snip

Four 6 volt golf cart batteries in series parallel will give you about
440 amp-hours of capacity but in actual practice it is a bad idea to
discharge below the 50% level since that will greatly shorten batter
life. You can get a rough idea of discharge level by looking at a
digital voltmeter when the batteries are under load. 12.6 volts is
fully charged, 11.6 volts is about 50%, and 10.5 is fully discharged.

I thought 12.2v was 50% charged (12v lead acid battery). No idea were I got
that idea from, but it would explain why I struggle to keep my batteries
above it. I have 4 x 12v 135amp batteries connected as two pairs of 12v
batteries wired in series then wired together in parallel, to give 24v.
Something I have often wonder about is if I should connect them to the
electrical system with both positive and negative coming off one pair, or
the positive off one pair and the negative off the other.