Can I connect 6 and 12-volt batteries together?

Well, sorta kind of, but perhaps not like you planned…..

Dear Mike,

Don’t know if this question is JAM-worthy or not, but here goes:

I just upgraded my house batteries on my 1993 Holiday Rambler Alumalite 5th Wheel. When I bought it the dealer had installed a new Interstate Hybrid Marine-RV 12V (100 Reserve rating=41.67 AHs). It was OK, but really able to maybe last the night when boondocking. And I do wear a CPAP.

So, I replaced the 12-volt battery this week with two 6V GC2s 220 AH batteries from Les Schwab. Wiring them in was pretty easy and they fit in the compartment where the one 12-volt had been. I reused the cutoff switch on the negative side.

Here’s my question: The old 12-volt battery is perfectly good; Schwab tested it for me. I have room to put that battery in as well, and could connect in parallel to the two 6-volt batteries. Probably the simplest way to do it. But should I? The 12-volt battery is 2 1/2 years old, so should still have some life left in it. Should the coach side of the B+ (red wire) come off the 12V or the 6V? Or does it matter? But, is mixing an older 12V with new 6Vs a good idea? (See my diagram below for an explanation of what Chris is asking.)

If it’s not OK, another option is to just have the 12V battery available, in case the 6Vs get low, then using something as simple as jumper cables could re-power the coach. This seems too “Rube Goldberg” to me, and a hassle.

If it is OK, I could put in a Blue Sea Systems marine battery switch, for instance the Selector 3 Position, that would disconnect the 6Vs and connect the 12V to the coach; so instead of in-parallel, it would be either 6Vs or 12V.

But then how would I keep all batteries charged? I guess by throwing the switch either way, the converter would charge whichever battery is connected to it? 

Finally, maybe a 4-way switch (Battery 1/Battery 2/Both/Off) to put on the positive side, but the wiring is trickier. I think it would parallel the 12V with the two 6Vs in the “Both” position, or switch them so either the 12V or the 6V is feeding the coach.  

I’ve looked and haven’t really seen a wiring diagram or discussion for any of these situations. I hope you understand what I just asked! What do you think? —Chris

Dear Chris,

That’s a great question and it is certainly JAM-session-worthy, so here goes.

The short answer is no, you cannot simply parallel your 12-volt battery with the new 6-volt batteries (in series). You can only parallel batteries that are exactly the same brand and chemistry, including age. Otherwise it will result in unbalanced charging as well as shortened battery life.

One possible solution

However, you can do a temporary parallel interconnect as a quickie jumper to start a generator or whatever. But that shouldn’t last more than a minute or two. Or you could do a Batt-1/Batt-2 selector that keeps these two battery banks separated at all times, but lets you switch in one or the other to your RV’s 12-volt system.

Here’s the diagram you sent me, which is actually pretty good except for the second disconnect switch. Note that the Blue Sea battery selector switch you’re showing allows you to connect your RV’s electrical system to Batt-1 or Batt-2, plus a third option of !+2 for both batteries.

But as you guessed, you only want to use the Batt-1 position or the Batt-2 position. Leaving it in 1+2 Mode with the 12-volt and 6-volt batteries connected together in parallel for more than a few minutes would likely cause battery life problems since they’ll try to equalize the charges between them. And you really don’t need the second disconnect switch since that function is included in the Blue Sea switch.

The Downside

However, you won’t be able to charge all three batteries at the same time even if the switch was left in “Both” mode. My suggestion is to only use the Batt-1 or Batt-2 positions and never run the two sets of batteries in parallel (Both), but go ahead and charge them separately by simply selecting one or the other battery selection for a few hours.

Note that I’ve looked around and can’t find a battery selector switch without the parallel (Both) battery function. So if you do use this switch, be careful not to use the 1+2 selection for longer than a few minutes at most. And if anyone reading this knows of a Batt-1/Batt-2 switch that doesn’t have a Both (parallel) selection, please let me know in the comments below.

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