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When Jumping start, I understand that I need to connect the positive terminal first due to the safety reason. And also if I connect negative terminal first and touching the body of the vehicle with the wrench by accidentally, I might damage the battery due to making a short circuit. And could you explain how the short circuit is created?

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  • \$\begingroup\$ The body of the vehicle is connected to the negative side of the battery. Some old cars use a positive-ground system, though, where the positive side of the battery is connected to the chassis. \$\endgroup\$ – Hearth Oct 27 '18 at 16:49
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    \$\begingroup\$ At least for US cars that'd have to be a really old car. I think the last positive ground Fords were produced in the 1950's. \$\endgroup\$ – TimWescott Oct 27 '18 at 16:52
  • \$\begingroup\$ The reason for connecting the cable to the chassis rather than the battery is a spark could cause an explosion. Lead acid batteries give off hydrogen. I know this to be true. My cousin's battery blew up in his face while connecting the cables. . \$\endgroup\$ – Misunderstood Oct 27 '18 at 19:07
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When you have vehicles with the battery negative connected to chassis (mainly the norm), it makes sense to attach the positve jump lead first because, if you accidentally and momentarily touched it to the chassis of the other vehicle nothing bad would happen. However, if you had already connected the negative jump-lead first, then you would short out the battery on one vehicle if you accidentally touched the positive jump-lead against the chassis.

And could you explain how the short circuit is created?

The battery in any vehicle has one terminal connected to the chassis of the vehicle. If the terminal that connected to chassis was positive (as in older model vehicles) then you would attach the negative jump lead first.

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  • \$\begingroup\$ Could you please make it a little clearer? However, if you had already connected the negative jump-lead first on both sides, or just one? , then you would short out the battery on one vehicle which one? if you accidentally touched the positive jump-lead against the chassis. so the positive jump-lead is already connected on one side, and touches the chassis on the other side? \$\endgroup\$ – Lodovik Jan 30 '19 at 21:38
  • \$\begingroup\$ @Lodovik if you have already connected the negative jump lead means FULLY connected between both cars. \$\endgroup\$ – Andy aka Jan 31 '19 at 10:38

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