36V connection <+ ->--<+ ->--<+ -> = "series" connected
(3 batteries)

24V connection <+ ->--<+ -> = "series" connected
(2 batteries)

12V connection <+ ->
(2 batteries) ( ( = "parallel" connected
<+ ->


assume 100 amp hour rating for each 12 volt battery
With series connected batteries the amp hour rating remains constant while the voltage is additive 12+12+12 =36 volts @ 100 amp hours
12+12 =24 volts @ 100 amp hours


With parallel connected batteries the voltage remains constant but the amp hour rating is additive 100 + 100 = 200 amp hours @ 12 Volts

the available energy is determined by the number of batteries you have and not the voltage so 3 batteries in parallel contain the same stored energy as 3 batteries in series - hope this offers some additional help on this topic