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Saturday, October 24, 2009

Battery Capacities

I currently have two 12-volt deep cycle batteries but will need to replace them probably this year. I have heard that it is better to use 6-volt batteries. I noticed in your reply to another question in an earlier column that it would be better, but you mention using them to replace one deep cycle 12 volt battery. Is it still better if you use two 6-volt batteries to replace two 12-volt batteries? That’s about as clear as mud!
Don, (Moab, UT)


Don, a lot depends on the brands and obviously the size (ampacity) of the batteries in comparison, but I do contend that, nominally speaking, two 6-volt batteries wired in series yields more storage capacity than two, Group 27, 12-volt batteries wired in parallel. It’s simply a matter of mathematics. Here's how I usually explain the math. When two 6-volt batteries are connected in series, the voltage doubles, but the current stays constant. A Trojan T-105, 6-volt battery, as an example, is rated at 225 amps, so two wired in series produces 12-volts at 225 amps. When two 12-volt batteries are connected in parallel, the voltage remains constant, but the current capacity doubles. A typical Group 27, 12-volt battery is rated at 105 amps, so two wired in parallel produces 12-volts at 210 amps; slightly less storage capacity than the 6-volters. The added benefit is that the 6-volt batteries are considered “true” deep cycle batteries; containing heavier plates and sturdier separators than a typical RV/marine deep cycle battery. Hope this helps clear that mud a little!

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