I run my 12V car battery trough a trimpot into my pro micro, micro controller. I am using (voltPinValue1 * 0.01465) to get the Volt out of analog in. I have trimmed the pots and getting the right V.

But now I want to calculate for how long I can run for, example the stereo and some LED:s, but don't know how.. Does anybody know how I can do this??

Sorry for my English

Thanks! //Rasmus

  • This would be easier if there was no engine involved. Throwing the engine in makes this very, very difficult to solve. May 15, 2015 at 15:56
  • Is is a camper van that has an extra battery. So the sweet part is that the remains of the battery can be calculated when the motor is not running. And its also then its the most important to know for how long I can run my stereo =)
    – MrHaze
    May 19, 2015 at 20:38
  • Take a look at electronics.stackexchange.com/questions/110104/…. You can measure battery percentage
    – PhillyNJ
    Jun 14, 2015 at 19:38
  • Thx @PhillyNJ!!
    – MrHaze
    Jun 16, 2015 at 10:44

1 Answer 1


You really need to know the current being drawn rather than the instantaneous voltage. A voltage drop profile of a battery changes depending on the current draw, and just using the voltage is very very hard indeed.

You could measure the voltage over time, and try and calculate the slope of the voltage and attempt to predict when it would reach a specific pre-determined cut-off point, but since that change in voltage will be non-linear it's not really something that can easily be calculated.

But, if you can measure the current instead, and you know the Ah of the battery, then you can get a better estimate of how long the battery will run until it's considered flat.

  • I think can get the current out of it using another calculation! But the thing is that its a camper van that has an extra battery. So the sweet part is that the remains it will be calculated when the motor is not running.
    – MrHaze
    May 17, 2015 at 16:08

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