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When jump starting a car A using a car B, can one connect the chassis of both cars together instead of connecting the negative battery terminal of car B to the chassis of car A. In theory this seems to work as the negative battery terminals of both cars are connected to the chassis but wouldn't there be extra electrical resistance ? I am asking this question because I want to jump start my car and the donor car's negative battery terminal is not accessible.

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Yes there will be extra electrical resistance, but this will be negligible compared to the extra cable length used in the jump cables.

As long as the connections are secure and not on painted surfaces it will work fine.

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Yes, you can connect the (-) cable to the donor car's chassis. There may be some additional resistance, but it should work fine. Make sure that the last connection you make (of the four, two on each vehicle) is as far away as possible from the battery in that vehicle.

You're providing a current path from the donor battery to the recipient battery. The donor battery's (-) pole is connected to the donor car's chassis (unless you're jumping a positive-ground vehicle like a British car from the 50s, in which case the polarity would be reversed and the battery (+) on the car being jumped would be connected to the chassis).

You'll get a better electrical connection if you open the hood of the donor car and put the jumper cable (-) on an accessible bare-metal point on the engine, or a large unpainted bolt that goes into the engine or the car's structure. Either will give the jumper cable a good electrical connection to the donor car chassis.

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  • I have done it by using contact of (steel) bumpers . Then you can put the cables together to have twice the length for the positive ( red) post. – blacksmith37 Jun 11 at 15:23
  • Yes, I've seen that done too. Risks some scarring of the bumpers at the connection point, and can't be done with most of today's cars' bumpers which are hidden behind plastic panels. – DavidSupportsMonica Jun 11 at 17:19

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