Power from the battery per cell P/# cells = 52.75 KW/192
= 274.7 W/cell
= 824.2 W/6 V monobloc
= 1648.4 W/12 V monobloc
Due to the well known Peukert relation „high current = lower capacityˇ° it is advisable to use the maximum allowable number of cells to supply the 52.75 KW power to the inverter.
With above data of 824 W/20 minutes and an operation at 20 ˇăC one would find in the constant power discharge tables, for 1.60Vpc end-of-discharge voltage, the monobloc of
the type 12HD-310 with 930 W for 20 minutes the most suitable. With this extra power available (+11%) a partial compensation of reduced back-up time due to the battery aging can be accommodated (effective back-up time = 23 min i.e. 115%). A total of 32 monoblocs is required. In the case of low temperature service the capacity derating is -1 % per ˇăC from 20 ˇăC.
