Is this a job for you?
Replacing a standard 12-volt battery is one of the more approachable home jobs, but it is not risk-free. A battery stores a lot of energy, can produce sparks and a short circuit, and contains acid. Some modern cars also add complications: the battery may sit under a seat or in the trunk, may need to be registered with the car’s computer after replacement, or may have a start-stop system that needs a specific type. If your owner’s manual says the work should be done by a dealer or technician, or if you feel unsure at any point, ask a mechanic. Many shops and parts stores will fit a battery for a small fee or none.
This article applies to a conventional 12-volt battery in a gasoline or diesel car. Hybrid and electric cars also have a 12-volt battery, but their high-voltage battery packs must never be touched by amateurs, and the instructions for those cars come from the manufacturer.
Before you start
First, make sure the battery is actually the problem. A car that will not start could have a weak battery, but a fault in the charging system or starter can look the same. A basic checklist of reasons for an engine not starting is a good place to begin, and if the battery warning light has been showing, see the article on the charging warning. If the car just needs a boost, a jump start may get you going, but a battery that keeps dying needs testing or replacing. Many parts stores will test one for free.
Gather these items:
- the replacement battery, with the correct size, terminal layout and power rating as listed in the manual
- a wrench or socket set that fits the terminal clamps and hold-down bracket
- safety glasses and gloves
- a terminal brush or cleaner, and clean rags
- optionally, a small memory-saver device if your car’s settings are lost on disconnection
Also note down any radio code or settings you may need, as some cars lose them when disconnected. Work outdoors or in a ventilated area, with the engine off, the key removed and the car in park or in gear with the parking brake set. Remove rings and metal jewelry, which can cause a short if they bridge the terminals.
The five steps
1. Locate and identify the battery. Open the hood and find the battery, or check the manual if it is elsewhere. Look at the two terminals: the positive is usually marked with a plus sign and often has a red cover, and the negative is marked with a minus sign. Take a photo of the layout and the way the cables run before touching anything.
2. Disconnect the negative terminal first. Loosen the clamp on the negative (minus) terminal and lift it off, keeping it clear of the battery. Then do the same with the positive (plus). Removing the negative first reduces the chance of a spark if your wrench touches metal on the car. Never let a tool touch both terminals or bridge from the positive terminal to the car body.
3. Remove the hold-down and lift out the old battery. Undo the bracket or clamp that secures the battery. Batteries are heavy, often in the range of tens of pounds, so lift with your legs and keep it upright to avoid spilling acid. If any fluid has leaked or there is crusty buildup, wear gloves and eye protection. Do not tip the battery.
4. Clean and fit the new battery. Clean the tray and the cable clamps with a terminal brush, removing any corrosion. Place the new battery in the tray in the same orientation as the old one, then refit the hold-down so it cannot move. A loose battery can be damaged by vibration.
5. Reconnect: positive first, then negative. Attach the positive clamp and tighten it, then the negative. This is the reverse of removal. The clamps should be snug but not overtightened, and should not twist when you try to move them. Some owners add a thin coating of terminal protectant to slow corrosion. Check that nothing is left loose in the engine bay, then start the car.
After the swap
- Check that the car starts and that no new warning lights appear. A few lights may flash briefly.
- Reset the clock, radio and any other settings that were lost. Some cars need to relearn idle or window settings after a short drive.
- If your vehicle has a battery management system, ask the manufacturer or a shop whether the new battery needs to be registered.
- Take the old battery to a store or recycling center. Batteries contain lead and acid and must never go in household trash. Many sellers accept the old one, and some charge a refundable deposit if you do not return it.
When to stop and ask for help
Stop and ask a mechanic if the terminals are badly corroded or stuck, if the battery case is cracked or swollen, if you smell rotten eggs or see leaking fluid, if you cannot tell which battery type your car needs, or if the new battery dies again soon. The problem may then be elsewhere in the electrical system. Battery lifespan varies with climate, driving pattern and age, so a test as a battery gets older is sensible, and cold starts are often when a weak one gives out.