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I'm having an issue where a fully charged battery doesn't necessarily report the proper voltage when checked from the socket within the car, because in order to have the socket activated, the car has to be ON, which, in turn, puts non-trivial load that drops the voltage below what should be the proper voltage without the load (as the 2.1V+ per cell rating, 12.6V+ total, is the full-charge at-rest rating of the battery without any recent charging or load).

In fact, I often find that it's even a problem when trying to measure the voltage of the battery directly from the battery after opening the hood, because the car continues to consume some extra above-idle power until a certain timeout of some number of seconds and/or minutes.

Is there any reliable calculation, formula or a rule of thumb to quickly assess the charge of the battery in such conditions, without fully disconnecting the battery, nor resorting to very expensive specialised measuring tools?

  • I'm thinking there must be some known estimates how much a given car is expected to consume when the key is ON, parking break is activated (i.e., all lights are off), radio and inside lights off etc.

  • Likewise, there should be some tables on how much the voltage would drop in the situation as above.

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    What exactly are you trying to measure? Just the battery voltage? I think your idea that when the car is on, or at a minimum is in the accessory position, the load is quite trivial. Reading the voltage directly from the battery with everything off is going to give you a very accurate reading of the battery. Unless there is a major draw (something which shouldn't be happening), you should get the voltage without issue.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jan 27, 2019 at 0:08
  • If this is a "big" problem, your battery is on the point of failing through high internal resistance anyway. Professional battery testers (that cost $100 or more) intentionally put a high load on the battery, to measure what it does in a realistic working situation.
    – alephzero
    Jan 27, 2019 at 1:16
  • I think you might find this page helpful
    – Carguy
    Jan 27, 2019 at 8:59
  • @Pᴀᴜʟsᴛᴇʀ2, I'm just trying to understand the math behind the whole thing. When I test the battery the next day after driving, it never reads more than something like 12.4V from within the socket within the car, even when it's supposed to be fully charged.
    – cnst
    Jan 27, 2019 at 16:38
  • 12.4vdc could be considered fully charged. Also, consider the source of the reading ... how much trust do you put in the socket reader? Put a DMM directly on the battery and see what the reading is ... if it's the exact same, then that's what it is. If it reads more, the socket reader is suspect.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jan 27, 2019 at 16:43

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If we had to test a battery that was on a car that had just been driven, then we would switch the headlights on for two minutes to remove the "surface" charge and then test the battery.

There are tables showing how the voltage changes with charge and it is easy to measure the current flowing in the car with an ammeter, but you will also need to account for the temperature because that will have a large effect on the battery's performance - while it shows the normal voltage its capacity can be massively reduced.

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  • I would be more thorough. I would pre-open the hood, let the car sit there for at least half an hour, and then fully open the pre-opened hood WITHOUT opening the driver's door to measure the voltage. Opening the driver's door can do various stuff like turn on some computers, turn on the inside lights, turn on a brake booster pump, etc. After all, I have time to wait, I'm not in a hurry to know the battery state.
    – juhist
    Feb 26, 2019 at 19:30

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