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)

  1. Replace the controller with on specifically capable of solving this. Best, but expensive
  2. 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.
  3. 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.

  • \$\begingroup\$ To the downvoter, or anyone else who can add some insight, please explain whats wrong with my question. \$\endgroup\$
    – user71573
    Commented Oct 10, 2013 at 0:14
  • \$\begingroup\$ The down-voter probably couldn't understand that this is an electrical design question - which it is. See my answer for possible solutions. \$\endgroup\$
    – Russell McMahon
    Commented Oct 10, 2013 at 2:40

1 Answer 1


If you trickle charge the starter battery using voltage from the panels rather than the MPPT controller output you have ample voltage to add a blocking diode and prevent backflow. The current drain is low enough to not significantly affect how the MPPT controller functions.

Long term trickle to a lead acid battery may cause damage. A simple voltage regulator set to a suitable "float" voltage is probably all that's needed to prevent this. This can be cheap simple and compact.

You could join the batteries with a "switch" for charging purposes, with the switch being activated only when the panels were providing energy. The switch could be a transistor but in this case a relay is probably an easy and good solution. Drive relay from panel with a "controller" that senses panel voltage - or use MPPT output or signal to switch relay.

Joining the batteries via a diode prevents back-feed but if the main battery is properly managed by the controller then the lower voltage on the other battery caused by the diode drop can noticeably affect battery lifetime. [I've seen this in practice]. Using a Schottky diode reduces the drop. The diode may try and handle starting currents in some cases so a little thought may be needed.

  • \$\begingroup\$ Thanks - enough useful ideas to give me some homework. I like the idea of tapping into the solar panels - will work on a solution using that. \$\endgroup\$
    – user71573
    Commented Oct 10, 2013 at 19:45

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