Solar Regulator

The Regulator:

I had the itch to play with solar electric power and I finally scratched it with a small 6W panel and a car battery. I needed a charge regulator to prevent overcharging the battery. This is what I came up with.

The general consensus says a 12 Volt battery should be charged up to 14 Volts while charging. After prematurely killing 2 batteries, I found that this consensus is pretty accurate.

When the battery Voltage gets to about 14V current starts to flow through D1, D2, D3, R1, and transistor base to emitter. This causes the transistor to open, allowing current from the solar panel to flow through R2 to ground instead of to the battery. R1 is quite small so when current does start to flow, a small voltage increase opens the transistor quickly.

This circuit is only practical for small panels since all the energy dumped from the solar panel is burned up as heat in the resistor and transistor. A heat sink on the transistor would be most helpful, if not required, depending on the size of the panel and the transistor used.

The Fully Charged Indicator:

I wanted an indicator to show if the battery was fully charged and the solar the regulator dumped some power during the day. Of course, that normally happens during working hours, so it has to have some sort of memory. It should not consume any more energy that needed. Here is the resulting circuit I added to the regulator. A and B show the connection points to the regulator circuit.

When the battery is full and the regulator starts dumping charge, current flows through the 10 Ohm resistor. Some of the current bypasses the resistor and flows through the 1N914 diode and charges the capacitor. The capacitor acts as the memory and the diode prevents the charge from discharging back through the 10 Ohm resistor when the sun goes down. When the switch is closed, the capacitor discharges through the LED causing it to light up.

When the capacitor is charging, there is a 0.6 V Voltage drop across the 1N914 diode, and when the capacitor is discharged through the LED, there is about a 1.6 V Voltage drop across the LED. Between the 2, the Voltage across the 10 Ohm resistor need to be about 2.2 V to make all this happen. That means the current must be at least 0.22 A to light the LED.