A Photovoltaic Battery - Charge Controller

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THE CONTROLLER

Obviously, if we're to match our photovoltaic power cycle to the battery's charging pattern, we'll have to bring the cells to the point of gassing within the four-hour period between 10:00 AM and 2:00 PM. Then, through the remainder of the day, a trickle charge can be applied to lift the battery above 80% of of capacity. And a simple controller can decide when to reduce the current.

Fortunately, the battery itself gives an electrical cue when the gassing point is reached. There's a well-defined relationship between the state of the cells' charge and their voltage, as shown in Fig. 1. For a 12-volt battery, gassing begins at 12.6 volts . . . and that battery will be fully charged at 13.2 volts. [EDITOR'S NOTE: For a more detailed analysis of batteries and their characteristics, see "MOTHER's Guide to Storage Batteries", page 114, MOTHER NO. 74.]

The controller, then, contains an electronic comparator that monitors the battery's voltage and, in turn, manages a relay. When the voltage is low, the comparator leaves the relay in its normally closed position, allowing the full photovoltaic output to go to the battery . . . but once it reaches that 12.6-volt threshold, the relay snaps open and shunts the charge through a current-limiting resistor. This produces a trickle charge that's low enough to continue indefinitely without damaging the battery.

To prevent the battery from discharging through the PV panel at night, a diode is included in a series circuit in the positive lead. This one-way valve also prevents the controller from drawing power from the battery . . . so that all of the circuit's needs come from the photovoltaic generator.

CONSTRUCTION

Building your own charge controller is a relatively straightforward electronics project, based on a printed circuit board. In MOTHER NO. 84, page 136, I discussed the easy preparation of these convenient circuit paths. But if you prefer, you can order one readymade from Danocinths (the address is included in the bill of materials that accompanies this article).

Once you've prepared or purchased the circuit board, simply insert the components in their appropriate holes, as shown in Fig. 4, and solder them in place, using a low-wattage iron. Make absolutely sure that the integrated circuit and semiconductors are facing the right direction. It's easy to reverse them, and doing so will lead to their rapid demise.

To help you keep tabs on how the charging process is progressing, a monitor has been designed into the controller. Indicator lamps LED 1 and 2 show, respectively, when the circuit is at full current and at a trickle. (This feature isn't essential to the controller's operation, but it can be handy. If, however, you decide to eliminate it — by removing resistors R6 and 7 and lamps LED 1 and 2 — the device will still do its job.)

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