The 6 volt battery has lots of AH built into it. when you put 2 together and get 12 volts and you still get the service of AH per time, it will provide more power longer.
If you try to do this long service time from a 12 volt battery the size gets bigger...weight..s--ampacity per cell.....
it's back to the design and what you want from the system. you could use a 12 volt vice 2---6 volts but suspect it goes dead sooner.(not enough AH total) volts per cell
notice the 6 volt has 3 cells (where the cap is for water) and each cell has a AH rating....if you did this for the 12volt battery each cell (and the number of plates to do the AH service) makes it really big.
Amp Hour (Ah or ah):
A unit of measure for a battery’s storage capacity obtained by the amount of amps a battery can be discharged (typically 1.75vpc) multiplied by the amount of hours @ 80F (26.7C) . Most auto/marine and light truck batteries are tested at a 20-hour discharge rate. Example: A 12-volt 100ah rated marine battery means that 5 amps can be discharged for 20 hours at 80°F (26.7C) before falling below 10.5 volts (6 cells X 1.75v)
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