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Old 26-09-2017, 12:14   #1
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Mixing batteries, 6V an 12V

I would like to upgrade my battery bank but don't have a lot of space. Currently I have 2 x 80AH 12V deep cycle batteries. They are connected in parallel of course, but are separated via an isolator switch and an ACR -- one of the two being prioritized for the engine.

I was thinking I'd replace this bank with 2 x 230AH 6V batteries connected in series, and move one of the 12V batteries to the engine compartment as a dedicated starting battery.

What problems would I run into in such a combination?
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Old 27-09-2017, 05:45   #2
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Re: Mixing batteries, 6V an 12V

Conceptually, 6V deep cycle batteries for the house and a 12V starter battery for the engine is decent enough. But maybe depends on some details.

By "mixing" (in your thread title), do you refer to having all the batteries on the same charger? Multiple bank charger, so one feed could be dedicated to the house bank and another feed to the engine bank? If so, that can work well enough.

Or, maybe your starter battery doesn't really need a charger feed anyway (especially if you have a combiner or parallel switch), and/or maybe the ACR feed would be sufficient. (Need others to comment about the ACR.)

One potential (and maybe minor) risk is that if one of your two 6V batteries were to crap out, you wouldn't have a 12V system for house functions until you can get that fixed. You could decide whether that's a big deal or not. And maybe the start battery in reserve could get you through easily enough until the fix happens.

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Old 27-09-2017, 08:00   #3
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Re: Mixing batteries, 6V an 12V

Your thinking is sound. Under your new setup, the ACR would go between the engine battery and the house bank with the house bank receiving the bulk of the charging. You would have to ensure that the cable is big enough to handle the charging current. You might also want to consider connecting the battery switch between the two banks as an emergency to start the engine. The ACR could go to each side of the battery terminals to connect them when there is a charge current.
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Old 27-09-2017, 08:26   #4
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Re: Mixing batteries, 6V an 12V

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Originally Posted by Ryban View Post
...I was thinking I'd replace this bank with 2 x 230AH 6V batteries connected in series, and move one of the 12V batteries to the engine compartment as a dedicated starting battery...
OK, as long as you get rid of the other 12-volt battery from the original pair.
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Old 28-09-2017, 07:06   #5
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Re: Mixing batteries, 6V an 12V

Thanks for the feedback!

Yes, the plan was to get rid of one of the 12V, and make the other a dedicated starting battery. Alternatively, I could buy a dedicated starting battery while I'm buying the 6V, but I don't think it would be super necessary.

So my plan is to have the 2 x 6V 230AH batteries connected in series as my house bank, and one 12V 80AH as my starter. I'll wire them up so they're separated by the ACR and the battery isolator switch. That way I'll be able to start my engine still if the engine battery poops out.
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Old 28-09-2017, 07:34   #6
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Re: Mixing batteries, 6V an 12V

One further consideration - Deep cycle batteries are rated in amp hours. Cranking batteries are rated in cold cranking amps (CCA). If you're going to use your old deep cycle battery to start your engine, make sure that the CCA rating is enough for your engine.
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Old 28-09-2017, 07:38   #7
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Re: Mixing batteries, 6V an 12V

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One further consideration - Deep cycle batteries are rated in amp hours. Cranking batteries are rated in cold cranking amps (CCA). If you're going to use your old deep cycle battery to start your engine, make sure that the CCA rating is enough for your engine.
Good point. It's already being used to start the engine, so I assume it's up to the task, but I better double check and make sure I'm not doing damage to the battery.
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