My setup has 2 domestic battery banks which are the same size being each 630 Ah, but the batteries are different ages.
Normally I use my 1,2,both domestic switch in the both position when on board but when I heave the boat for some time I switch this switch to the off position. I also each bank has a permanently connected 80 watt solar panel through a simple on off regulator to keep the batteries topped up when I am away.
Due to the different ages the resting voltage will and I can see are slightly as I have seperate volt meters and current meters for each bank.
So when I return to the boat and I switch my 1,2,both switch to both I can see a relatively current flow from one battery to the other as 1 battery ammeter will show a positive flow and the other a negative flow due to the small voltage difference and if you consider that a fully charges battery resting voltage will be about 12.7/12/8 volts and a battery of say 75% charge will be about 12.5/12.6 the volt difference is small so the current will be small depending on the internal and external resistance of the batteries and cables.
