Build a Solar Charge Controller

For an efficient, long-lasting solar electric system, learn how to construct a solar charge controller.

| March/April 1984

  • Batter Charge to Battery Voltage
    Battery charge in relation to battery voltage.
    Illustration by MOTHER EARTH NEWS staff
  • Chematic Diagram
    A schematic diagram for building a solar battery charge controller.
    Illustration by MOTHER EARTH NEWS staff
  • Circuit Board
    Positive of circuit board.
    Illustration by MOTHER EARTH NEWS staff
  • Battery Charge Controller Setup
    Parts layout.
    Illustration by MOTHER EARTH NEWS staff
  • Temperature Vs. Battery Voltage
    Temperature versus battery voltage.
    Illustration by MOTHER EARTH NEWS staff

  • Batter Charge to Battery Voltage
  • Chematic Diagram
  • Circuit Board
  • Battery Charge Controller Setup
  • Temperature Vs. Battery Voltage

Solar Charge Controller

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 percent 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.

1/8/2015 8:13:42 PM

If anyone is looking for solar charge controllers for a DIY home solar project. I would like to recommend we have a large number of brands with discount prices.

3/23/2014 11:43:25 PM

hi i couldnt see the circuit

Kyle Nicoletti
3/21/2014 1:09:01 AM

hey wayne check out he has a pretty simple charge controller works great too.

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