It is important to note here that a 12 volt battery is usually made up of six 2 volt “cells” wired in series to give us 12 volts. The size of the cells and consequently the size of the entire battery will usually determine the amount of electricity the battery can hold. The measure of battery capacity is the “Amp Hour”.

A typical Marine/RV, deep cycle battery which is similar to an automotive starter battery will have around 80 amp hours of storage capacity. To charge our 12 volt battery (bring it up to 14.5 volts) requires that we provide some amperage to “drive” the voltage of the battery upward. For this reason our charging source (solar, wind, fuel cell or hydro) has to generate amperage as well as voltage. The more amperage it generates, the faster it will push the battery voltage to the 14.5 volt stage.

As an example, let’s say that we have a solar panel capable of putting out 5 amps in full sun and generating 17 volts. Using the manufacturer’s rating system, we can multiply the amps times the volts of the solar panel.

Thus, 5 amps times 17 volts = 85 watts. If our 80 amp hour battery is 100% dead (100% depth of discharge) we can assume that it will take the 85 watt solar module 16 hours of full sun to completely restore the depleted 80 amp hours (80 amp hours divided by 5 amps per hour). In reality because of battery inefficiency and other factors such as temperature and battery type it will take slightly longer. We typically use a 15% inefficiency factor to estimate the recharge time for wet cell batteries. This means that we would have to provide 115% of the amp hours (92 amp hours) taken from the battery to recharge it. So it would actually take 18.4 hours of full sun equivalent to completely recharge the dead battery with our 85 watt panel. If we choose to wire two 85 watt solar panels in parallel, we will now have 10 amps output with a maximum of 17 volts. Recharging of the 80 amp hour battery will be reduced to ½ of the time it took with only one solar panel.

We could take this discussion into 24 or 48 volt battery systems or systems which use 2 volt or 6 volt batteries in series and at some point we hope to expand this site to include more information on those types of systems. There are some excellent publications in our book department as well as in the Sunelco Planning Guide and Product Catalog that provide extensive information on batteries and battery charging. We hope you will take the opportunity to add one or more of them to your library.