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So I have a car A with dead battery but good everything else and a car B with good battery.

What I did was disconnect car A's dead battery, connect car A's terminals directly to car B's battery. When I connect the two batteries together (+ to +, - to -), I cannot immediately start the engine of the dead car, even if the good car's alternator is running. Why is that? Shouldn't the power from car B's battery be enough to start car A?

The power from car B should be diverted to car A to start its engine.

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  • \$\begingroup\$ Possibly the dead battery is pulling the voltage low enough that the ECU isn't happy? \$\endgroup\$ Jun 8, 2019 at 22:16
  • \$\begingroup\$ I tried a case where the dead battery is completely disconnected. I connected the batteryless car's terminal to good car. \$\endgroup\$ Jun 8, 2019 at 22:18
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    \$\begingroup\$ 800 to 1kRPM min and 1.5k to 2k RPM for max charge current \$\endgroup\$ Jun 8, 2019 at 22:50
  • \$\begingroup\$ A really dead battery in car A can be an extra load for car B to overcome. \$\endgroup\$
    – Mattman944
    Jun 9, 2019 at 0:13
  • \$\begingroup\$ Thank you @ShuhengZheng I have wondered this my entire life and I can't get a satisfying anywhere. I'm glad to know I'm not the only with this question. Why can't car A just start immediately, off of B's battery??? \$\endgroup\$
    – hraban
    Aug 28, 2023 at 22:57

5 Answers 5

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Even though the jumper cables are very thick they often can't supply all the current needed by the starter.

If you wait a while for the battery in the dead car to charge even through the jumper cable from the good car it can provide some of the starting current to assist the process.

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  • \$\begingroup\$ But how come jump starter packs have seemingly thin cables and can still supply the current needed? \$\endgroup\$ Jun 8, 2019 at 22:04
  • \$\begingroup\$ I didn't know the current is limited by the jumper cable. I always thought thicker jumper cable is just to lower resistance so the cables don't melt when the high current is passed through. \$\endgroup\$ Jun 8, 2019 at 22:05
  • \$\begingroup\$ Isn't it more likely to be the alternator than the cables? After all this is peak current, not sustained so it doesn't matter if they heat up a bit. \$\endgroup\$ Jun 8, 2019 at 22:06
  • \$\begingroup\$ It should work even if the good car's alternator is off right? The good car battery should supply enough current for the bad car to start. \$\endgroup\$ Jun 8, 2019 at 22:09
  • \$\begingroup\$ Depends what the dead battery is. If its just empty, then it will also be charging. \$\endgroup\$ Jun 8, 2019 at 22:09
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Did you try to slightly increase rpm of the good car by pushing down the accelerator pedal a tiny bit? Increase rpm to about 2000 before trying to start the engine? Last time I tried to jump start a car, the engine of the good car was audible having trouble running stationary. Starting a car requires substantial amounts of energy.

Good quality jumper cables and clamps can make a difference too as they bring low resistance. You are essentially loading the known good vehicle with a large charge current for the battery and a large starting current for the motor. Cheap, thin (often aluminum rather than copper) wires and poor clamps will not only heat up, with that they'll also drop the power available to the starting vehicle substantially. Think order magnitude in excess of 100 Ampère to start the engine.

And yes if the battery is still able to charge and discharge at a substantial current, which is often the case if it hasn't been too long at low voltage, then indeed the low battery impedance will help a lot. Here is a video on YouTube where a battery is charged from penlights. https://www.electroboom.com/?p=602

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So I did some thinking and I theorize the following:

  1. When you connect the good battery to the dead battery's car using long jumper cables for the purpose of charging the dead battery, the jumper cable has a low but non-negligible resistance. During charge, the charge current is quite low and since R=(delta)V/I, the drop in voltage is still quite low so there is enough potential difference to charge the dead battery.

  2. However, when jump-starting the car, the car tries to draw a current in the range of 100s of amps, in that case, even though R is small but because I is large the drop in voltage across the jumper cable is non-trivial. Then there isn't enough potential difference at the dead battery's terminal to start the car.

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Placing two similar batteries in parallel especially if the bad battery is dead can lead to some difficulties. If the dead battery is in reasonable condition that is its internal resistance is still quite low, when you connect up the jumper leads the dead battery will take out from the good battery quite a bit of current, so due to internal resistance and jumper lead resistance it will take some time to bring the dead battery back up to a level where you can start the car. Just be patient. If the dead battery is really dead that is it is open circuit, then all of the starting current will come from the good battery. The trouble is when you take away the jumper leads the car may come to a stop. Hopefully the alternator system will be able to handle this and get you home.

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The good battery is doing two jobs:

  1. charge the dead battery from low voltage to some reasonable voltage
  2. provide energy for the starter.

The amount of power flowing to the dead car is limited by the jumper cables, and is split among the two jobs.

Initially, the dead battery draws a lot of charging current, but as time passes, it becomes more charged, draws less current, and more of the power that the jumper cables can supply is available for the starter.

Suppose the cables can carry 100A, and the dead battery requires 90A initially. The dead battery charges quickly enough at that current, and eventually requires only 20A. This means that the immediately available current for the starter is 10A, but if you wait a bit for the dead battery to charge a bit, the current available to the starter increases to 80A (out of the 100A the cables carry).

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