You may recall from an earlier post that the Solar Powered Rainwater Project requires a solar battery bank. In our original calculations, we decided we would need 15,796 watt/hours of storage to provide three days of autonomy (or days without sun). See the system sizing guide for how we calculated this. We would need 16, 190-amp hour golf cart batteries to reach this goal. This becomes not only a cost issue (about $1800) but also a logistical and space issue. Each battery weighs around 60 pounds. That’s nearly a half ton of lead!

In this post, we’ll look at how to size, configure, and wire a practical solar battery bank for an off-grid system.

To test my ideas and get the project running more easily, I decided to cut the number back to eight batteries. It turned out that Interstate 6-volt golf cart batteries were available from Costco for $83 each, and were rated at 205 ampere hours. Eight batteries would yield 9840 watt hours of storage at 48 volts — nearly 2/3rds of our original goal. Plenty to get the project started.

Why 48 volts? We chose a 48-volt system to keep current manageable and reduce cable losses.

Battery Bank Layout: Storing a 1/2 ton of lead

The next problem was, where would they go? The garden shed has a concrete floor, and I realized they would partially fit under shelving located inside. If I had a way to roll them out for easy service, they wouldn’t take too much space. My friends at Vintage Auto agreed to weld up the cart I designed. Storage problem solved for the time being.

300 Amp Wire

Now on to cabling. The inverter is rated at 4000 watts, and accounting for inverter efficiency and the 48-volt DC supply, we would expect a 95-ampere current drain. The inverter can supply more power  though — up to 12,000 watts peak. Brief surges of up to 285 amps could be expected. Just to be safe, I decided on 0/1 rubber-covered welding cable. Perhaps overkill, but I like that. The cables weren’t hard to assemble with the copper terminals and hammer-actuated crimper, and I had the them done in no time. I also added heat-shrink tubing with internal adhesive to prevent corrosion inside the crimp.

With a shunt for metering installed on the negative lead, and a 200-amp breaker, we were ready to power the inverter!

Designing a solar battery bank is always a balance between capacity, cost, weight, and space. Start with your ideal design, then adjust to what’s practical.

Next: Final Assembly