Submitted: Monday, Oct 01, 2012 at 07:50
ThreadID: 98338 Views:2047 Replies:2 FollowUps:2
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I was wondering if someone could tell me if it was ok to run an agm battery alongside a deep cycle wet cell battery. Setting up my camper trailer with Anderson plug from car batteries, a multi stage 240v charger and possibly solar added later. I was always going for just the one 120ah agm but have been given a deep cycle wet battery as well so is it a case of the more the merrier or does it complicate things if they aren't both the same type battery?
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Reply By: Notso - Monday, Oct 01, 2012 at 10:36

Monday, Oct 01, 2012 at 10:36
I've done a bit of research about this as well and from what I can see on the web and general advice from people wiser than I is that linking the two for charging will shorten the life of both batteries.
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Reply By: Allan B (Member, SunCoast) - Monday, Oct 01, 2012 at 11:42

Monday, Oct 01, 2012 at 11:42
It is perhaps not the ideal arrangement as the two battery types have slightly differing charging requirements. However, neither the AGM nor the Flooded Deep Cycle will receive the "ideal" charge from your alternator even if used alone so it hardly matters anyway.

Some people will maintain that connecting a pair of dissimilar batteries in parallel will somehow cause some interaction that harms the batteries. It does not. If healthy, each battery will absorb an appropriate amount during charging and each will contribute an appropriate amount to the load when discharging.

The vehicle alternator, as designed, recharges the cranking battery using a simple constant-voltage method (too crude to be described as an "algorithm"). This is less than ideal for the deep cycle auxiliary batteries but that is what they get in most installations.
A better way is to leave the alternator and cranking battery arrangement as-is and install a DC-DC multi-stage charger to charge the deep-cycle battery from the alternator supply. Such a charger will provide the charging algorithm that is better for the deep-cycle batteries.

Joining two batteries in parallel when being charged by a multi-stage charger is also not ideal even if the two batteries are identical, so it is better to have a DC-DC charger for each battery (as I do) but it is now becoming a bit expensive. Also with this arrangement it is not possible the have the auxiliary batteries in parallel to feed the load. The load must be split and allocated to each battery. (as I have)

So in summary, I would say go for it. Just connect the two batteries in parallel and charge them from the alternator via a battery-isolator. You have little to lose.


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Follow Up By: JTD - Monday, Oct 01, 2012 at 11:59

Monday, Oct 01, 2012 at 11:59
How will they go if I have a multi stage 240v charger connected to them?
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Follow Up By: Allan B (Member, SunCoast) - Monday, Oct 01, 2012 at 14:51

Monday, Oct 01, 2012 at 14:51
I just re-read your original post and realised that I had missed that these were in a camper. Sorry.

My earlier comments still apply re batteries in parallel and when charging the camper batteries from the car alternator. In addition, it is necessary to use large capacity cable to overcome voltage drop which will limit the charge rate. Alternatively use a DC-DC charger at the camper end to compensate for losses along the cable.

A 25amp 240v multi-stage charger should be fine. There should be no problem with connection of a solar panel, but be sure that it has a good MPPT regulator.


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