I don't think I've had more than a week's worth of sunny days since mid October. With my solar panels working at barely a dribble, how was I going to keep my batteries charged? I came up with a circuit that lets me hook a grid powered charger to my batteries. If my solar panels can't keep the batteries charged, the circuit kicks in and turns on the charger. The rest of the time, it stays turned off. The beauty of this switch is it's fully automatic. I've been running it for a couple years now and I've never had to worry about dead or undercharged batteries. It's been completely trouble-free.
In many respects it's like a reverse grid-tie system. You're hooked to the grid, but instead of sending excess electricity out onto it, you draw grid power when your alternative energy production falls off. It protects the batteries and allows you to run loads with your inverter even though you can't produce energy. The benefits of a reverse grid tie system are that you don't need the expensive equipment, and you don't go down when the grid goes down, at least not until your batteries go dead.
The Grid Charger Voltage Switch uses a PIC microcontroller to monitor battery voltage. It turns on a solid state relay when it detects a lower than optimum battery voltage. The relay, in turn, controls a grid powered battery charger. It stays on for a couple hours. Then it turns off again and waits for the next low battery event. There's also an over voltage shut-off in case the charger turns on and then the sun comes out. If I lived in the southwest, I wouldn't need this circuit, but I live in New England, and it has kept me from giving up completely on solar power. I don't get a lot of solar electricity, but when the sun does shine, I get a free ride.
Several people here have asked about the specifics of the Grid Charger Voltage Switch. It has taken me awhile to get everything together, but now I have the directions, schematics, parts lists, and PIC programming ready for anyone who would like it. I put it on my web log, http://altenergyweb.com Do-it-yourselfers will find everything they need to build their own circuit. There are kits and circuit boards available if you need them.
I hope some of you will take a look at the plans and tell me if I need to make anything clearer. Thanks, --tom
