A group 24 is 75 amp hours give or take an amp hour.
A 27 is like 95
29 105
31 130
A pair of GC-2 Golf car batteries in series (These are six volt when you lug them but once wired in Series 6+6=12) are 230 amp hours. And each costs about the same (or less) than the G-24.
As to mixing batteries by size.. IN series, a bad idea. But in parallel. works just fine.
There are many who yammer about the dangers, ,, And yes there is a very very very slight danger, however ... IT happened to me and guess what, was not a problem.
That danger, and this applies anytime you parallel batteries, is if one of the batteries fails "Shorted Cell" it can, in theory, suck enough power out of the remaining battery + Converter to "explode" (Rupture the case).
I lost a Group 29 that way this summer
and my GC-2's this winter
And no rupture.
However even if you only pair "Twins" this can happen.. (Identical batteries), so size mixing.. Well as implied above I do it.
The myth: "The smaller battery will charge faster and signal the converter "FULL so the larger battery never reaches full charge".
The Fact: Batteries in parallel, after the first charge/discharge cycle are always 100% in LOCK STEP with each other.. Here is why:
As a battery charges the internal voltage goes up.. Since electricity behaves much like water (In this case "Seeks it's own level") this means the battery with the LOWER STATE OF CHARGE will charge faster.. Much much faster relative to the other till it catches up.. Does not matter if it is the larger or smaller it will take nearly all the charge current till they match.. From that point on they can not be anything but the same state of charge.
Likewise when discharging if one is "Hotter" (better charged) it will provide ALL or very nearly all the load current till it's down to it's partner's state, and from then on they are locked.
So after the first charge/discharge cycle, they are locked. Can't be anything else, until one of them fails.
