If mitochondria are the powerhouse of the cell, a car battery could be called the powerhouse of the engine. A charged battery is needed to start the car engine, but when it’s time for a replacement, it’s important to know how to disconnect a car battery.

Luckily, disconnecting a car battery is relatively simple when you have the right tools and basic electrical knowledge. The main objective is to stay safe while working with any active current and disconnecting battery terminals in the correct sequence, both of which we will go over in detail.

Disclaimer: Working with car batteries poses risks such as electric shock and potential damage to vehicle systems if improperly handled. Please use caution while attempting any vehicle maintenance and always consult your car manufacturer’s manual.

A car battery in an engine bay.

When Should I Disconnect A Car Battery?

There are a few circumstances where you should disconnect a car battery:

  • The old battery can no longer hold a charge 
  • You are replacing the battery tray
  • You are cleaning substantial corrosion from the existing battery and cable connectors
  • You are performing maintenance on the car’s electrical system
  • You will be keeping a vehicle in storage for several months or years
  • You are servicing the alternator
  • You are replacing battery cables/battery posts

How Does A Car Battery Work?

Car batteries work by converting a chemical reaction into an electric current. Inside every battery are an anode (often lead or carbon), a cathode (lead oxide or lithium oxide), and an electrolyte (sulphuric acid or lithium salt). The electrolyte acts as a catalyst for the anodes and cathodes to produce electrons, which create electricity. That electricity is then transferred from the battery terminals to the engine and other parts of the vehicle.

It is worth noting that this process also works in reverse order, which is why you can use a battery charger to revive a dead battery.

How Do I Know If I Need A New Car Battery

There are several ways to measure your car battery to see if it is still good for use. Most car batteries last between three to five years, though neglect from a car owner can necessitate a faster battery replacement.

Here are some ways to determine if you need a new battery:

  • The engine regularly cranks or sputters before starting
  • You regularly have to jump-start the car
  • Headlights are dim/flickering
  • Dashboard lights are dim/flickering

Another way to check your battery’s charge is via an automotive multimeter. A multimeter is a handheld device that can read electric currents, voltages, and several other measurements of electricity.

To assess your car battery’s health with a multimeter, set it to 20.0 DC volts and touch the negative and positive probes to the appropriate battery terminal. If fully charged, the battery should read approximately 12.6 V. If the vehicle is running or was recently driven, you can expect a higher reading because it will still have an excess charge.

If you don’t have a multimeter, you can typically have your car battery assessed at an auto parts store.

How To Disconnect A Car Battery

While disconnecting a car battery, you may want to use gloves and safety goggles as a precaution. Even with fading power, the old battery may still have enough residual charge to cause a shock if someone touches both terminals simultaneously.

  1. Make sure there is no power: Turn off the vehicle.
  2. Locate the battery: Usually, car batteries are under the hood of the car, although some are located in a vehicle’s trunk. Consult the owner’s manual if necessary.
  3. Use the right tools: Disconnecting the terminals will require an open-end wrench or socket wrench to disable the bolts housed under the plastic covers.
  4. Find the negative terminal: It is recommended to always disconnect the negative battery terminal first to avoid an electrical short. Both the positive terminal and negative terminals should be marked with plastic caps
  5. Loosen the cables: After unbolting the negative battery cable, loosen and remove it from the battery. Do the same with the positive cable
  6. Free the battery: Once both cables are disconnected, you can remove the battery hold-down (which may require a socket wrench to free) and remove the battery from the engine bay.

How To Disconnect A Motorcycle Battery

Motorcycle batteries are similar in build and construction, but accessibility is slightly different. Many motorcycle batteries are housed under the motorcycle seat or other body panels, so it can take some finesse to remove them.


A heavily corroded car battery.

Another thing that can weaken your battery connection is corrosion. Car batteries corrode as the sulfuric acid within the battery releases hydrogen gas. As the hydrogen gas interacts with air and other sources of moisture, it begins to build up on top of the battery and around the terminals. 

To preserve your battery’s electrical connection, an old remedy is using a mixture of 1.0 tablespoon of baking soda with 1.0 cup of water. This basic remedy will work quickly to neutralize the corroded acid. Tough corrosion may require using a wire brush to remove. After brushing, just spray it down with cool water and dry with a clean cloth. You can also apply a lubricant such as petroleum jelly to protect from future corrosion.

What To Look For In A Car Battery

Car batteries are not one size fits all. When shopping for a car battery, make sure your replacement fits all the following criteria:

  • Cold cranking amps: Cold cranking amps (CCA) is the amount of power a car battery can produce to start a cold engine. The colder your environment, the higher the CCAs you’re going to want for your battery.
  • Reserve capacity: This is the amount of time a car battery can power critical systems before dropping to unsustainable levels. This is especially important if you plan to run your battery for long periods of time.
  • Warranty: If your battery fails before the warranty expires, the manufacturer will often replace it free of charge.

Group Size

Group size is a number that should be listed on the battery itself. It refers to the dimensions of the battery and the size of the battery post it will fit. Below are common group sizes. 

Group Size

Common Vehicle Fit

(top terminal)

Acura, Honda, Lexus, Nissan, Toyota

(top terminal)

Mazda, Nissan, Subaru, Honda, Toyota

(top terminal)

Buick, Chevy, Volkswagen

(top terminal)

Audi, BMW, Cadillac, GMC, Jeep, Mercedes-Benz, Volkswagen, Volvo

(top terminal)

Audi, BMW, Mercedes-Benz, Hyundai

(top terminal)

Honda, Nissan

(top terminal)

Large vehicles and SUVs

Disconnecting A Car Battery: FAQ

*Data accurate at time of publication.