How to Size a Battery Bank

Selecting batteries to meet the Amp-hour capacity

Now that you know the Amp-hour (Ah) capacity that will give you the storage you need, you may need a little guidance in selecting specific batteries. Keep in mind that it’s best to keep the number of parallel strings of batteries to three or fewer. If you parallel more than three strings of batteries, you risk shortening battery life due to uneven charging¹. The ideal battery bank has no parallel connections but is comprised of one or more series-connected batteries. Though parallel connections are not to be avoided at all cost, fewer such connections tends to reduce the chance of charging problems over time.

When batteries are cabled together in series, the voltage is additive. For example, you can put two 12V, 100 Ah batteries in series for a 24V bank. The capacity of that bank would still be 100 Ah. When batteries are connected in parallel, the voltage remains constant and the Ah capacity is additive. In our example with the 12V, 100 Ah batteries, connecting them in parallel would result in a 12V system with a capacity of 200 Ah.

The batteries you select must meet both your system voltage requirements AND the Ah capacity you calculated. In our example of the 48V system, we calculated that we needed 1,040 Ah to produce 6,000 Wh per day with 3 days of storage. More than one configuration of batteries can meet this need. For example, you could have four 12V batteries in series, each with a capacity of 1,040 Ah or more. Or you could use eight 12V batteries wired in two parallel strings where each battery had a 520 Ah capacity. Or you could use twelve 2V batteries in series, again with appropriate Ah capacities. In any given case, there may be multiple solutions. Your choices will be limited by battery availability and budget.

Building the bank: Amps, then Volts

To build your bank, try first to select a battery that is rated close to the Ah capacity you calculated in Step 5 above. Ignore voltage for a moment. If you can’t find one that’s very close, look for one that has a capacity either one-half or one-third your needed Ah figure. These fractions represent the number of series strings of such batteries you would need, in parallel, to complete your bank (1/2 = 2 strings, 1/3 = 3 strings). Once you find a candidate battery, divide your system voltage by the battery’s voltage. This will give you the number of such batteries you would need in each series string.

The total number of individual batteries you will need to complete your battery bank will be the product of the number of strings needed to meet your Ah requirement and the number of batteries per string needed to meet your system voltage requirement.

You can then compare your candidate battery banks against price, size and availability. You may want to talk with people who have used these batteries and learn what their experiences have been, compare warranties and advertised features, and finally buy the batteries you feel are best for you.

In any battery-based RE system, batteries are a major component investment—second in cost only to the PV modules in most cases—and they are a critical part of the system. Careful planning and battery selection is vital to ensure that your battery bank meets your needs and provides many hundreds or thousands of charge cycles. Take your time, run the numbers more than once, and you’ll avoid the worst pitfalls of RE system design.