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I have seen the procedure to jump start a car when the battery is dead.

What I don't understand is that why is one end of the cable connected to the metal frame chassis of the car which has the dead battery?

Why don't we connect the positive negative of the working battery to the positive and negative of the dead battery and start the car?

Can someone explain why do we do this connection and what's wrong in shorting the both battery terminals? How does the battery get charged during the correct procedure?

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  • \$\begingroup\$ What is "the positive negative of the working battery"? \$\endgroup\$
    – Transistor
    Nov 7, 2019 at 7:45
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    \$\begingroup\$ positive and negative of the working battery. \$\endgroup\$
    – user220456
    Nov 7, 2019 at 8:51

3 Answers 3

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In addition to what others have mentioned there's also less resistance if you connect it to the frame. Everything is negative-ground including the 'starter'. This means that during a jumpstart the current flows through the positive into the car frame and then back to the negative terminal. If you put the negative jumper onto the 'dead negative terminal' you add some resistance to the path. It might be small but it's some nonetheless. This leads to higher voltage drops and more heating.

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The negative terminal of a car battery is connected to the car's chassis and engine block, so connecting the negative jumper cable to the chassis does (eventually) make an electrical connection to the negative terminal of the battery.

The lead-acid batteries used in cars can generate hydrogen and oxygen gas while being charged. If you make a spark in the presence of these gases they may explode. It is recommended that you make the last connection of jumper cables some distance from the battery to reduce the danger of explosion as making that last connection will very often make a spark.

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Normally, the last of the 4 clamps is connected away from the battery. That is because there could be a spark, and one or both batteries do emit flammable hydrogen gas. This way, that spark is away from the battery. I have seen a battery explode, messy and dangerous.

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  • \$\begingroup\$ Could you answer my question please? \$\endgroup\$
    – user220456
    Nov 7, 2019 at 3:07
  • \$\begingroup\$ I saw this answer, but it wasn't clear enough for me. electronics.stackexchange.com/questions/74956/… \$\endgroup\$
    – user220456
    Nov 7, 2019 at 3:13
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    \$\begingroup\$ Could you boil your 4 questions down to 1 question? \$\endgroup\$
    – John Canon
    Nov 7, 2019 at 3:15
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    \$\begingroup\$ You can . . . as long as the last connection is away from the battery. Cars today are mostly negative-ground, meaning that the negative terminal of the battery is connected directly to the body/frame/engine. For example, this allows each headlight to be powered by just one wire. The body/frame/engine provides the negative side of the circuit. \$\endgroup\$
    – John Canon
    Nov 7, 2019 at 3:26
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    \$\begingroup\$ Spark . . . plus hydrogen . . . equals . . . boom. \$\endgroup\$
    – John Canon
    Nov 7, 2019 at 3:29

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