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@alternator_ada ·

Battery read 10V dead, 14.5V while running after a jump, then dropped again once I switched off — which part is failing?

Sequence over one morning: car would not start, meter across the battery read about 10V. Jump started it, and with the engine running the same meter read 14.5V. Switched the engine off, waited ten minutes, and it was back down to about 11.5V.

It started again that afternoon, then failed again the next morning.

I have read that 14.5V means the alternator is fine, and also that a battery holding 11.5V is finished. Both cannot be the whole story or the car would behave consistently.

  1. What does each of those three readings actually tell me?
  2. Is this the battery, the alternator, or something draining it?
  3. Is there a test I can do with just a multimeter?
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  • @terminal_tess · 4h ago · 2 replies
    @terminal_tess ·

    Take them one at a time, because each reading answers a different question and people run them together.

    14.5V running tells you the alternator is producing. That is a genuine pass and you can stop suspecting it. It does not tell you the battery is accepting the charge — those are different things, and this is the confusion in what you have read.

    11.5V after sitting is the important one. A healthy lead-acid battery at rest sits around 12.6-12.7V. 12.4 is about half charged. 11.5 is essentially flat, and getting there ten minutes after being charged means it is not holding what the alternator put in.

    10V under the starter's load is then just the consequence.

    So the shape of your evidence is: charging system fine, battery not retaining. That is a failing battery or something pulling it down overnight, which is your question 2.

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    • @alternator_ada · 4h ago
      @alternator_ada OP ·

      "Producing" versus "the battery accepting it" is exactly the distinction I was missing. I had treated 14.5V as proof the whole system was healthy.

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  • @creep_speed_cem · 21h ago
    @creep_speed_cem ·

    The test that separates a dead battery from a parasitic drain, with just a multimeter:

    Charge it fully, then leave it disconnected overnight. Take the negative lead off the battery entirely, note the resting voltage, and check it again in the morning.

    • Still around 12.6V → the battery holds charge. Something in the car is draining it, and that is a current test next.
    • Dropped to 11.5V again with nothing connected → the battery is the fault. Nothing else can be, because nothing else was attached.

    This one test is worth more than a week of guessing and it needs no special equipment. Most people skip it and replace parts in order of price.

    If it turns out to be a drain, the follow-up is a current measurement in series with the negative lead — but do that only after the overnight test, because half the time it is not needed.

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