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Mike F
I recently purchased a motorhome that has a large commercial battery for boondocking. The battery is about 7 years old. It still functions, but drains down fairly quickly, maybe a day.

I am wondering, what option is best; replace the large commercial battery, or connect 2- 6 volt batteries in series.

There are a lot of passionate feelings about battery voltage. Unfortunately there are no definitive tests. My suggestion is to choose what ever voltage will allow the greatest number of amp-hours of storage @ 12 volts without overloading the weight limit for the compartment.

For more detailed thoughts surf here:


and click on "Battery Selection”.
What is most important is that you choose a true, deep cycle battery that is intended for this type of use. Very likely the large battery you have is not such a battery, but it would require more information to be certain.

As an electrical professional, if you are selecting between two commonly found 12V or two 6V, both in deep cycle class batteries, I would always choose two 6V of the golf cart type. Trojan makes the top rated ones but for the money, those from Sam's Club or Costco will come close for about 1/2 the cost.

You can find a great deal of expert advice from very experienced boondocking fulltimers if you visit this technical forum.
batteries are more magic than science smile.gif

I have never seen a 24 or 36 volt system in a motor vehicle, so I will assume that you have two 6V batteries, wired in series to produce 12V (12.6v at rest).

Batteries are a touchy subject. I am kinda partial to the care and feeding of low voltage systems... I run a battery backup system in my house as well as the MH. My backup system powers my sump pump, emergency lighting, ham radios and computers....

If you are replacing existing batteries, I would check out the Trojan 105 batteries. I have a pair in my battery bank - as well as some 12V AGM, 12V 7Ah, 12V 4Ah and some 12V 25Ah cells. The batteries are used for other uses when not in the battery bank...

The most important thing is connections.

Every interconnecting cable has resistance. If you want to test the resistance, take a DMM (Digital MultiMeter) on the lowest VOLTAGE range, and with a load on the battery, measure the voltage ACROSS the jumper. You should not read any voltage, in a perfect world, you won't read a voltage, but you will, and the goal is to get it as low as possible. Using Ohm's Law, you can calculate the jumpers resistance as well as the total current draw using the voltage across the jumper.

Ohm's Law is E = I x R
E = Electromotive force - voltage - EMF in volts
I = current in amperes
R = Resistance in Ohms

Power is P = I x E
P = Power in Watts

To make connections, make sure the posts are clean, and use bolts or terminals that are made for the job at hand.

House batteries need to be deep cycle or AGM. DC or AGM are made to be charged, discharged, recharged again. Car batteries hate being discharged, they are made for momentary high discharge to force a starter motor to turn over the engine/motor, while deep cycle are made to be run down and recharged over and over again, if you tried this with a car battery, the plates will warp and you will be replacing the battery soon.

A hint to keep the terminal clean. I use a thin skin of white/lithium greese. This will keep the corrosion down, and corrosion is bad for the cables.
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