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Question [Solved] How to maintain multiple 12v batteries connected in parallel

 

 Randy M.
(@Randy M.)
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Joined: 7 months ago
Posts: 2
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The chassis on my RV is a diesel.  So the chassis (not the coach) has two maintenance-free lead-acid 12v batteries connected in parallel.  Some questions here related to maintaining the batteries:

- Previous owner has the on-board battery charger connected with charger (-) connected to battery_1 (-), and charger (+) connected to battery_2 (+).  Is that optimal?  Or should both charge connections go to the *same* battery?

- Is it OK to re-charge two 12v batteries connected in parallel?  Or, should each battery be charged separately?  (This would be difficult, because the chassis batteries are not easy to access)

- Is it OK to de-sulfate two 12v batteries connected in parallel?  Or, should each battery be de-sulfated separately?  (Again, this is difficult)

- Should the two 12v batteries occasionally be swapped with each other, to ensure they both experience the same electrical loading/resistance?


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(@mike)
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Joined: 5 years ago
Posts: 94
 

Actually, I think that everything you're asking about is correct. I'll draw up a diagram tomorrow showing how your battery charger is connected for the rest of the readers to see, and I'll comment on each of your questions. 


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 Rick
(@Rick)
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Joined: 7 months ago
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I can't see your answer and diagram? Was it posted anywhere?


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 Micheal Whelan
(@Micheal Whelan)
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Joined: 7 months ago
Posts: 13
 

Our coach uses four 6V batteries for the house 12 V system. 2 are under the entrance step and 2 are up by the generator. I would like to pulse charge them. Can I use one charger for all 6 (being as they are all interconnected) or should I use 1 charger for each pair of 6 V batteries, or worse (and most expensive) use four 6V chargers, one for each battery?


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 Randy M.
(@Randy M.)
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Joined: 7 months ago
Posts: 2
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@mike Looking forward to seeing your reply.  Not sure if you posted something, and I can't see it, or if you are still working on the reply.

Thanks,

Randy


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(@mike)
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Joined: 5 years ago
Posts: 94
 

I’ll try to post a graphic tonight. I have to draw one up from scratch…


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 Steve Hericks
(@Steve Hericks)
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Joined: 6 months ago
Posts: 7
 

Randy,

I'm a retired engineer and former RV Plant engineer;

1) The cross terminal connection of the charger probably has little to no effect as the batteries are most likely using a large gauge cable connecting them in parallel. The voltage drop caused by what is likely a small current is unlikely to be a significant factor in charging.

2) 'Pulse chargers' (aka desulfators) are a 'poor mans' method of equalization charging which is the 4th stage of proper lead acid battery charging. The pulse is a low power/current attempt to produce a high charging voltage that can reverse lead sulfate crystallization (otherwise known as 'sulfation'). They are relatively ineffective on large batteries and completely ineffective on a battery bank because the pulse does not achieve the high plate voltage needed. The proper way of achieving sulfate breakdown is to use the equalization charge on a high-performance (20A) battery charger. Equalization charges at 15.5-16V and can reverse crystallization that naturally occurs when batteries are not recharged soon after use. It also causes water loss in the electrolyte so after the equalization charge, top of the battery (yes, even a 'maintenance free' one). Be careful not to use equalization charging frequently (more than monthly). Typically only flooded types are ok with equalization charging and AGM and Gel can be damaged by it so check your manufacturer's charging recommendations. The best method to prevent sulfation is to recharge fully, soon after use. As a starting battery, they should be well charged unless you make frequent short trips.

3) Swapping chassis batteries periodically is (unfortunately) 'a thing' but if your chassis is susceptible to unequal demand/charging, you would be better served by installing the appropriate (usually) ground cables to insure equal demand. The problem of unequal demand is usually a result of one of the battery negative terminals connecting to the engine block and one to the frame (a higher resistance path) with a common cable in between. This causes the starting load to be carried more by the battery with the engine block connection. Adding a cable connection from both batteries to the engine block with the same cable is the best solution. 


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 CHARLES CONWAY
(@CHARLES CONWAY)
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Joined: 2 months ago
Posts: 1
 

I have them connected correctly but having issues with a large discharge.  Over night it drops to 50%. Nothing is on. Only draw I can see is from the fridge (runs mostly on propane) and the radio which is off but has some lights on.  Help any thoughts?


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