Forum Discussion

ron_dittmer's avatar
ron_dittmer
Explorer III
Oct 13, 2014

Batteries: Two-6V In Series, or Two-12V In Parallel ???

Before spring I will be replacing our two 12V Walmart coach batteries. Our 7 year old rig has it's original Tripplite 2000 watt inverter/charger similar to THIS ONE. I was thinking of replacing the ...
Class C
DiploStrat's avatar
DiploStrat
Explorer
May 19, 2015
Gang,

You are all making this WAY too complicated, but you are also ignoring some variables.

-- Wiring size and length: Most RV's are very poorly wired with a wire of 6 AWG or less. Assume a 14v alternator producing 100A, a wire of 6 AWG, and a run of 15 feet one way. Your battery will only see about 12.8v. (12.7v is "fully charged.") You can drive all day and you will not see much happen. And pnichols is right; none of this works if you have dirty, roached out wiring, no matter what the size.

-- State of charge: The ability of a battery to accept a charge depends on its state if charge. (Expressed as resting voltage.) Batteries accept a charge fastest when discharged about 50%, say a resting voltage of about 12.3v . Above or below this, the charge rate drops off.

To charge a battery, you have to provide it way a higher voltage source. (That is why two batteries connected together will reach the same voltage.) As you can see from the exampe above, instead of giving your battery 14v, you are only giving it less than 13v.

Now replace the cable with a 1/0 AWG wire and rerun the numbers. Now your battery will see 13.7v and I assure you that you will get a much, much faster charge, at least initially. (I use 2x1/0 AWG cables and routinely top 200A)

The trick is, however, that you won't see it of long. As the battery's voltage rises, its ability to accept a charge drops and your maximum charge rate will quickly end up being below 50A or so. This is why a good shore power or, better yet, solar charger, is so important. More batteries die from not being charged LONG enough than for any other reason.

Run the numbers to your heart's content: Really nice voltage drop calculator

You can also download an appalling amount of information on this subject from my website, look under Documents.

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