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Originally posted by techntrek Fred is correct.  Adnd if they are two 12 volt batteries they must be wired correctly (+ to one battery, - to the other battery).  Then they will be fine.

techntrek, I know you know what you are talking about, your infois always spot on.  This post is a bit unclear.  Just to clarify matters, if you have two 12 volt batteries and want a 12 volt bank of batteries, connect the positive of each battery to the positive of the other and the negative of each battery to the negative of each battery.  Take your power from the positive and negative of either battery.  Actually, I like to take the power from the positive of one battery and the negative of the other.  I attach the charging cable the same way, positive of one battery and the negative of the other. 

If you have two 6 volt batteries and want a 12 volt bank, connect the positive of one battery to the negative of the other, then take your power from the remaining posts.

I know that you know this but I felt your post didn't state that clearly.  I just didn't want someone to become confused.

12 volt batteries wired in parallel.
This is to answer Fwunder question about two 12 batteries wired in parallel. When you start with new batteries from the same manufacture and the same group size and you wire them in parallel they will be in voltage balance, but as the batteries age and the cells get weaker, and the specific gravity of the electrolyte changes the two batteries won’t have the same internal resistances, so the higher voltage battery will discharge through the weaker one. Once the stronger battery voltage drops to what the weaker battery is they will be in balance and the discharge will stop. If one of the batteries failed early and has to be replaced you have the problem of, do you replace both at the same time or have a brand new battery and an older battery?

Single 12 volt batteries and two 6 volt batteries wired in series do not have this voltage balance drain problem. Two 6 volt batteries wired in series has 6 cells at 2.1 volts each (12.6 volts) just like a single 12 volt battery. For me the single battery also means less tongue weight. A group 27 battery is about 51 pounds and 90 amp hours, a group 31 battery is 60 pounds and 100 amp hours. A group GC2 6 volt battery is 67 pounds (x2=134 pounds) and 200+ amp hours, that is the same amp hour rating and weight as a single group 4D battery. If I was to go to a 2 battery setup I would use two 6 volts wired in series. At 67 pounds each I think my wife and I could lift them in place. We could not lift a 4D battery.

So for me the advantage of a single battery is it is simpler to maintain and check one battery, less weight on the tongue, you don’t have the batteries trying to match the voltage of the weaker one as they age and if the battery has to be replaced you don’t have the issue of do you replace both of them.

Another thing to remember about normal wet cell batteries and it doesn’t matter if you have one or many, is they all have a self-discharge rate, about 2-4% per month, so this is why you must maintain your batteries and kept them charged, because if you just let them sit they will die on their own.

Pretty happy with the new battery layout..

Furpod,

Nice looking installation. Where did you find the nifty battery box??

Nice box, eh!