Battery/System Sizing Examples

Following are examples to assist you in choosing the correct Discover battery for your needs. In the first 2 examples the constant current load is given in amperes. In the third example the load is a constant power in kilowatts. Finally, the last example is slightly more involved as an inverter power factor and its efficiency need to be accounted for.

Example 1: Constant current battery load at 20°C (68°F)

Load: 170 amps
Support time: 60 minutes
System Voltage requirements: 240V
End of Discharge Cell Voltage (ECV): 1.75 VPC

Calculation

By checking the discharge tables for an ECV of 1.75 VPC and a support time of 60 minutes under a constant current load of 170 amps, we find that the EVL16A-A will meet the requirements. The next step is to calculate the number of batteries per series string. In this case there will be 40 batteries per series string as the EVL16A-A has a nominal terminal voltage of 6 volts (240V/6V per battery = 40 batteries).

Example 2: Constant current battery load at 20°C (68°F)

Load: 400 amps
Support time: 45 minutes
System Voltage requirements: 240V
End of Discharge Cell Voltage (ECV): 1.65 VPC

Calculation

By checking the discharge tables for an ECV of 1.65 VPC and a support time of 45 minutes, we find that no single battery within the EV product line is capable of delivering 400A for 45 minutes to 1.65 VPC. Therefore we must add another 240V string of batteries in parallel so the load can be halved to 200A per string. By checking the discharge tables again we find that the EV8DA-A will support 200A for 45 minutes and maintain an ECV at or above 1.65VPC. We can then calculate that we will need 2 strings of 20 EV8DA-A batteries (20X12V=240V) connected in parallel to support the constant current load requirements while maintaining and ECV at or above 1.65VPC.

Example 3: Constant power battery load at 20°C (68°F)

Load: 55 kilowatts (55,000 watts)
Support time: 15 minutes
System Battery voltage: 360V
End of Discharge Cell Voltage (ECV): 1.75 VPC

Calculation

Since the EV battery discharge tables give the constant discharge ratings in watts per cell (WPC) the first step is to calculate the number of cells we will need. In this example we will use 180 cells per series string (360V/2V per cell = 180 cells) since each cell has a nominal voltage of 2V. The next step is to convert the load to a per cell basis. In this example the load is 305 WPC (55,000 watts / 180 cells = 305 WPC). We can now check the tables to find a support time of 15 minutes and an ECV of 1.75 VPC for our 305wpc needs. The smallest battery that can support this load is the EV31A-A, which is capable of delivering 310 WPC for 15 minutes to 1.75 VPC. Therefore we will use 30 EV31A-A 12V batteries to support the load (360V/12V=30).

Example 4: Constant kilovolt-ampere (KVA) battery load at 20°C (68°F)

Load: 55 KVA (55,000 VA)
Inverter power factor: 0.85
Inverter efficiency: 90%
Support time: 15 minutes
System Battery voltage: 360V
End of Discharge Cell Voltage (ECV): 1.75 VPC

Calculation

The first step is to convert the KVA into an equivalent KW by using the following formula:

Kilowatt = (Kilovolt ampere x Power Factor) ÷ Efficiency

55 x .85 / 90% = 51.9

Using this formula the above numbers translate into a kilowatt requirement of 51.9 kilowatts. The subsequent steps are identical to those outlined in the third sizing example given above.