Re: 49 engine stopped

Posted by humanpotatohybrid On 2022/12/31 17:45:39

That's a good point, though I didn't suggest it because I don't know a normal voltage drop under load conditions, and it varies a bit by battery and car.

All batteries have internal resistance, you can visualize as a voltage source (like a power supply) in series with a small resistance. The internal resistance increases slowly but permanently with age (and excessive heat), and temporarily from discharge (and cold temperatures). This is why a battery is considered discharged at like 11½ volts instead of 0... because the resistance gets so high that there is not much power available. (An axiom of battery power is that the maximum power available under an optimal circumstance is V^2/R where R is the internal resistance. As R goes up, power drops.) This is why, even though you could crank over a car just fine on a charged up 11V battery (if there were such a thing), you would have no chance if you have a 12V battery reading 11V.

So that's why, given the engine won't turn over, a simple voltage reading is useful on its own. 12V batteries go roughly from 12.6V full charge to 11.6V empty. So if you read a voltage of like 11.7V then probably you have a charging system issue and ran the battery dead. If it's like 12.1V and still won't start then the battery is toast.

To be clear, nothing of what Howard said is wrong, I just wanted to explain the slightly different path of reasoning that I like to use.