# Maximum resting 12V car battery voltage

I've been trying to find the theoretical maximum resting car battery voltage so I can determine a good voltage threshold to trigger off in a car.

The threshold voltage must be above the theoretical maximum resting car battery but below the alternator voltage.

example 1

maximum resting car battery voltage = 13V alternator voltage = 13.5

Threshold switching voltage = 13.25V

In [1], it is 12.9V but optima is 13.1V and Ballistic Performance Components EVO2 Battery is 13.7V

[1] Denton, T (2012). Automotive and Electrical Systems. USA: Routledge. P186.

Obviously if there was a Ballistic EVO2 Battery installed example 1 would not function.

Any ideas what the most typical maximum resting voltage is in a car?

• If the vehicle is at rest, the voltage will be at peak when the vehicle is turned off. It will never get higher than that without being charged. My point is, the way you have this worded, the device you are trying to turn on will never turn on because it will never reach the given voltage. Either that, or it will turn on when you shut the car off because it has met your threshold. One more guess would be, you need to put more information into your question as to what you are actually trying to accomplish. Mar 20, 2015 at 13:01
• Hopefully it is edited better now! Mar 20, 2015 at 17:28
• While the added information helps, I don't think you are going to get one right answer. There are too many factors involved, the largest of which is the health of the battery itself. As a battery gets older, it will lose the ability to "fully charge", meaning that brand new, the battery may sit at 13.1vdc, while when it comes close to end of life, resting voltage may be 12.3vdc. Your setup would work on the new end of things, but fail as it became older. A catch all just is not going to work as far as I can see at this point. Mar 20, 2015 at 17:32
• It sounds like you want to power an electrical device in your car only when the engine is running. If checking the charging system voltage isn't working, there should be another circuit that you could use. Ignition for one, although you probably don't want to touch that. Tach/engine speed sensor or fuel pump might be an option for your "engine on" signal.
– jscs
Mar 21, 2015 at 8:11