How to Reconnect a Car Battery


If you’ve had your battery connected to a maintainer over the winter or had to remove it during other repairs, reinstalling it is a pretty simple process. All you need are some simple hand tools and a few other supplies you probably have around the house. It’s important to not only connect the battery, but to make sure the connections are clean and the battery is secure.

Part 1
Cleaning and Inspecting the Battery and Connections

Cleaning and Inspecting the Battery and Connections on How to Reconnect a Car Battery

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Protecting your eyes is especially important when reinstalling your battery. If you accidentally mix up the positive and negative cables, for instance, the battery could overheat and “burst,” spraying the water or chemicals stored in the battery.[1] Latex gloves will protect your hands if your battery leaks or bursts, but mechanic gloves will also offer protection from scratches and pinches. Goggles offer the most protection, but traditional safety glasses are also fine.

Cleaning and Inspecting the Battery and Connections on How to Reconnect a Car Battery

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It’s important that none of the vehicle’s electronics are set to “on” when you reconnect the power from the battery, otherwise, the power surge could cause damage. If the keys are in the vehicle, turn the ignition to “off” and remove them for safety.[2] Just having the key inserted in the ignition will turn on the door chime in many vehicles.

Cleaning and Inspecting the Battery and Connections on How to Reconnect a Car Battery

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Use a wire brush to remove any corrosion or buildup on the battery terminals before you put it back in the car. Add 1 tablespoon (15 mL) of baking soda to 2 cups (470 mL) of water to create a cleaning solution to help you remove stuck on corrosion.[3] The baking soda and water mixture is abrasive enough to scrape away rust or old oil. If you don’t have baking soda, you can use 100-grit sandpaper to get the same result. Use a damp rag to remove the baking soda mixture when you’re done.

Cleaning and Inspecting the Battery and Connections on How to Reconnect a Car Battery

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Use the same mixture and wire brush to clean the connections at the end of the cables in your car as well. There has to be a good metal on metal connection for current to flow from the battery and into the car.[4] The outside of the wire connectors are often painted. It’s fine to leave the paint intact, just ensure the inside of the connection is clean and clear of any debris. Use a damp rag to remove the baking soda mixture when you’re done.

Cleaning and Inspecting the Battery and Connections on How to Reconnect a Car Battery

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You can find a variety of electrically conductive corrosion prevention creams or sprays at your local auto parts store. Choose one and apply it to the positive (+) and negative (-) terminals of the battery, as well as the inside of the wire connectors coming from the car.[5] These products are sometimes also called battery terminal protectors. Spray the wire connectors and terminals liberally, or apply a generous amount of cream or gel, then wipe off any excess.

Cleaning and Inspecting the Battery and Connections on How to Reconnect a Car Battery

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There are two cables that connect to your battery. The positive cable’s end connector is often either painted red or has a red plastic clip attached to it. It runs to the engine’s alternator. The second cable is a ground cable, which connects the negative terminal of the battery to the body of the car. Look both cables over closely for signs of damage or cracking.[6] If the covering on the cable is cracked, or the cable itself shows signs of wear or damage. It should be replaced.

Part 2
Securing the Battery in Place

Securing the Battery in Place on How to Reconnect a Car Battery

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To replace the positive cable, use the appropriately sized socket with a socket wrench to remove the nut that secures it to the top of the alternator. Slide the loop at the end of the cable off of the nut, then slide the replacement cable loop in its place and secure it with the nut. The negative cable can be replaced in the same fashion, but by removing the bolt holding the cable on to the body of the car, then swapping the new cable in its place and screwing the bolt back in.[7] You can purchase both cables at your local auto parts store. If neither cable is damaged, you can skip this step.

Securing the Battery in Place on How to Reconnect a Car Battery

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In most vehicles, the battery’s tray is located toward the front of the engine bay and off to one side (behind one of the headlights). However, many newer vehicles place the battery in the trunk, or even in the cabin of the vehicle, for better weight distribution.[8] If you have trouble locating where to install your battery, refer to the vehicle’s owner’s manual or manufacturer’s website for guidance.

Securing the Battery in Place on How to Reconnect a Car Battery

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Most car batteries have their terminals close to one edge of the battery. The positive cable will be coming from one side of the engine bay, and the negative cable will come from the other. Orient the battery so that its positive (+) terminal is on the same side as the positive cable, and the negative (-) is on the same side as the negative cable.[9] The positive terminal on the battery will be labeled with a (+) sign. The negative terminal will be labeled with a (-) sign.

Securing the Battery in Place on How to Reconnect a Car Battery

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Be careful, batteries often weigh .20 pounds (0.091 kg) or more. Grip the battery from the sides as you lower it into its tray, being careful not to pinch fingers as you do.[10] Make sure there’s nothing in the tray before you lower the battery into it. If need be, tuck the battery cables off to the side as you place the battery in the car to prevent them from getting in the way.

Securing the Battery in Place on How to Reconnect a Car Battery

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Some car batteries use a metal hook you can loosen and tighten by hand, while others may use a metal or rubber strap. Look for the strap or hook in your vehicle and then use it to secure the battery in place.[11] Straps should be pulled over the battery and then secured on the other side, often using a bolt you can tighten with a ratchet. Hooks can sometimes be rotated by hand or with pliers, but some vehicles have a bolt you tighten to move the hook. If you are unsure how to secure your battery, refer to a vehicle specific repair manual or the manufacturer’s website for further guidance.

Part 3
Connecting the Battery Terminals

Connecting the Battery Terminals on How to Reconnect a Car Battery

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The positive cable will come from the alternator. Press the connector onto the terminal with your hand until it lies flat against the battery.[12] If that connector won’t slip over the terminal post, use a wrench or socket and ratchet to loosen the bolt on the connector, then try again.

Connecting the Battery Terminals on How to Reconnect a Car Battery

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Once the positive cable is on the terminal, it needs to be tightened to ensure it can’t vibrate off while you’re driving. Use a wrench or socket to turn the bolt on the connector clockwise until it’s snug.[13] On many batteries, it’s easier to use an open-ended wrench than a socket, but either will work. Wiggle the wire on the terminal a bit with your hand to make sure it’s secure. If it moves at all, tighten the connector further.

Connecting the Battery Terminals on How to Reconnect a Car Battery

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The negative cable should be installed just like the positive one was. Press it onto the terminal post with your hand, then tighten it with a wrench.[14] If either cable can’t reach the appropriate terminal, it means the battery wasn’t oriented properly when you lowered it in. Wiggle the connection on the negative terminal with your hand and tighten it some more if there’s any movement.

Connecting the Battery Terminals on How to Reconnect a Car Battery

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With the battery connected, the dome light should come on when you open the driver’s side door. Insert the key into the ignition and turn it to start the vehicle and ensure the battery is properly connected.[15] If the vehicle does not start, check the connections at the battery as well as at the alternator and where the negative cable touches the body. If they are all good, try again. If it still doesn't start, the battery itself may be dead. If so, try jump starting it.