How to Disconnect a Car Battery
- by Joe Weber - updated on 8/23/2024
Every one of us who owns a car or truck will need to replace the battery in our vehicle at one point or another. While countless service centers and garages will do this for you, it is always a good idea to learn how to do this yourself.
There is no better feeling than successfully doing a job yourself and saving some money. Keep reading to learn the detailed steps of this process, and some do's and don'ts to pay attention to along the way.
When Would I Need to Disconnect My Car Battery?
The obvious answer here is when you are replacing the battery. But there is one more reason, a big one, why you would want to disconnect your car or truck battery.
For your safety, you should always disconnect the battery when working with anything electrical in the vehicle that could cause a spark or injury, like wiring, ignition work, airbags and the vehicle's computer.
A sudden jolt of electricity to the airbag control could cause it to deploy and cause injuries, damaging not only you (potentially) but also the steering wheel and components and even the engine computer, which can cost thousands to replace.
As a general rule, always disconnect the battery when working on electronics. Replacing light bulbs and things of that nature are safe to replace without disconnecting the battery.
Are There Times When I Shouldn't Disconnect A Car Battery?
For the most part, disconnecting the battery before you begin working on your vehicle is a good idea. When is it not? When the engine is running. Why? This is more about the electronics in the vehicle than anything else.
While the engine may still run for a short time after disconnecting the battery, the biggest threat will be voltage fluctuations that occur when the system switches to running off the alternator. These fluctuations can fry many electrical components in the vehicle. It is best to be safe and shut the car off before disconnecting the battery.
How Do I Disconnect My Car Battery?
First, remove any rings and jewelry to avoid creating a potentially dangerous situation if you accidentally connect the hot (positive) and ground with your metal jewelry.
Prepare Your Workspace
Before doing any type of work on a vehicle you should take precautions and prepare your space so you have everything you need at your fingertips.
- Protect your skin and eyes by always wearing safety glasses and gloves.
- Gather all of the wrenches and other tools that will be required to remove the battery.
- It's a good idea to have a rag or two on hand to wipe away dirt and grime.
- Have a car battery terminal protection kit ready to clean and protect the battery terminals and terminal connectors.
Disconnect the Battery
Ready to disconnect your battery? Let's begin:
- Park and turn off the engine.
- Open and prop the hood of the car.
- Connect a memory saver to the OBD-II port in the car, under the dashboard to preserve radio presets and other settings that may get erased after disconnecting the battery.
This is optional and if you don't have a memory saver, don't worry. You will just need to reset your radio presets and other settings after reconnecting the battery.
- Locate the battery along with the positive and negative battery terminals.
- Disconnect the negative cable from the battery first, using a screwdriver or wrench, then the positive terminal.
- Be sure to secure the cables away from the battery so they cannot come in contact with any metal or accidentally touch the terminals as you remove the battery.
- Loosen the battery hold-down clamp so you can remove the battery.
- Lift the battery out of the battery tray and place it in a safe place.
- Clean out any dirt and debris, like dirt and leaves, from the battery tray.
- Use a wire brush with the terminal cleaner spray to clean the battery terminals and connectors.
- Spray the terminal clamps and battery terminals with terminal protection spray to prevent any corrosion from building up.
- To reconnect the battery or install a new one, proceed to connect the battery terminals in the reverse order. Connect the positive (+) terminal first, then the negative.
- Make sure to tighten the battery hold-down bracket so the battery is secured in place.
For more detailed steps on how to change a car battery please read our handy online guide "Simple Steps to Changing Your Car Battery".
Do I Need to Disconnect The Battery When Charging?
Most modern chargers are automatic and microprocessor-controlled. You do not need to disconnect the battery in your car before connecting it when using this type of charger.
Automatic chargers will 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 a healthy level but it will not overcharge it.
What If I'm Not Using An Automatic Charger?
If you are using an older charger or one that is not automatic you don't need to disconnect the car battery from the vehicle before charging but you do need to keep an eye on the charge as it's charging.
Chargers that are not automatic 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 easy to overcharge a battery with traditional chargers which can reduce its lifespan, so it's important to disconnect the charger when the battery reaches a full charge.
There are also other safety concerns with many older chargers as they don't have reverse polarity protection. What does that mean? Connecting the clamps to the wrong terminal could result in bad things happening, including damage to car electronics and even injury to yourself.
You never know when you may need to connect your car battery to a charger. That's why we recommend 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?
Car batteries will naturally self-discharge at a rate of roughly 5% per month with at least the negative terminal disconnected.
However, leaving the battery connected completely while it isn't being used will drain at close to 20% per week. Why? Leaving the battery connected will still draw energy to power things like the clock, power lock/unlock, keep radio presets active and other electronics while the car sits idle.
5% and 20% may not seem like a huge difference but leaving your car battery connected could mean your battery is dead in only about a month.
If you intend on not using the car for a longer period it is worth disconnecting the negative battery terminal as the battery will discharge much more slowly.
Leaving your car or truck sitting unused for long periods can be completely harmless though and some of us have to do it every winter. If you do, it's wise to add a battery maintainer to your list of tools to keep the battery healthy while it sits.
What Is A Battery Maintainer?
A battery maintainer is a tool that we should all have in our toolbox. A battery maintainer sends small amounts of energy to the battery to keep it charged and healthy while it's being stored for a longer period.
They help extend the life of the battery by preventing damage that can occur when leaving the battery undercharged for a long period.
Using a battery maintainer while storing your batteries for longer periods will ensure your battery is always charged at the optimum level when you start using your vehicle again.
What's The Difference Between A Battery Maintainer and A Battery Charger?
They both send energy to the battery, they're the same, right? Wrong. A battery maintainer and a battery charger are different. Here's why:
- Battery chargers use a variety of charge rates that range from 1 amp to 25 amps or higher.
- Some battery chargers need to be monitored during the charging process to ensure the battery is not overcharged. (Most new chargers are automatic and will switch to a float mode when the battery is fully charged to prevent overcharging.)
- Battery maintainers use a smaller amount of charge amps, usually around .75 to 2 amps.
- Battery maintainers prevent overcharging and keep the battery healthy, whereas battery chargers can get you back on the road quickly with higher output to charge faster.
- Battery maintainers are smart and fully automatic so you can safely leave the battery connected at all times during storage.
Shop For New Car Batteries at Batteries Plus
If you need a replacement car or truck battery look no further than Batteries Plus. Your local store offers free battery testing if you are starting to notice the signs of an aging battery.
We'll even install your new battery while you wait (on most vehicles) when you purchase a new battery. Check out our large selection of car batteries today!
Don't forget to keep an eye on our blog to stay up-to-date on all things car battery and more!