How To Disconnect a Car Battery

You may need to disconnect your car battery when your old battery needs replacing or when you’re working on your car’s electric wiring or simply to clean the battery terminals and connectors,… 

It’s very important to do this in a safe way. Many people are reluctant to disconnect their car battery themselves as they don’t know exactly how and have heard horror stories of what can go wrong. 

In this step-by-step guide we’ll show you how to safely disconnect your car battery:

Preparation

Everything you need to disconnect a car battery  

• wrench
• power supply  (optional)

Proceed With Caution

Although a car battery’s 12V current is too weak to pass through the human body and “electrocute” you, the high amperage can cause severe burns and injury. 

Short circuiting battery terminals will cause extreme, instant heat and sparks that are so hot they will melt metal in a split second. This extreme heat can cause severe skin burns and eye injuries too. Worst case, the spark will ignite the hydrogen gas a car battery constantly vents which might make the car battery explode, splashing sulfuric acid everywhere.

That’s why when working on a car battery, you want to wear eye protection and keep any open flames away from the battery and surrounding area. This includes cigarettes and other smoking products. You also want to wear latex gloves to keep battery acid from burning your skin.

Furthermore, if you randomly disconnect your car battery, your car’s settings and presets might get lost. You might even ruin your car’s electronics. When power loss is detected, your car alarm might go off too. That’s why you need to disconnect your car battery in a safe, controlled manner.

In this “How To” we’ll explain how to disconnect and reconnect a car battery in a few simple steps without damaging your car or injuring yourself or others.

Here’s how to disconnect a car battery in 8 easy steps

Step 1: Connect a power supply (optional)

If you don’t want to risk losing your radio presets, your car’s settings and presets, ruining your car’s electronics or triggering your car alarm, you’ll first have to connect a power supply. 

Most older cars that aren’t equipped with the latest high tech, aren’t that sensitive to power loss and will need no special procedures or settings to reboot them. For this type of vehicle, it’s not strictly necessary to use a power supply. Check your car owner’s manual to make sure. 

If a simple disconnect without power supply will do for your vehicle, you can skip this step and go straight to step 2 and ignore the alligator clamps on the battery terminals in the illustrative pictures.

Connecting a power supply is very easy to do: 

With your battery still connected to your car terminals, connect a mains powered power supply with its red alligator clamp to the metal sleeve on the red battery cable, but in a way that you can still reach and undo the terminal bolt. 

Because the power supply we recommend and use is spark-proof, it is perfectly safe for us to connect the black alligator clamp to the metal sleeve on the black battery cable (as seen in all the pictures).

If you’re going to use a power supply that is not spark-proof (check the owner’s manual), you want to clamp your power supply’s black alligator clamp to any blank metal part under the hood of your car like the suspension posts or engine. You want to choose a location as far away from the battery as possible to avoid contact sparks near the battery, because lead-acid car batteries produce hydrogen gas which is highly explosive. 

Step 2: Unscrew and undo the negative (black/ – ) battery terminal clamp

To disconnect the car battery, you want to start by unscrewing the negative (black / -) battery terminal clamp bolt first with a wrench and work the terminal clamp loose until it comes off.

You want to put the battery cable in the engine bay next to the battery tray on a dry towel for example, but make sure it can’t touch any metal parts of the car nor the other terminal.

NOTE: Virtually all post-war cars are negative “ground” or “earth”. Meaning the engine, chassis and body are connected with the negative car battery terminal via welds, bolts, screws… This has become the international modern standard.

Step 3: Unscrew and undo the positive (red / + ) battery terminal

After you’ve disconnected the negative (black / -) terminal, you can now safely disconnect the positive (red / +) terminal. 

Unscrew the battery terminal clamp bolt just like you did with the negative (black / -) battery terminal clamp.

Again, you want to put the battery cable in the engine bay next to the battery tray on a dry towel for example, but make sure it can’t touch any metal parts of the car nor the other terminal.

Step 4: Test and/or replace the battery 

Now that the battery terminals are tucked away securely beside the battery and they don’t touch any metal parts, each other or the battery posts, you can test your battery and replace it if necessary. Check out our posts on how to test a car battery and best battery testers.

NOTE: Theoretically, by disconnecting the negative (black / -) terminal first and the positive (red / +) terminal clamp last, you can’t accidentally cause a short circuit by touching a metal part of the car because the negative “earth” is already disconnected. 

Nevertheless, it is good practice to avoid contacting any metal parts with the battery clamps. 

To (re)connect the car battery follow the above steps in reverse:

Step 5: (re)connect the positive (red / + ) battery clamp to the positive (red / + ) battery terminal

Step 6: (re)connect the negative (black / – ) “earth” clamp to the negative (black / – ) battery terminal.

Step 7: Bolt them on tightly for optimal contact.

Step 8: Disconnect the power supply (if connected)

Disconnect the power supply clamps in any order, as long as they don’t touch any metal parts or each other in the process. You can now unplug the power supply’s mains and remove it.

Great job! 

Now that your car battery has been disconnected, tested or replaced and reconnected in a safe professional way, your vehicle is ready to hit the road again. Check out our recommended car batteries if your battery needs replacing, 

Mission accomplished!