My setup in my motorhome is 240W solar panels connected to 200Amp Hour house battery through an MPPT solar controller. The starting battery in the cab has a constant current draw around 200mA, flatting it if not used after a month or two.
The problem seems simple to solve on the surface - connect the solar to the starter battery, but it turns out to be a bit more tricky than that. I don't want one battery going flat dragging the other down, so can't just connect them. My question is what is a good (as in simple and cost effective) way to connect the solar output to the starter battery so the starter is maintained in a charged state.
What I have considered so far (an within my limited electronic skill range)
- Replace the controller with on specifically capable of solving this. Best, but expensive
- Connect though a low value resistor (say 3 Ohm, 50W ) as a current limiter - Cheap, easy but could flatten house batteries if cab lights left on etc, and flatting the house battery could drag down the starter.
- A second small solar panel - costs add up by the time you factor mounting and wiring it in, and there is not much space left on the roof anyway.
Although I am OK with a soldering iron, ideally I could do this with one or two the shelf components for a few 10's of dollar.
I am looking for suggestions that I may not have come across.