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How to disconnect a car battery

Done properly, disconnecting a battery is simple and safe. But get the sequence wrong and you could damage the car or yourself.

A typical car battery might produce 'just' 12 volts (the same as a torch) but it can also produce lots and lots of current.

In fact, many a bush mechanic has used a pair of car batteries to weld metal back together, so there’s some serious power involved.

Which means if you need to disconnect a car’s battery (particularly a modern car with lots of on-board computers and electronics) you need to follow the correct procedure to avoid blowing up those electronics.

And since a battery also produces explosive gasses as it charges and discharges, blowing yourself up is a real possibility, too, if it’s done incorrectly.

There’s no real trick in how to disconnect car battery terminals, but there is a definite sequence that you ignore at your peril.

So, how do you avoid those problems and disconnect car battery terminals correctly? Really, it all boils down to knowing which battery terminal to disconnect first. Is it positive or negative first? Okay, let’s go through it step by step.

  • Make sure the car’s engine is stopped, the ignition is turned off and all the electrical systems like the lights are switched off.
  • Identify the negative and positive battery terminals. The positive will usually have a small + symbol next to the terminal in the top of the battery, the negative will have a – symbol. Beyond that, the global car industry has made it even simpler by colour-coding battery terminals: Red positive black negative. If you’re still unsure, the owner’s manual might include a car battery diagram that identifies the terminals and cables.
  • And here’s the critical bit: Remove the negative terminal first and the positive terminal last.
  • How to remove car battery terminals involves using the correct size spanner, loosening the negative terminal and removing it from the battery. Tuck it away somewhere where it can’t flail (battery cables can be pretty springy things with a distinct memory of where they live) about and accidentally touch the battery terminal at any stage.
  • Then you can loosen the positive terminal and remove that lead, again, tucking it somewhere safe where it can’t move.

Why remove the negative terminal before the positive? Because it’s the positive cable that is most likely to throw a spark and ignite gasses or send an electrical spike into the car’s electronics and damage them.

By removing the negative cable first, you remove the chance for the positive terminal to throw that spark, or arc, because the battery is no longer grounded to the car.

And that’s another reason to tackle the negative cable first: If you accidentally touch the spanner you’re using to loosen the positive cable with the negative cable still connected, you’ll learn all about impromptu welding techniques.

But if the negative is already removed, even if you bang the spanner against the car, there can’t be an arc as the negative is no longer connected to complete the circuit.

Knowing how to properly disconnect a car battery is the first step in removing the battery if, for instance it has gone flat or needs to be replaced altogether or, as in some modern cars, if you simply need access to the headlights to change a bulb.

The first step when changing a car battery is to make sure the car’s engine has been stopped. The first step when changing a car battery is to make sure the car’s engine has been stopped.

But how to take a battery out of a car can vary enormously depending on the make and model.

Some have simple clips that are easy to access, others have the fasteners buried deep in the engine bay. The change car battery section of a good workshop manual should detail the process.

The first step, though, is understanding whether you need to fully remove the battery in the first place.

If you only need to recharge the battery via a charger, and the charger will reach the car, then you can connect the charger without removing the battery.

Which leads us to the other big question of can I charge my car battery without disconnecting it?

The positive battery terminal will usually have a small + symbol. The positive battery terminal will usually have a small + symbol.

The answer is yes, you won’t do any harm provided you don’t get the terminals mixed up. Even then, most modern battery chargers have protection circuits to stop this, but you could still throw a spark if you get it wrong.

How to reconnect a car battery when you need to drive it again is the reverse of this procedure. That is; the positive cable should be connected first, followed by the negative cable.

Once you’ve reconnected the battery, you will naturally have to reset the car’s clock and perhaps enter the security code into the stereo system before it will work again.

Finally, don’t forget to refit any insulating covers for the battery terminals and make sure the battery is securely fastened in place.