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What would cause the negative side of jumper cables to get so hot the rubber sloths off when charging a deep cell battery off your car?

2 Answers2

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Cables get hot due to large amounts of current. When you connect a full battery to an empty one, a large current can flow. The current depends mainly on contact resistance, at 0.1 Ohm you can get 60 A, with good contact the current can be much higher, into hundreds of Amps. Thin jumper wires can get hot enough to melt their insulation at those currents. This is why good jumper cables are very thick (10 mm2 supports 60 A continuously, and you can get them much thicker than that).

Using jumper cables is the worst way to charge a battery and should only be used as a last resort, due to that large current. A battery charger is much better, as it will gradually charge the battery.

Hobbes
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  • 50-100 A is nothing. I have 50mm2 cables which support a load of 125-170A continuously depending on what temperature the insulator is capable of withstanding, and the fusing current for 10 seconds is 1900A. However, I agree with your claim that battery charger is a better alternative. – juhist Feb 05 '17 at 16:27
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    As was recently pointed out to me, even a tiny amount of contact resistance (0.1 Ohm) on the jumper cables or their clamps will limit the current to 60A. – Hobbes Feb 05 '17 at 16:32
  • So hot they smoke? –  Feb 05 '17 at 16:34
  • @Hobbes - Your contact resistance number is way too high. High-quality copper clamps do not have contact resistance of 0.1 ohm. You can verify this with a multimeter: connect the jumper cables from one end to each other and from the other ends to a multimeter. Let's assume the multimeter reads 0.9 ohm. Now try what is the resistance of the multimeter's cables by using only them. Let's assume the reading is 0.8 ohm. Now you have three connections using the clamps, and the resistance of one is (0.9-0.8)/3 ohm = 0.033 ohm. This was just an example, I believe even 0.033 ohm is way too high. – juhist Feb 05 '17 at 16:38
  • @juhist it got just as hot putting a little tooth on it. –  Feb 05 '17 at 16:43
  • @juhist: fair enough, amended my answer. I'll try measuring my jumper cables tomorrow. – Hobbes Feb 05 '17 at 17:12
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If you're experiencing heating of the jumper cables as you've described when you jump to another battery, you've put the jumper cables on backwards (positive lead to negative terminal and visa-versa).

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