RV Electricity – Just Ask Mike (J.A.M.): One battery tender for two 12-volt batteries?

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By Mike Sokol

Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.


Dear Mike,
I have two questions for you. I have two 12V batteries on my rig hooked up in parallel. Can I use a standard battery minder to keep them over the winter or do I need a special one for two batteries? And, what is the best way to connect them, bring them inside? Thanks for your help! —Kelly

Dear Kelly,
This is an easy one. Since they’re connected in parallel already, they’ll have identical charges (assuming they were installed at the same time). So you can simply connect one Battery Tender (or Minder) across either of the batteries (from Negative to Positive) and both batteries will be equally charged.

As far as bringing them inside, I think that’s a big waste of time. While Lithium batteries may not like being charged at very low temperatures, all modern ones should include some sort of heating system, either via a blanket wrap on the outside of the batteries or heaters inside of the case.

So my recommendation is to leave the batteries in your RV, connect them to a Battery Tender (or perhaps your RV’s 3-stage charger) and periodically inspect any flooded cell battery for electrolyte level once a month, adding distilled water as needed.

Messing with battery cables can be very dangerous if you’re not careful, and I really don’t see the benefit of bringing them into a cold garage, let alone a warm basement.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

Let’s play safe out there….

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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Thomas D
1 month ago

You might want to be sure that the cigarette lighter outlet remains on when engine is not. My Honda crv and wifes Toyota rav4 both have cigarette lighter outlets that go off with the key.

Grant Carroll
1 month ago

Thanks so much for the information about hooking up one Battery Tender (or Minder) to our coach’s house batteries! I am assuming (which always gets us in trouble of course) that the same would be true if we have four (4) 6VDC deep cycle batteries hooked up in series/parallel…? If we attach the Batter Tener/Minder to one of the them then they should all charge equally, correct? What about a battery desulfator? I purchased a PowerPulse a few years back and have been disconnecting each battery, letting it do its thing, disconnecting it and then moving onto the next battery. Can this also simply be attached to one battery to desulfate them all simultaneously? THANKS!

Brian
1 month ago
Reply to  Grant Carroll

Deltran, who makes Battery Tender, recommends for two batteries (connected in parallel or series) to connect the tender to the positive side of one of the batteries and to the negative side of the other battery to provide an even charge across both.

Mike Sokol
1 month ago
Reply to  Brian

That’s certainly true for inverter connections and fast chargers, but the potential voltage drop at 1 ampere of current would be minuscule, and not affect battery balance. Still, it’s a good habit to get into.

Mike Sokol
1 month ago
Reply to  Grant Carroll

I’ll check on the desulfate cycle, but I assume you can do them all at once. Stand by.

Marvin
1 month ago
Reply to  Grant Carroll

If I am reading Grant’s question correctly, he is asking to connect the tender to ONE battery. No, you want to connect any one pair of 6V batteries (12V across the pair).