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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. – Pᴀᴜʟsᴛᴇʀ2 Mar 20 '15 at 13:01
  • Hopefully it is edited better now! – SeanJ Mar 20 '15 at 17:28
  • 1
    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. – Pᴀᴜʟsᴛᴇʀ2 Mar 20 '15 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. – Josh Caswell Mar 21 '15 at 8:11
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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.

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.

  • Hi Paulster2, thanks for the feedback. In the example you give I would set the threshold at 13.25VDC. This means any voltage below 13.25V would indicate Engine OFF and any voltage above would indicate Engine ON, (alternator running). The resting voltage would drop over time but this doesn't matter if the alternator, (Engine ON), voltage remains constant. Does what I am saying make sense or maybe you are saying alternator voltage drops over time? – SeanJ May 18 '15 at 13:25
  • The 13.25vdc would probably be good. One of the issues you could possibly run into would be if there is an extreme load put onto the battery/alternator during operation. Theoretically, if you have a bunch of load on the system, it could fall below that threshold. Maybe something you should consider in the design of your system. – Pᴀᴜʟsᴛᴇʀ2 May 18 '15 at 14:56
  • Yep, I think you are right. I was considering a self calibrating threshold value that adjusts over time, but now the movement is more to utilise CAN communications in order to determine whether the engine is on. The CAN don't lie.... – SeanJ May 18 '15 at 17:52

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