Batteries Unlimited
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Methods of Calculation
The examples shown below are intended to illustrate the steps to be taken to determine the cell size to meet the duty specified.
Example A (A constant Current Discharge)
DUTY
200 Amps for 2 hours
Nominal voltage: 48 volts
Maximum battery discharge voltage 44.4 volts
Maximum battery float voltage 54 volts
Temperature 25 degrees centigrade
Maximum number of cells = maximum battery float voltage/2.25
Volts (nominal cell float voltage) = 54 volts/2.25 volts = 24 cells.
Minimum cell discharge voltage= Minimum battery discharge voltage/ maximum
Number of cells = 44.4 volts/24 cells = 1.85 volts
From data sheet for Constant Current discharges to 1.85 volts
YHP13 gives 199 Amps for 2 hours to 1.85 volts
YHP15 gives 232 Amps for 2 hours to 1.85 volts
ZHP15 gives 214 Amps for 2 hours to 1.85 volts
BRCT8)) gives 216 Amps for 2 hours to 1.85 volts
Example B (Constant Power Discharge)
Duty
400 KVA for 15 minutes
Power factor: 0.80
Efficiency: 0.90
Maximum Voltage: 435 Volts
Minimum Voltage: 306 Volts
Power Requirement = KVA x Power Factor divided by Efficiency = 400 x 0.80/0.90 = 355.6 Kw
Maximum number of Cells = Maximum Voltage divided by 2.25 (nominal cell float voltage) = 193
Minimum Cell Voltage = Minimum Battery Voltage divided by Number of Cells = 1.58 volts
Watts Per Cell Required = Power Requirement divided by the Number of Cells = 1843 Watts
From the data sheets for Constant Power discharges to 1.60 volts
YHP19 gives 1973 watts per cell 1.60 volts
ZHF21 gives 1924 watts per cell 1.60 volts
11RCT1100 gives 1900 watts to 1.60 volts
Because the YHP19 gives greater than requirement it is possible to reduce the number of cell as follows:-
Try 186 cells,
Minimum cell voltage = 306/186 = 1.645 volts
Watts Per Cell Required = 355600/186 = 1912 watts
From the data sheets for Constant Power discharges to 1.65 volts
YHP19 gives 1944 watts per cell to 1.65 volts per cell similarly
Try 180 cells,
Minimum cell voltage = 306/180 = 1.70 volts
Watts Per Cell Required = 355600/180 = 1976 watts
From the data sheets for constant power discharges to 1.70 volts
YHP19 gives 1773 watts per cell to 1.70 volts per cell
Therefore 180 YHP19 will not meet the duty
Use 186 YHP19 cells
Alternatively it may be preferred to meet the duty with two parallel banks of smaller cells, In this case the load is divided by 2 giving 177.8 Kw per bank, or 921 watts per cell.
Again, from the data sheets for constant power discharges to 1.60 volts
YCP33 gives 1005 watts per cell to 1.60 volts
ZCF31 gives 978 watts per cell to 1.60 volts
6RCT600 gives 1036 watts per cell to 1.60 volts
Using the same logic for YHP19 above
Try 186 cells,
Minimum cell voltage = 306/186 = 1.645 volts
Watts Per Cell Required = 177800/186 = 956 watts
From the data sheets for Constant Power discharges to 1.85 volts
YCP33 gives 1116 watts per cell to 1.65 volts per cell
Similarly,
Try 180 cells,
Minimum cell voltage = 306/180 = 1.70 volts
Watts per cell required = 177800/180 = 988 watts
From the data sheets for Constant power discharges to 1.70 volts
YCP33 gives 988 watts per cell to 1.70 per cell
Therefore 180 YCP33 will meet the duty
Use two parallel banks of 180 YCP33 cells
