Shop Parts keyboard_double_arrow_right
Need car parts? Select your vehicle
Reading Time: 4 minutes
Summary
  • If you remove the positive terminal before the negative terminal, the battery will continue to discharge. 
  • Removing the positive terminal first will cause it to touch the chassis through a spanner or wrench or some other metal tool, creating the same effect as putting both battery terminals together. 
  • Fire and physical injury can occur when you fail to disconnect the negative battery terminal while working on your vehicle. Disconnecting both terminals isn’t a bad idea, but make sure they can’t move over there and touch the terminals because the cables are stiff.

Disconnecting the battery during repairs is a pretty standard procedure regardless of the type of issue you’re dealing with.

Along with this protocol, disconnecting the negative terminal first before the positive terminal is a must.

But what’s the reason behind this practice?

Why Disconnect the Car’s Negative Battery Terminal When Working on Repairs

The negative battery terminal, also known as the ground strap or cable, is directly connected to the vehicle’s engine, body, and chassis. The positive terminal is connected to the starter, the alternator, and the fuse boxes.

close up of car battery negative terminal
Disconnecting the negative terminal first before the positive terminal is a must.

Removing the positive terminal without removing the negative terminal can create a situation where a tool that is touching the engine or body can still short out against the positive battery terminal. While this isn’t likely to happen, it’s still a good idea to remove the negative terminal, even if only one of the two terminals is being disconnected.

See also  How to Tell Positive and Negative Terminals on a Car Battery

If you’re just going to remove one terminal, remove the negative terminal rather than the positive terminal.

Removing the positive battery terminal without removing the negative terminal can create a situation where a tool that is touching the engine or body can still short out against the positive battery terminal. While this isn’t likely to happen, it’s still a good idea to remove the negative terminal, even if only one of the two terminals is being disconnected.

If you’re just going to remove one terminal, remove the negative terminal rather than the positive terminal.

Richard McCuistian, ASE Certified Master Automobile Technician

Here are other problems that can arise when you fail to disconnect the car battery’s negative terminal first:

  • Damaged electrical components
  • Physical injury
  • Fire 
diagram of a car battery wiring
In the Illustration shown, the green part of the vehicle (engine, body, and frame) is all still grounded to the battery if you only disconnect the positive battery terminal. But if you disconnect the negative terminal, neither the engine, the body, nor the frame are still battery connected. | Image Source: Richard McCuistian
, Here’s Why You Need to Disconnect the Negative Battery Terminal When Working on Your Car

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: If you only disconnect the positive terminal, the entire engine and car body are still connected to the battery. If you disconnect only the negative terminal, the likelihood of an accidental short circuit is practically zero.

How to Disconnect a Car Battery

When disconnecting a car battery, it’s always important to make sure that the engine is turned off. Attempting to remove a battery while the engine is on can spike sensitive electronics and cause a lot of damage.

See also  Underhood Checks

The next step is to locate the battery. Most vehicles have it under the hood, while some have it under the trunk. Always consult your owner’s manual to determine the right location.

The next step is to locate the battery terminals. The negative terminal is usually denoted by a “-” symbol, while the positive terminal has a “+” symbol. In most cases, the positive terminal also comes with a red plastic cover.

Using a wrench, loosen the nut on the negative terminal and take it off the battery post. Make sure that the cable isn’t anywhere near the terminals. Then, proceed with removing the nut on the positive terminal.

Charging the Battery the Right Way

Dead batteries aren’t always a lost cause. In some cases, you only need to recharge them to get them up and running. Here’s the right way to charge your car’s battery.

Step 1: Determine the charge rate. This is usually based on the state of charge (SOC) and charging rate.

Step 2: Connect the charger to the battery, but make sure that the charger isn’t plugged in yet.

Step 3: Set the charging rate. The initial charge rate should be about 35A for 30 minutes to kickstart the charging process.

Keep in mind that charging a battery too fast can increase its temperature at a rapid rate, warping the plates in the process.

Note: Never charge or jump-start a frozen battery. A car battery can freeze up in its discharged state because the electrolyte turns into water.

A frozen battery often bulges at the sides because of water expansion. As a result, the gas pockets in the battery can explode.

See also  Bad Starter Solenoid Symptoms

Battery Maintenance Guide

Most new car batteries are made with a maintenance-free design in mind. Still, it pays to keep an eye out for your battery’s condition to avoid unwanted repairs down the road.

Battery maintenance includes a thorough inspection of the electrolyte level. If the electrolyte level is low, most manufacturers recommend adding a bit of distilled water.

Proper battery maintenance also involves making sure that the battery terminals are free from corrosion, the battery case is clean, and the hold-downs are secured.

Check for loose or corroded connections at the battery terminals or the ground connector on the engine block.

If you have loose or corroded connectors, clean them using a tablespoon of baking soda mixed with a liter of water. Brush the mixture onto the battery and housing to neutralize the acid.

Then, you can proceed with cleaning the connections and washing the area with water.

Also, make sure that the battery hold-downs are secured enough to prevent the battery from moving, but not tight enough to cause the case to crack.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

File Under : Electrical System , DIY Tagged With : ,
NTMS Racer
Subscribe
Notify of
guest
0 Comments
Inline Feedbacks
View all comments

View all Questions & Answers

expand_more
CarParts.com Answers BE PART OF OUR COMMUNITY: Share your knowledge & help fellow drivers Join Now