Originally Posted by garbonz
Yes, I am sure there are many reasons to run batteries separately, and that is a good one and it would be a good reason to have 2-12volts in parallel instead of 2-6 volt in series.
It is also true that you get somewhat reduced capacity because of Peukart's law and so in a situation where you really need all the amphours you can get, you may want to hook the batteries together.
Just some ideas to consider.
Here is a great article on the Peukart Effect.
Since amp draw determines total available capacity, (more amps; less capacity), sharing the amp draw across more batteries will result in a SIGNIFICANT improvement in battery life under load.
Remember using 2 6 volt batteries double the voltage and not the capacity, so the amps out of both are the same and the effect is felt equally on both batteries.
With 2 12 volt batteries you double the capacity and halve the amp draw on each battery.
So using the graph below with a 100 AH "stack" of "notional" batteries (2 100 AH 6 volt batteries or 2 12 volt 50 AH batteries), examine the capacity loss across the stack for a 30 amp continuous draw.
In the 6 volt scenario, your 100 AH stack will have its total available capacity reduced to 54 AH.
In the 12 volt scenario, the 100 AH stack (since the 30 amp draw is shared between the 2 12 volt batteries with each matched battery
contributing 15 amps) will only be reduced to 68 AH.
Obviously, if the 12 volt batteries are not matched internally, they will have dissimilar amperage flows and the capacity will be reduced from the optimum 50% share. Matched means similar manufacturer, size, capacity, charge discharge cycles, and age.
If one (the lower internal resistance) is contributing 75% (22.5 amps) and one (the higher internal resistance) is "loafing" at 25% (7.5 amps), the effect will have a serious degradation on overall battery "stack" life, as the higher voltage (loafing) battery will start trying to charge (equalize with) the lower voltage (harder working) battery.
This "self charging"/reduced capacity effect with non-matched batteries does not happen with 2 six volt batteries. The older 6 volt one will just die even though the other is still "good" resulting in the entire stack dropping offline.
This is why changing out the entire stack (rather than just one) is critical in multi-battery installations in order to keep things in balance whether 6 or 12 volt.