Batteries and Jumper Leads

What’s the difference between these jumper leads and these ones?

Does it matter? Aren’t they all the same? And when you are jumping your car, why does the engine have to be running on the other car?

The answer to all of these lies in the voltage drop across the wires.  Voltage drop in wires is not something that we normally have to deal with, as copper is highly conducting and therefore by definition experiences little voltage drop.

In the case of jumper leads however the factor simply is the very massive current that is flowing.  With several hundred amps flowing the tiny resistance of the copper becomes significant and it can result in a significant voltage drop from one and of the cable to the other.  The other thing that determines the magnitude of the voltage drop is how thick the cables are and how long they are – the thicker the cables the lower the voltage drop and the longer the cables the greater the voltage drop.

In the example above the 750 amp cables would be noticeably thicker than the 100 amp cables.  And this is why the tables never seem long enough – the longer they are the greater the voltage drop.

So if they are connected to your car with your battery in good condition at say 13.4 volts, the voltage may have dropped down to say 12.2 V at the other end, which may not be  enough to start the other car.  But if you start your own car, the voltage from your car is now what the alternator is supplying, typically over 14 V.  This results in a healthy 13 V at the other end and increases the chance of a successful start.

Of course one way of knowing that your cables are too thin is if they melt, as they can get very hot if the cables are too thin.

One way around the problem of cables being too short is to make your own.  Go to a welding supply shop and buy some thick gauge welding cable, typically 10 mm in diameter, then get someone to braise on to the end some heavy duty welding clamps.

My jumper leads made this way are 5 m long, and I don’t have to start my engine when jump starting another car, as the heavy gauge cables mean there is negligible voltage drop from one end to the other, and as the other car starts it is able to draw power through the cables directly from my battery.  If you’ve ever had your car started by the RAC you will notice that they use the same type of cables.

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