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So I have a car B with a dead battery and a good functioning car A. I remove the dead battery from B and connect the terminal directly to car A's battery terminals.

I cannot start car B. I tried to start car A, and then start car B but to no avail.

Is it because the jump cables I use introduces such a significant resistance that insufficient power reaches the bad car?

I measured the voltage drop across the good battery and the voltage drop across the jumper cables that are connected to the good battery, and they were identical.

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  • \$\begingroup\$ Do the headlights dim when the start is turned? That’s a clue your connections are poor. Oxidized contacts on a small surface area contribute lots of series milliohms which with >x00 Amps will make it slow turning. if it turns fast, something else is damaged. Gas ECU or ? Not having a battery may also damage some electronics without something to absorb the current pulses from the alternator to limit the voltage. Return the battery or get a new one. Squeeze the clamps and rotate to cut thru the oxide. \$\endgroup\$ Commented Jun 9, 2019 at 0:54
  • \$\begingroup\$ "I tried to start car A". Don't just start car A, rev its engine. this will increase voltage to (hopefully) compensate for resistance of the jumper leads. \$\endgroup\$ Commented Jun 9, 2019 at 4:52
  • \$\begingroup\$ Can the jumper leads have that much significance? They are probably like 12 feet long and very thick (which I thought reduces resistance) \$\endgroup\$ Commented Jun 9, 2019 at 16:07
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    \$\begingroup\$ Yes the leads can have a big difference. But it is mostly from cheap leads that are thinner and often have poorly connected clips. I would be more suspicious of the contact points first though. In the engine bay there is almost always lots of oxidization build up. It's also difficult to get much surface area in contact with the leads. \$\endgroup\$
    – hekete
    Commented Jun 11, 2019 at 8:37

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Usually it's better to leave the old battery in there as it can still help with starting the car even if it can't manage on its own. Inexpensive thin jumper cables or jumper cables connected to sub-optimal connection points can easily have too much voltage drop for the starter motor to be able turn over an engine. Sometimes you can let the old battery charge for some time (maybe 30-60 minutes) by connecting it with a running car or using an external charger and that will be enough to get it moving.

The problems are worse again in winter when batteries have reduced current capacity and oils are thick. High quality jumper cables can make all the difference. You'll know there is a voltage drop problem when the lights on the dash just about go out when the engine is supposed to be cranking. Maybe the starter just clicks. If the lights don't dim and the engine isn't turning over it could be the starter itself. There are only a few things in the circuit and it's pretty easy to troubleshoot (but if the starter is shot it can be a job better suited to a pro to replace it- some of them are very hard to access without a hoist).

However, I did spend a good part of an evening with friends trying to jump start a small crossover SUV without success, the motor just wouldn't turn over even with a freshly charged brand-new battery and high quality connections. Turned out that the engine had seized internally (it was apparently thunking lustily when shut off) and the vehicle was practically a write-off.

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  • \$\begingroup\$ Checking that the engine itself turns is easy - just put a spanner on it - a test I would do as soon as the starter failed to spin the engine when using good batteries or battery... \$\endgroup\$
    – Solar Mike
    Commented Jun 9, 2019 at 6:52
  • \$\begingroup\$ The jumper cables are used are about 12 feet and quite thick. I measured the voltage drop of the good car without jumper and after jumper, there was no discernible difference. \$\endgroup\$ Commented Jun 9, 2019 at 16:39
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Consider it as if you are starting car A with remote battery. Since you've made sure that the cables are original the only reason comes is that the capacity of car A is bigger than car B. for example 8 cylinder car A and 4 cylinder car B. this could be the reason.

Usually the best practice to do that is to connect two batteries with jumper wire wile the car B is working. High the riv of the car B for about 5 to 7 minutes. Then ask the other person to turn on the car A. it should work well.

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