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I connected the jump leads red to the two positives and the black cable from flat negative to a metal earth point on the charged battery car.

Only figuring out the black should be the other way round, on the charged negative to an earth point on the flat battery car.

Would this mess up the jump start?

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No, it won't mess up the jump start, the only possible issue may be caused by the order that you make the connections due to the possibility of a spark : the last connection should be to the body / earth point.

  • Thanks. Also I've heard people recommend crossing the cables between positive and negative IE positive on charged to negative on flat and vice versa. Does that give a greater chance of charging? – user1561108 Apr 22 '17 at 7:30
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    NO, don't cross the positives and negatives - big sparks, excessive heating fire and explosion can result as the current gets very high. – Solar Mike Apr 22 '17 at 7:49
  • Thanks, but in theory would that actually work too or just do nothing? – user1561108 Apr 22 '17 at 7:55
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    What it does is to allow both batteries to discharge through each other - at the maximum current flow limited only by the internal resistance of the battery. Can lead to fire etc as above. As I said don't do it or don't even attempt it. – Solar Mike Apr 22 '17 at 7:57
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It's fine: the current will be the same either way. There is only a single situation where you do not connect plus-plus and minus-minus (note that there are some really old cars where ground/chassis is not minus, and you need to change your startup wiring then to still have plus-plus and minus-minus). This special situation is when you use two cars for jump-starting a large bus or other vehicle with 24V battery voltage.

In that case you connect one (and only one!) plus of one car to the minus of the other. The remaining plus and minus (which are then in two different cars) are then 24V apart and are sufficient for jump-starting a 24V vehicle.

If you are not totally sure what you are doing, don't though. And have really solid jump starter cables (of which you then need three, two running to the bus, namely one from each car, and one for connecting the cars).

However, busses usually have two 12V batteries in series and they rarely go dead exactly at the same time. So it's often sufficient to have one car (and one pair of cables) to jumpstart the bus: you just need to support the deader battery of the pair. If aiding one battery doesn't help, try the other.

This is a situation where it makes more sense to connect to the ground of the donor car only: only one of the two batteries in the bus actually has minus as its ground, and you don't want to get confused.

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