Solar Dump Mode Controller

In the solar and wind generation area, people talk about Dump Mode Controllers. What this really means is that you bring the batteries up to a certain level of charge and then dump the remaining power into some other load. One of the more common loads is a 12 Volt hot water heater. In this situation you top off the batteries and then heat water until the solar or wind stops. The controller then resets back to the first battery.

The RV problem is that you want to charge your house battery and your engine battery as both get discharged by different electrical loads while you are dry camping. In my case I added a third battery which was going to be used during cloudy weather to replace my house battery. This battery was actually in a battery box that was not connected in any way to the RT. We had very good weather this year in Baja. The battery only got used to start a friends pickup when his battery went bad and to recharge the laptop battery through a small inverter a few times.

Below you will find a diagram of the controller showing 3 solar panels and 3 relays. The numbers refer to a connector that was placed along the edge of the circuit board. The zener diodes are listed as 6.2 volts but 6.6 would work better. The trip point for each relay is the pull in voltage of the relay plus the zener voltage. The battery sees one diode drop less. My relays pulled in at 8 volts so add 6.2 and you get 14.2 but the battery is only seeing about 13.5 volts. Some people say it should be just a little higher than that so a zener voltage of 6.6 would work also. The diodes that are in series with each battery must be large enough to handle the full charging current. The reset switch forces the controller back to battery number one.

The size of the relays would depend on how much total current you planned to supply. In my case 5 amps was enough so I used some small relays. The current to run the relays comes from the solar panel and not from the battery.

I added 3 LEDs so I could tell which battery was being charged. This made understanding what was going on a lot easier. Each one was a different color. Green was battery number 1, red was battery number 2, and white was battery number 3. Blue meant that all the batteries were full.

Wiring Diagram
There are two down sides to this type of controller. Once the controller has moved to the second or third battery, someone could come and discharge the first battery and it would not get any more charge until the next day when the controller starts over. To solve this problem I added a switch which reset the controller to the first battery. Sometimes we were onto the second battery before lunch and we wanted to use some electricity to make lunch. After lunch I would hit the switch and let it start over on the first battery.

The second disadvatage is that a pulse width modulated controller can probably get 5% to 10% more charge into a battery than a on/off type controller. The downside of these controllers is when your battery gets all the way up, the rest of the power is wasted. I decided to save the wasted power and not worry about the last 5% to 10%. This is a trade off that works for me but might not work for everyone.