How to Reconnect a Car Battery

Connecting a battery on your car is an easy job that can be accomplished with just a few tools.

There are many reasons why a battery would have been removed from the car. Maybe it was flat and needed to be charged. Perhaps you will fit a replacement battery because yours has come to the end of its life.

Whatever the reason, by following our easy guide, you’ll reconnect your battery with no problems.

Tools You Need to Connect a Car Battery

The tools you require depend on the type of terminal fixing used. On larger terminals, it is usually a nut and bolt that tighten the clamp.

Depending on the terminal size, you’ll probably need a socket or spanner of between 3/8 inch and 5/8 inch. Some smaller connectors may have a crosshead cut in the bolt’s top so you can use a screwdriver.

To be sure, have the following tools available:

  • Large crosshead screwdriver
  • Large flathead screwdriver
  • Spanner set
  • Socket wrench and sockets
  • Wire brush
  • Petroleum Jelly or White Lithium Grease

Process of Reconnecting a Car Battery

1. Follow the handling and safety procedures

Following a few simple guidelines helps you reconnect your battery safely:

  • Read the owner’s manual section on the battery connection.
  • Wear overalls that cover your arms and body.
  • Wear gloves and eye protection.
  • Work in a well-ventilated area.
  • Be careful not to cause a short-circuit when using metal tools.
  • If you spill battery acid, clean the area immediately.
  • Should you splash yourself with battery acid, wash the affected area thoroughly and call a physician.
  • Battery fumes can be explosive, so keep the battery away from flames, sparks or cigarettes.

If the battery is a sealed type, then you are good to go. If it has caps along the top, make sure they are correctly closed.

2. Clean

While the battery is out, it’s an excellent opportunity to clean both the mounting tray area of the battery and the securing strap.

Check the mounting tray for damage or rust, and repair as required.

Make sure the battery cable terminals are clean, to give a good connection to the battery. You can use a wire brush on the cable connectors and battery terminals.

RELATED: What Does Corrosion On Car Battery Mean and How to Get Rid of It

3. Determine Positive and Negative Battery Terminals

This is very important. If you connect the positive and the negative round the wrong way, current from the battery flows in the wrong direction.

Most cars have a fuse to protect against this, but if there is no fuse, you could easily damage your car’s electronic systems.

The battery terminals are marked, red is positive with a “+” or “POS” symbol, and black is negative with a “-” or “NEG” symbol. When reconnecting your car battery, attach the cable to the positive terminal first, then the negative one.

Connect the red cable to the positive terminal (+) first, then the black one to the negative (-).

4. Place the battery in the battery tray

Make sure the retaining strap is out of the way and place the battery in the mounting tray. Take care doing this, as large batteries are cumbersome, and sometimes the tray can be awkward to reach.

Now secure the retaining strap in place to hold the battery tight.

5. Apply grease to the terminals

Before we attach the cables, smear a light film of petroleum jelly around the terminals. Not too much, as the majority is pushed out when you tighten the clamps.

The grease protects the connectors and terminals from corrosion and prevents them from bonding together.

6. Attach the cables

The cables should be attached to the corresponding terminals on the battery. Red cable to red terminal and black cable to black terminal. Attach the positive, red cable first, then do the same for the black, negative one. It is essential to connect the cables in this order from a safety perspective.

If you connect the negative terminal first, it is possible to short the battery to earth while connecting the positive accidentally. This can cause all kinds of problems.

7. Hand-tighten the nuts

Secure the nuts hand-tight first to prevent them from popping off. Now tighten the nuts to the recommended torque setting in the owner’s manual.

8. Final check

Make a final check of the battery mounting by trying to move it around in the tray. Tighten as required.

Also, double-check that the cables are securely attached. If not, they can cause a poor connection, and the car may not start.

Tips for Reconnecting a Battery

  • Make sure the battery is fully charged
  • If reconnecting an old battery, it may be worth having it checked first if it is a few years old
  • Make sure you have your car radio security code if it has one.
  • Don’t forget to reset your clock!


1. Is red or black positive?

On car batteries, red is always positive, and black is always negative (ground). Usually, you will also see + and – symbols.

2. Where does the positive battery cable connect adjusted?

The positive cable must always connect to the red (positive) terminal on your car battery. You may also see a + symbol on the positive terminal.

3. When reconnecting a battery, why connect the positive first?

You should always connect the positive cable first when reconnecting a car battery. The simple reason is safety. If you connect the negative terminal first, this makes anything metal on the car grounded, which by itself is fine.

However, while connecting the positive cable, you may accidentally touch the spanner on a piece of bodywork. This causes a short circuit between the positive and negative terminals of the battery.

In the next FAQ, we describe why this is a bad thing!

4. What happens if you connect positive to negative on a battery?

If you accidentally connect the positive terminal to the negative terminal on a car battery, several things happen.

You create a short circuit between the two terminals, and a current flows through the connection. The current can be very high, even welding the connecting item to the terminals. The current also causes the release of hydrogen gas inside the battery.

If the battery is vented, the released gas can be highly explosive. If the battery is not vented, then the pressure build-up could cause the battery casing to crack or blow apart.

5. What do I need to do after reconnecting the battery?

Once reconnected, the systems in your car should work as usual. There are no specific tasks required to complete. However, you’ll probably need to re-enter your car radio security information, and the digital clock may need to.

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