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Please see pic. I was able to jump start on day 1 on first attempt. After the bad battery depleted more over the weekend (day 3), I was not able to jump start easily. I kept trying for 30 minutes, then it jump started. Why? (The setup (source car, cables) was same on day 1 vs. day 3). On day 3, when it was not starting, I did measure the voltage on the wire, it was 14V.

The source battery must be having, say, a capacity to source 200A or more at start. The bad battery on day 1 must have been drawing, say 10A. If the explanation is that the bad battery on day 3 is drawing more current, how much? Let us assume it draws 50A. Wouldn't the source battery be able to source that additional current, because that is a small percentage of the high start current required?

pic of source car alternator, battery, bad battery, starter

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  • 25% is not small. Also, 200A is only the peak I think. When the current falls that becomes a much larger percentage.
    – DKNguyen
    Commented Mar 13 at 14:50
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    It isn't the bad battery that's drawing current, it's the starter. Jumper cables (especially cheap ones) cannot generally handle the full starter current; they rely on some capacity in the dead battery to supply the rest. The 30 minutes was the good car putting some charge into the bad battery. Your battery is too dead to handle that. Replace it already! Why didn't you get a battery the last time you got the car started?
    – Dave Tweed
    Commented Mar 13 at 15:07
  • Got appt. only after 2 days 😁 battery was under warranty
    – physicsist
    Commented Mar 13 at 15:10
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    @physicsist you might go into that appt with the strong feeling that your alternator is shot. Commented Mar 13 at 15:15

1 Answer 1

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We will pretend that we are still on Electrical Engineering, because Ohm's law will answer most of your questions. When you are dealing with current levels beyond 50 amps, you discover those wires are actually resistors. Maybe only a tenth of an ohm, but V = I x R is 0.1 x 50 which means a voltage drop of 5 volts. And voltage drop is the second important concept. You may measure 14 volts on the left side where the battery is, but go to the other end where the motor is and you would measure 9 volts!

One more important concept: When your switch is open, you will see 14 volts and not 9, because the voltage drop only occurs when the current is flowing. You have to measure the voltage while actually cranking.

On a more automotive note, the usual situation is that after you've connected the jumper cables, the dead battery begins to charge, and after it has accumulated some charge, it will actually be a big contributor to the current trying to get the starter to turn. Because of all that resistance-of-wires stuff, it's hard to pull enough current through the jumper cables to do the job, so the nearby battery helps out. Unless, of course, it really is a bad battery. Then you have a situation like you experienced.

Numbers were made up as examples. Many factors will affect these things like wire size, how good the connections are, age of the battery, and so on.

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