Disconnecting The Battery

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» Parts List » How Do I/Tune Weber Carburettors » Emissions And The MOT Test » How Do I » Good Engineering Practice/Batteries/Disconnecting The Battery

When disconnecting the battery before commencing work on the car, you should disconnect the negative, black lead first, here's why:

The battery has approximately a 12v "potential difference" between its positive and negative lead and, due to the construction of the battery, it is able to deliver extremely high currents (around 300A to start the car for example). By contrast an AA torch battery will deliver only a few hundred milliamps (thousandths of an amp). Your car battery is capable of delivering around 2.7KW (KILOWATTS!)for a short time so the potential for excitement and heat are large. Treat your battery with respect! It can't electrocute you in the acknowledged sense as the volts are too low, but it can and will produce enormous heat if connected in a way which shorts the two terminals together.

All circuits need a complete loop of wire from one terminal of the battery through the circuit itself (eg: the headlights say) and back to the other battery terminal. So basically two wires are needed for each circuit - a "flow" wire and a "return" wire.

In order to save on lots of unnecessary wiring, all cars with metal bodies use the chassis of the car as one of the wires. This is called "grounding" or "earthing" in car electrical parlance and thus saves 50% of the wiring (and associated expense) that would otherwise be needed. (Thus fully fibre-glass cars need lots more wire). It doesn't actually matter which terminal, either positive or negative, is grounded to the chassis. Both will work equally as well. However, in order to have standardisation in the motor industry worldwide in terms of how components and equipment are constructed, virtually all vehicles today use a negative earth. So don't go swopping yours over!

Now to the crux..... you will only get a big bang if you connect the two terminals together. So if you leave the negative lead connected and, whilst disconnecting the red positive lead, your spanner happens to touch the chassis while its other end is around the bolt on the positive, you will have connected the two terminals together through your spanner and it will become a permanent part of your chassis as 300 amps flow through it and weld it to the car! You will also probably let go of it very smartly as its temperature reaches that of the Sun!!

Conversely, if you disconnect the negative lead first, thus also disconnecting the chassis from having anything to do with the battery, then your spanner can be round the positive and touch the chassis with no ill effects. I would suggest that, once you have disconnected the battery negative, you close the protective cap on it so that nothing metal can accidentally fall across the two terminals. There is no need to disconnect the positive terminal of the battery unless of course you are taking the battery out for some reason. If you do undo both, always remember, UNDO THE NEGATIVE FIRST AND RECONNECT THE NEGATIVE LAST. When you have undone the negative lead, tuck it safely out of the way so that it can't suddenly spring back and touch the negative of the battery again before the work is finished. And do undo the negative everytime you work on the electrics. A minute's extra work is better than a fire.

Chris Wyles

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