The Solar Design Book

Example 5, Battery System

This is an example of a battery system. It is for a cabin in the mountains near Kalispell, Montana. The cabin is use in the summer from June through August. The cabin will have a 12-Volt system to run lights, a small refrigerator, a music center, and some other 12-Volt appliances. The refrigerator is an energy efficient model and the lights are LED type. The energy use is 1,500 Wh per day and the maximum draw on the battery is 480 Watts or 40 Amps. The array will be ground mounted, oriented south and tilted to the most optimum angle for the summer use. Two parallel sets of two each series connected 6-Volt batteries are planned (four 6-Volt batteries total).

The SolarSizer program is used to calculate the battery and array size. Losses are assumed to be 10% each for the wiring, soiling and aging. The efficiency of the charge controller is assumed to be 96%. The batteries return 80% and 50% discharge is planned. The battery bank is sized to run the cabin for 3 days and the array is sized to charge the battery bank in 3 days. There is a standby generator in case there is a string of cloudy days and to run the washing machine and power tools.

The SolarSizer program calculates the required battery size to be 375 Amp-hours each for a total of 9,000 Wh. It also calculates the array size as 802 Watts when tilted at the optimum angle of 27°. The latitude of Kalispell is 48°. If the cabin was used year round, the array size and optimum tilt would be much different. With the 12 volt appliances off, the system produces enough energy to keep the batteries charged during the winter. The owner tilts the array up to 60° to keep the snow off during the winter.

Details:

The following inputs are used in the SolarSizer program:

1. Task chosen: Size a stand-alone battery system
2. State: Montana
3. City: Kalispell
4. Fixed or tracking: Fixed
5. Tilt of Panel: 27°
6. Orientation of Panel: 180°
7. Temperature Coefficient: -0.50 %/°C (default in program)
8. Junction Temp above Amb @ 1,000 W/m2: 34.00°C (default in program)
9. Fraction of Diffuse Used: 100.00% (never changes except for concentrators)
10. Daily DC usage: 1,500 Wh/day
11. Soiling loss: 10%
12. Aging loss: 10%
13. Wiring loss: 10%

In the Battery tab:

1. Depth of discharge: 50%
2. Battery charge return: 80%
3. Battery bank voltage: 12 Volts
4. Parallel strings: 2
5. Maximum charge current: C/5
6. Maximum discharge current: C/5
7. State of Charge after bulk: 75%
8. Controller efficiency: 96%
9. Days of battery power: 3 days
10. Days to charge: 3 days

The SolarSizer program shows the required battery bank to be 9,000 Wh or 375 Ah per battery. Four 6-Volt batteries of 375 Ah each were chosen. The SolarSizer program shows the maximum allowable battery bank drain to be 300 Amps total, which is far below the anticipated 40 Amps. The SolarSizer program shows the required array size to be 802 Watts and the optimum tilt angle to be 27° for the summer months of June, July and August.

The battery will last a long time if only used during the summer and kept charged curing the winter. There is reserve capacity because the battery bank could be discharged to 80% instead of 50%. The generator provides emergency power if needed and power to run large loads. The combination of generator and solar keep the overall cost low because the generator is not run constantly and the solar system does not have to be sized for the occasional long string of cloudy days.

The battery recovery chart shows, in June, that the battery is fully charged in 3 days from 50% state of charge. The chart shows that the battery is charged sooner in July and August due to the increased solar resource.

The WireSizer program can be used to size the conductors and breakers.