Why do diesel trucks have two batteries?

Diesel trucks (and many other heavier engines) require more power than the standard 12-volt system can provide, thus requiring two batteries. One battery charges while you drive and provides power for starting the engine; this is called the “cranking” battery. The second battery acts as a reserve; once the vehicle’s initial electrical needs are met (lights, stereo, wipers), it starts charging to provide continuous power.

If you own or work on diesel trucks, then you know that they have two batteries – one for starting the engine and another that provides additional power when the vehicle is running. The reason for this arrangement actually has to do with an automotive engineering problem – it’s called “jacking down” or “dieseling.”

Engine manufacturers are always looking for ways to make more power, get better fuel economy, etc. To get higher performance from engines, automakers try all kinds of tricks. For example, they use smaller pistons in the cylinders of a V8 engine or exhaust gas recirculation (EGR), in which they route part of an engine’s exhaust back through the combustion chambers.

One thing automakers don’t want to do increase the weight of their engines. After all, adding more metal makes an engine less fuel-efficient than it would otherwise be, so manufacturers usually try to avoid this when possible.

With that in mind, you can probably guess what happened when diesel engines started to replace gasoline engines in heavy trucks. Gasoline engines are designed to operate at lower rpm, thus creating less power and requiring lighter components.

The first diesels, on the other hand, were built for high-rpm operation; since they used the same engine blocks as their gasoline counterparts, they produced plenty of power but weighed more than the typical gasoline engine.

And here’s where the problem comes in: Trucks with diesel engines are heavy enough already; adding weight with a larger engine block can cause problems with shifting and steering – not to mention increased fuel consumption. While it might be possible to use smaller pistons, that would lower the engine’s power-producing capabilities and require more gears in the transmission to get the same performance.

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Benefits of two batteries?

Dual batteries do have some benefits. The primary battery provides power for starting the engine; once you turn the key, this battery gets charged by either your alternator or an additional belt-driven generator (known as a “DG unit”).

The reserve battery, meanwhile, is charging whenever the engine is running – including when it’s idling – and this is why it’s sometimes called the “idle battery.” If either battery goes dead, you can still start your engine – because the alternator will recharge the one that’s installed under the hood.

Dual batteries also provide an extra layer of protection against accidents or bad design. For example, if there was a problem with one of the batteries or the wiring, you could still start your vehicle with the other battery.

Not surprisingly, this arrangement isn’t ideal for anyone who works on truck batteries. Having to raise a heavy vehicle may be difficult for some technicians – especially those working out of their garages or shops – and accessing the battery under the hood is often impossible without first raising the vehicle.

One possible solution is using an external battery charger, but these devices only work when the truck is parked.

The solution will probably never be perfect, but it’s nice to know why trucks have two batteries.

AdBlue: A Solution for Emissions, But a Problem for Battery Circuits

So what did automakers do? They developed another solution: urea injection. This process sprays diesel exhaust fluid (aka “DEF”) into the exhaust stream, where it reacts with the NOx emissions and turns into harmless nitrogen and water. This fluid is injected either by a separate urea injection pump (usually located on the frame rail near one of the batteries) or by a mechanism under the hood (known as Engine Braking Assist, which replaces gear oil with DEF).

As you might already know, DEF is corrosive – especially to the circuits in a battery. This is why some diesel vehicles have two batteries: One that’s accessible for servicing and another that’s permanently mounted on the frame rail near the urea injection pump. The former charges while you drive, while the latter provides power when needed.