# 24V truck battery - voltage when the engine is running?

What's the actual voltage when the engine is running for a 24V truck?

I know that for regular cars, the 12V battery voltage is above 13V, but I was wondering what the voltage is for a 24V truck battery when the engine is running.

A float charging voltage for 12V lead acid battery is 13.8V (2.25V to 2.3V per cell). In a 24 system you have to multiply by two, which gives 27.6V. However the battery can be charged also with higher voltages for fast charge, which is not implemented in automotive alternator, so it should output the float charging voltage.

I am familiar with buses (especially Mercedes Benz) -they are 24V-vehicles as well. Anyway, most of buses I've seen have around 28VDC (Actual value is between 27.6 ~ 27.8V) while the engine is running. Shouldn't be different from trucks.

PS: ISO-16949 and ISO-16750-2 suggest that electrical tests should be performed with 28V supply for 24V-products (and with 14V supply for 12V-products) since the battery voltage is 28VDC while the engine is running.

• Thank you. I didn't know that there's specific standard for this.
– ShP
May 7, 2017 at 20:03
• Ideally any devices for a 12V system should operate over 9-16V. Twice that for a 24v system. May 7, 2017 at 20:52

Minimum: 24V -30%.
Normal: 24V +15%.
Maximum 24V +25%.
Surge: 24V +/- 40% (max 1 second)

Then there are all sorts of guidelines regarding switch-over, load dump and crank duration. Buy the standard if you want to know all the details.