Could be lots of reasons:
> cost -- 12V batteries are probably easier to find
> reliability -- a single cell fail results in replacing a 12V battery instead of a 24V battery
> installation -- two 12V batteries could be easier to find mounting spots for compared to a single, large, 24V battery
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The historical reason had to do with early incandescent lamp technology. 24 Volt systems were tried to provide more reliable starting for large truck engines. 24 Volt filaments were a lot thinner than 12 Volt filaments. The early 24 Volt filaments and the early roads were incompatible.
The solution to frequent lamp burnout was to use a 12 Volt system with a 24 Volt starter.
Two 12 Volt batteries were connected in parallel, charged in parallel and all the lamps and accessories ran on 12 Volts.
For starting, one battery was connected in series to provide 24 Volts for the 24 Volt starter.
This was accomplished with a device known as the "Series-Parallel Switch".
Two 12 Volt batteries came to be the standard arrangement for large trucks.
It was realized that replacing a battery due to one failed cell was about half the price for a 12 Volt battery compared to a 24 Volt battery.
24 Volt batteries are not a stock item.
And some older truck makers went one better, using two or four six Volt batteries, rather than 12 Volt batteries. Cheaper to replace when one cell fails.
Bill
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"Why not the best?"
Jimmy Carter
I don't think it would be for reliability. If they're in series and one fails would not the circuit attached to both fail?
And even if they're in series you should be able to have two independent circuits from each battery and one, from both batteries, dedicated to the engine.
