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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\$ – user1937198 Jun 8 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\$ – Shuheng Zheng Jun 8 at 22:18
  • \$\begingroup\$ 800 to 1kRPM min and 1.5k to 2k RPM for max charge current \$\endgroup\$ – Sunnyskyguy EE75 Jun 8 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 at 0:13
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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\$ – Shuheng Zheng Jun 8 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\$ – Shuheng Zheng Jun 8 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\$ – user1937198 Jun 8 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\$ – Shuheng Zheng Jun 8 at 22:09
  • \$\begingroup\$ Depends what the dead battery is. If its just empty, then it will also be charging. \$\endgroup\$ – user1937198 Jun 8 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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