Heavy Vehicle Battery Usage

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Most of the heavy vehicles are using two 12 volt batteries instead of one 24 volt battery. What’s the reason for this?


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.

Sorry Bill, I know this is kinda calling you out as “old” but…

It’s probably worth mentioning that the battery standard was decided in the middle of the 20th century, and nobody has the guts to change it (yet).

Battery technology was limited in certain ways by manufacturing and tooling available in the 1950’s through the 1970’s. This has gradually changed in recent decades. The economic sweet-spot to use 12V in automobiles and trucks in the middle of the 20th century is pretty much gone now. Batteries used to be more expensive “per pound” and they required regular maintenance (they weren’t sealed, nor recyclable). The relatively recent “maintenance free” batteries are one of many automobile technologies that has contributed to the “disposable” cars that we have today. But that’s a separate issue.

Also a separate issue is the point somebody could make about Aviation batteries. “They’re much more expensive than auto batteries, and they are 24V so obviously 24V batteries are more expensive.” Well, yes and no. Yes they are more expensive, but that’s because they have to be safe for use in aircraft. That means testing to much more rigorous standards than auto batteries. Plus all of the other parts and quality controls necessary to tag an aviation battery, and it costs way more to produce and sell an aviation battery for aircraft use. But the sheets of lead in the plastic bucket of acid is not more expensive.

Sealed lead-acid batteries in 6V, 12V, and 24V combinations are commonly available and the cost is nearly the same “per pound” or “per Watt-hour” of capacity. This trend applies to many Lead-acid, NiCad, and Lithium batteries. One place where this trend becomes obvious is in renewable energy batteries. There are so many different RE systems, there’s no single standard battery system. So you can get RE batteries at many different voltages: 2V, 6V, 12V, 24V, and there’s no price penalty per unit of energy.

If the world was to choose the auto battery technology standard all over again, today, I’m not sure it would be 12V lead, this time around.