there is not too much difference in the price between a 6v and a 12 v so it seems more sense to go for the 12v?
On the contrary, there are HUGE differences between typical 12V "marine batteries" and 6V deep cycle batteries.
When buying batteries one must consider well beyond just Ah capacity. The most critical aspect to longevity will be the cycle life or how many cycles to 50% DOD you can pull off.
Most all 12V "deep cycle" marine batteries are imposters
including top end brands like the Trojan 12V G-24, G-27 & G-31... They are not a true deep cycle battery and will yield far less cycles than a 6V GC2 battery.
Some brands will give more cycles
than others in these group sizes but none compete directly with a GC2 in terms of cycle life..
You are not just buying Ah capacity you are buying cycle life. There are 12V golf cart batteries such as the Trojan T1275, and they do have the same cycle life as a 6V, but they are also taller, like the 6V, and a built to use the same plates as a GC2 battery. Group 24, 27 & 31 batteries simply do not have the plate thickness a GC2 battery does.....
For example in the Deka / Sea Volt / West Marine line you have starting
, dual purpose
and deep cycle
G-24, 27 & 31 12V batteries which all share the same case.
However, they are only really deep cycle
when compared to the starting or dual purpose batteries which share the identical case. They are not deep cycle
when compared to the GC2 or other true deep cycle batteries..
Here's the data across the Deka / East Penn / West Marine line up:
Flooded Batteries - Group 24, 27, 31 & 6V GC2
12V Starting - Cycles to 50% = Not Rated
12V Dual Purpose - Cycles to 50% = 200
12V Deep Cycle - Cycles to 50% = 350
6V Golf Cart - Cycle to 50% = 700-1000
The above are LAB RATED cycles. Expect at least 50% less in the real world
As you can see the 12V "deep cycle" battery is only a "deep cycle" when compared to the starting or dual purpose batteries it shares a case with. If you absolutely must buy a 12V Group 24, 27 or 31 buy the deep cycle
version. In many cases this is all that will fit. It is easy to see that these are NOT deep cycle
when compared to a real
deep cycle battery.
The only ones that have lab rated cycles the same or close, to their own brands 6V batteries, are Lifeline and the US Battery DCXC 12V series (at least those are the claims). I know of no other brand that will rate their G-24, G-27 or G-31 at even half the cycles of their GC2 6V batteries. The Deka / East Penn / WM product is less than half and these are by far the most popular marine batteries out there....