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Anyone have experience with parallel batteries in a vehicle -- maybe like a diesel or RVs.

I'm wondering how close the batteries have to be in capacity to avoid having to limit charging current to the smaller battery -- and also how close in type to avoid damage to one. Am thinking an Optima or AGM type lead acid might not appreciate the high charge current possible with an ordinary lead acid but maybe that is not the case.

Was hoping not to need two automobile size batteries but rather one auto size and one that is smaller and rated at around 60AH that can be easily hidden...The question is made more difficult by the difference in how batteries are rated -- CCA for auto vs AH for most everything else. The lack of an accurate way to make a direct correlation between the two ratings presents a problem. Any info or advice would be appreciated.

I am wanting to run two batteries in parallel part of the time and in series other times. Have made a circuit and module that seems to work quite nicely for the original plan of just operating the clutch but am thinking of increasing the capacity, changing a couple of wires around and going in a different direction by powering the entire unit.

I know they sell a gizmo that isolates the two batteries a bit so only one is charging at a time with a bias toward the main battery. Maybe called a battery isolator?

I had one in a car many years ago when I had an over the top stereo system. I had a deep cycle battery in the trunk that ran the stereo amps. So I could let it play at cars shows and what not, and it would drain the deep cycle, but not the starting battery.

But it was many years ago, so I don't remember all the specifics about how it worked from the charging side.

Thanks. The isolators I've found are all 12v which would be unusable in the 6v system and they do exactly as you describe-- allow charging of both but cut the main off if voltage or demand was in danger of dropping the main below a certain point. Ones I looked at didn't seem to be adjustable.

Not quite sure how it would handle the situation when batteries were placed in series but do believe the electronic sensing would react if one battery was effectively disconnected from parallel for a period and then reconnected. Probably sense that as a fault or bad connection and cut the second one off completely.

vanner makes a battery isolator they also make a d.c. voltage step up module. i don't know if that is something you are looking for but it might be a start.

You might want to check with your local RV dealer. Lots of the large trailers have two batteries with the required electronic gadgets to make things work properly.

Usually what happens is that if one battery is in poor condition it takes the second battery down with it.

If both batteries are in like condition running them in parallel should be no different than using a single bigger battery. However, using two different sized batteries may cause the smaller unit to be overcharged because the regulator is sending a single current output.

I suppose the isolator/cutout thing will work if you charge the bigger battery first and the smaller unit second. That would prevent the combination from being charged like a bigger battery with weak cells.