12V vs 24V: What Is the Difference?

12V vs 24V describes the two nominal supply voltages used on road vehicles. A 12 V system runs from a single 12 V lead-acid battery, made of six cells, and is used on almost every car and van. A 24 V system connects two 12 V batteries in series and is used on heavy goods vehicles, buses and coaches, most construction and agricultural machinery and many military vehicles.

The nominal figure is a label rather than the voltage the electronics actually see. A healthy 12 V battery rests at around 12.6 V, and with the engine running the alternator holds the system at roughly 13.8 to 14.4 V. A 24 V system rests near 25 V and charges at roughly 27.5 to 28.5 V, which is why military and aviation equipment often calls the same supply 28 V.

Both systems also see short events well outside those figures: deep dips while the starter motor cranks the engine, spikes when inductive loads switch off, and surges if a battery is disconnected while the alternator is charging. Anything permanently wired to the vehicle has to cope with all of them.

Electrical power is voltage multiplied by current, so doubling the voltage halves the current for the same load. That has several knock-on effects:

The table sets out the differences that matter when you fit electronics to a vehicle.

The transient levels are why a 24 V vehicle is the harder environment. Test standards set their 24 V severity levels well above the 12 V ones, and an input stage that is merely adequate on a car may not survive a lorry.

A computer built for vehicles avoids the question with a wide-range DC input. Many of our in-vehicle computers accept 9 to 36 V DC, which covers a 12 V car during a mild cranking dip and a 24 V lorry at full charge, and some accept 9 to 48 V or more. One model can then serve a mixed fleet of vans and heavy vehicles without a converter.

The input stage does more than accept the range. It filters and clamps transients, protects against reverse polarity, and on most vehicle computers adds ignition sensing so the computer starts with the engine and shuts down cleanly after it stops. Our guide to configuring ignition control on rugged vehicle computers covers the start and shutdown delays and the low-voltage cut-off that protects the battery.

Where a computer must keep running through a cold start or protect its data if the supply fails, a supercapacitor ride-through module helps. Our industrial UPS modules include supercapacitor designs with ignition control, among them a DIN rail module with a 12 to 35 V DC input that suits either system.

When a device only accepts 12 V and the vehicle is 24 V, the answer is a DC-DC converter, often sold as a 24 V to 12 V converter or voltage dropper, sized for the device’s current with some headroom. Two shortcuts are common and should be avoided. Taking 12 V from one battery of a 24 V pair unbalances the batteries and shortens their life. Feeding a 12 V device directly from 24 V will usually destroy it.

Military vehicles are a special case: their 28 V supplies are tested to MIL-STD-1275, which sets its own spike and surge levels. Cars, meanwhile, are adding 48 V mild-hybrid systems alongside the 12 V supply, but accessories and computers are still normally fed from the 12 V side.

We supply in-vehicle computers as part of a complete system, and as standard we match the input range, ignition settings and any ride-through module to the vehicle’s supply before the system ships.