If the starter and house batteries are both essentially the same type - ie one is not a gel type
- AND provided that if the starter
battery is an AGM and the house is a flooded cell type (an unlikely combination), then the
solar regulator is not set to provide a periodic equalising cycle
- AND if you are just looking to keep the starter
battery charged when the vehicle is sitting for long periods
THEN it should be satisfactory (but not necessarily convenient) to just connect both positive terminals of the batteries together. The lead you use should have a
fuse at each end of the lead - say 30 amps - and the jumper should have a cross-sectional area large enough to carry the 30 amps without overheating.
This will not work if one
battery is fully charged and the other well down when you connect them and you also need to be careful not to ground one end when the other end is still connected to the
battery.
If you want to do the job properly, there are
battery isolating units that work both ways - ie wait until the engine
battery is charged before connecting the house
battery to the alternator OR wait until the house
battery is charged before connecting the engine
battery to the alternator.
If you already have a one-way unit installed to charge the house
battery from the engine alternator when on the road, then you could (depending on the unit and exactly how it was connected) add some simple switching to reverse the connections when you put the vehicle in storage.
There are also
solar regulators that do a similar job by charging two banks of batteries while keeping them isolated from each other but they are quite expensive.
