Your math doesn't work for me.Generators in general have much much smaller alternators then cars or trucks as they were not designed for start and stop. The general rule of thumb on MEP’s with the correct battery size that they need to run for four hours after just one start to bring the charge of the batteries back to 100% and about 8 hours if the genset went through several start attempts or extreme cold weather.
Charging two large batteries connected in series will double the needed charging time. The current displayed on the charge indicator or ampmeter is misleading because you are charging a total of 12 cells instead of 6 cells in a single battery system.
constant trickle / float charging is your friend and a good trickle / float charger will do that without damaging the batteries.
The most often made mistake is the constant undercharging of the genset batteries. Nothing will kill
your batteries faster.
the math is simple:
each batterie has 120 amp hour capacity
thats 120 * 2 = 240 Ah
this means if your batteries connected in series are completely empty and you apply a charging current of 1 Amp then it will take 240 Hours to charge the batteries to 99.99%
at 10Amps it would take 24 hours
so take your time to charge your batteries properly
I thought that if they are in series it's still only 120 amp hours, but at 24V. If you put them parallel then it's 240 amp hours at 12V.
1 amp of 24V power is twice as many watts as 1 amp of 12V, or equal to 2 amps 12V as there is 1 amp 12V going to each of the batteries.
1 amp 24V = 24 watts power
2 amps 12V = 24 watts power.
The total available watts is the same either way you do it, but in one configuration it's higher voltage, the other configuration it's higher amperage.