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How to Disconnect a Car Battery

Batteries Plus Blog - Power - by Joe Weber - updated on 8/9/2022

Disconnecting a car battery

Every car owner will need to replace their car battery at one time or another. While there are countless service centers that can do this for you, it is always a good idea to learn how to complete this task yourself. Read on to learn the detailed steps of this process.

When Do I Need to Disconnect My Car Battery?

This may seem like a pretty straightforward question but in reality, there is more to it than just when taking the battery out. You absolutely need to disconnect the battery if you want to replace the battery. However, there are many other reasons to disconnect your car battery especially if you are a do-it-yourself repair person.

To be on the safe side, disconnect the battery whenever you are working with or replacing anything in the vehicle that is electronic that could cause a spark or injury. Light bulbs and things of that nature are safe to replace without disconnecting the battery but airbags, the engine's computer, etc. are not safe to work on with the battery connected. A sudden jolt of electricity to the airbag control could cause it to deploy and cause injuries. Damaging the engine computer can end up costing thousands to replace. As a general rule, always disconnect the battery when working on electronics.

How Do I Disconnect My Car Battery?

First and foremost before working with a car battery remove all metal rings and jewelry to avoid creating a dangerous situation if you accidentally make a connection between a hot (positive) and ground with your metal jewelry.

Prepare Your Workspace

  • Wear safety glasses and gloves.
  • Gather all of the tools that will be required to remove and replace the battery.
  • Have a car battery terminal protection kit ready to clean and protect the battery terminals and terminal connectors.

Disconnect the Battery

  1. Open and prop the hood of the car.
  2. Locate the battery and positive and negative battery terminals.
  3. Disconnect the negative terminal first, then the positive terminal.
  4. Lift the battery out of the battery tray and place it in a safe place.
  5. Clean out any dirt and debris in the battery tray and the battery terminal clamps.
  6. Use a wire brush with the terminal cleaner spray to clean the battery terminals and connectors
  7. Spray the terminal clamps and battery terminals with terminal protection spray.

For detailed steps on how to change a car battery please read our online guide "Simple Steps to Changing Your Car Battery".

Do I Need to Disconnect Both Battery Terminals When Charging?

With most modern chargers being automatic and microprocessor controlled it is not necessary to disconnect the battery in your car before charging it when you have this type of charger. These chargers will automatically detect the battery's charge levels and adjust the amperage accordingly. Once the battery reaches the optimal charge, the charger will switch to a maintenance mode to keep the battery at that level but it will not overcharge or it will shut off.

If you have an older charger or one that is not automatic it would be best to disconnect the car battery prior to charging. Old chargers will not power off or switch to a maintenance mode when the battery is charged. It will continue to charge the battery as long as it is connected. It is very easy to overcharge a battery and reduce its lifespan. There are also some safety concerns with older chargers as they don't have reverse polarity protection. If you accidentally connect the clamps to the wrong terminal, bad things could happen, including damage to car electronics and even injury to yourself.

It is recommended that you have a good quality automatic charger in your arsenal to safely charge your car batteries.

Will the Car Battery Drain if the Negative Cable Is Disconnected?

A car battery will self-discharge at a rate of 5-15% per month with the negative terminal disconnected. However, the alternative option of leaving the battery connected while it isn't being used will drain at close to 20% per week. That's a very large difference and if you intend on not using the car for a longer period of time it is definitely worth disconnecting the negative battery terminal as the battery will discharge much more slowly.

Should a Car Battery Be Disconnected When Not in Use?

As seen above there is a big deterrent to leaving your battery fully connected when stored. The reason for such a drastic difference is that when both terminals are connected, even though you are not using any of the electronics in the car, there is still a draw from the battery to power things like the clock, radio presets, etc. This draw will quickly leave you with a dead or dying battery that may need to be replaced if it sat long enough.

If you do intend to leave your car or truck sitting unused for long periods of time it is wise to add a battery maintainer to your list of tools. This will ensure your battery is always charged at the optimum level, even if it is sitting unused.

Shop For New Car Batteries at Batteries Plus

If you are in need of a replacement car or truck battery look no further than Batteries Plus. Your local store offers free battery testing and installation (on most vehicles) if you need a new battery. Check out our large selection of car batteries today! Read more about our battery installation services in our blog articles titled "Does Batteries Plus Install Batteries?" and "Batteries Plus Offers Free Car Battery Installation".