Location: Two 6v vs Two 12v Batteries

Which is better? Two 6v or Two 12v Marine Batteries

The eldest electrical debate on the water is the debate over which is better?
A pair of 6v deep cycle batteries, or a pair of 12 volt deep cycle batteries...
This page should dispel a few of the myths, and perpetuate the discussion.
Hopefully with a little sanity, knowledge and humor for all our sake.
Should you have a comment, the contact form is here.

Two 6v batteries, rated at the double the amp hours as two 12 volt batteries, should have an identical amount of power. However, here begins the debate, so let's address the simple and obvious first argument, based in fiction. Two 6volt batteries at 160 ah, are actually less power than two 12 volt batteries at 100ah. YUP! REALLY! Smaller 6 volt batteries will never beat a larger pair of 12 volt batteries. When you compare weight and size, you would find the 100 ah 12volt would be bigger than a 160 ah 6 volt because the lead is what stores the power. There is no way to hid the lead, so you see it. The remainder is the configuration. Should the batteries be the same size, but different voltages, there is virtually no difference, but we'll get to that.

Here is the math for 2 small 6 volt batteries vs. 2 larger 12 marine batteries;
(Remember, you can add amps or volts, but not both, in any equation.)
6v @ 160 ah + 6v @ 160 ah = 12v @ 160 ah (doubling voltage)
12v @ 100 ah + 12v @ 100 ah = 12v @ 200 ah (double amperage)

Had you invested in two 6 volt batteries at 200ah each, they would be identical in size and weight to the 12 volt batteries above. You could expect them to have a similar amount of power too.
6v @ 200 ah + 6v @ 200 ah = 12v @ 200 ah (doubling voltage)

So since we see that batteries store the power, based on lead content, the configuration gets less relevant, as long as you have efficient batteries. Today's AGM Batteries remove the resistance from elder battery designs, allowing you to remove much more math for inefficiency from the equations. Therefore, you can use today's AGM batteries either as a pair of 6 volts, or a pair of 12 volts.

My arguments in the 12v vs. 6v marine battery discussion; AKA What I THINK!
Yes, I expect comments
1. A pair of 6 volts charge better, or not.
We don't really think so. For 12 volt configurations, the charger setup is supposedly less efficient because of one extra wire, and fewer intercell connections. When you string positive of battery, and negative of battery b to the charger, the resistance is equal throughout the circuit, I don't think it really matters much. When you talk about intercell connections, we are talking tenths of Ohms of resistance in today's newer AGM designs. I think fewest connections is the rule. Buy batteries that are the right size, and keep no more than 3 strings of whatever you want.

2. Redundant 12 volt systems for Marine Applications.
I haven't seen a good argument from the 6 volt camp regarding the ability for a single 12 volt battery, in a redundant system, to get you back to port. Now having been stranded, and having walked the boat back to port by towing it along the shore, I won't ever go without redundant battery systems. This is why I run 2 twelve volts for marine applications, even my little fishing boat. I don't have the same valid argument with the RV crowd, and I know it. When it comes to RV applications, there is not nearly as clear cut a winning choice.


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