Build a Solar Battery Charge Controller

Enlarge Image Image Gallery Battery charge in relation to battery voltage. ILLUSTRATION: MOTHER EARTH NEWS STAFF

Photovoltaics, the process of making electricity from sunlight, is growing in popularity among alternative-energy enthusiasts . . . and for good reasons. In operation, PV panels are absolutely pollution-free (the same can't be said for their manufacture, of course) and require very little maintenance. What's more, solar cells are steadily dropping in price and are now competitive with other energy alternatives in many situations.

As is the case with so many of these independent power-generation systems, however, a photovoltaic setup requires some means of energy storage . . . and the most popular medium now is the lead-acid battery. During the day, when sunlight is plentiful, the electricity generated by the PV panel produces chemical changes in the battery cells. Then at night—and during other nonproductive hours—that chemical process can be reversed to retrieve the stored power from the battery.

But charging a lead-acid battery isn't a simple chore. These sensitive electrical instruments require specialized care: There must be a harmonious relationship between the photovoltaic generator and the storage battery if the system is to perform efficiently and provide the years of service that it's capable of. 

Battery Charging

Sunlight, like wind, isn't a constant force. Fortunately, though, it's much more predictable than wind! Seasonal changes and weather notwithstanding, we receive about six hours of productive sunlight each day. Of those hours, the period between 10:00 AM and 2:00 PM offers peak solar radiation and the bulk of the photovoltaic-accessible energy. 

Because charging occurs, at most, for only a quarter of the day, we should stuff as much power as possible into the cells during that period. On the other hand, we also must respect the requirements of the battery in order to insure that it gets fully charged and isn't damaged.

A dead lead-acid battery will accept a very heavy initial charge with little trouble . . . but only at first. As the battery progresses through its topping cycle and its chemical makeup changes, it takes on a completely different set of charging characteristics. When 70 to 80% of the total capacity has been placed in the cells, the electricity being forced in will begin to decompose the water inside the battery . . . breaking it down into its elemental components of hydrogen and oxygen.

You may have noticed this effect without being aware of what was actually going on. The situation is often called "boiling", a misnomer that refers to the percolating appearance of the rising gas bubbles. The process is more properly called gassing. . . and if allowed to continue, it can permanently damage the cells. To prevent this from happening, current is normally reduced just as gassing begins. At the lower rate (often referred to as a trickle charge), the battery can be lifted to 100% capacity without danger.