How to Change a Car Battery

202211-how-to-change-a-car-battery illustration by Paul Dolan

While replacing a car battery used to be a fairly simple process, it’s often much more complicated in modern cars. After reading some of the potential problems noted below, you may decide it’s a chore best left up to a shop.

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But first, an important safety caution: Car batteries contain caustic acid that can also be found on the outside of the battery — particularly an old one — and you don’t want to get this acid on your skin, in your eyes or even on your clothes. Therefore, you should wear safety glasses and nonfabric gloves when working with a car battery.

Then Vs. Now

Back in the old days, batteries were typically found under the hood, out in the open and placed next to a fender. Now, however, they could be under the hood, under the floor of the trunk, under a seat, hidden beneath a cover, or have components placed above them and mounted inboard. This not only makes them harder to find, but also harder to remove. You may need to consult the owner’s manual (or search online) to find where your car’s battery is located.

Car batteries usually weigh at least 40 pounds, so having to bend over to lift one out that isn’t on the edge of the vehicle can be tough on your back. Some batteries don’t even have a handle on them, making it tougher still and often requiring a special plier-like tool to grab and lift them out. You also may have to remove some components, such as sensitive electronics, to access the battery.

Another potential problem is that modern cars rely on the battery to provide power for such things as the clock, radio presets, seat and mirror memory, and even some “learned” drivability functions when the car is off. Therefore, disconnecting the battery will lose all of these, unless some other means of powering them is provided — which we’ll get to in a bit. Furthermore, if the battery is disconnected, some radios have to be reset with a code in order to function (this to discourage theft), so make sure you either have the code or keep power connected to the radio.

If your battery is easy to get to and you don’t have or don’t care if you lose any memory functions, feel free to skip down to “Removing and Replacing the Battery.”

If, on the other hand, you want to save your memory functions, you can do so with another 12-volt battery and some special tools.

Supplemental Power for Memory

In some cases, you can simply connect another 12-volt battery to your car’s battery clamps with jumper cables (small ones would be fine) to provide power after you disconnect the main battery. But while that’s fairly easy to do with top-post batteries and their large clamps, it’s trickier with side-post batteries, which have much smaller clamps. Either way, you want to make sure you don’t accidentally short the battery out by having the positive clamp or cable come in contact with anything metal on the car. That’s because the battery’s negative cable is grounded to something metal (usually the engine or frame), making almost everything that’s metal essentially connected to the negative battery post, as metal conducts electricity.

Easier than trying to connect a supplemental battery to the battery clamps are connectors that transfer supplemental battery power to either a 12-volt power socket inside the car or to the car’s OBD II port, which is typically found under the driver’s side of the dashboard.(Virtually all passenger vehicles starting with the 1996 model year have an OBD II port.) Note that with the former, you have to make sure the socket is “live” even with the ignition turned off; many are disconnected from the car’s electric circuit when the ignition is off and thus won’t keep memory functions alive. Furthermore, some 12-volt sockets that are live at first may automatically disconnect power after 30 minutes or so, and if it takes longer than that to replace the battery, you’d lose the memory functions anyway. Because of this, a memory saver utilizing the OBD II port is the preferred method.

With either method, it’s a good idea to clamp the battery clips onto something plastic or wrap them in a rag to prevent the clips from touching each other after plugging in the memory saver but before connecting the clips to the battery. If they touch, you’ll short something out.

You may be able to find memory savers at your local auto-parts store, but you’ll likely find a better selection online — though you’d have to plan the purchase in advance. Try searching for “memory saver for car battery” or “12-volt memory saver.” Some have clamps that connect to the posts of another battery, while others have a 12-volt plug that can be plugged into another car’s power socket. Those that use the OBD II port typically run about $10 to $30.

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Removing and Replacing the Battery

Once you’ve found the battery and determined how you’re going to lift it out — either with the battery’s built-in handle or a special clamp or strap — you’ll likely need to remove a retainer that holds the battery in place. This might be in the form of a bar that goes over the top of the battery or a small clamp that goes over a protruding lip at the battery’s base. The latter will likely require a ratchet wrench, appropriate-sized socket and long extension bar.

Next, remove the negative cable first. This prevents accidentally grounding the wrench against anything metal when removing the positive cable, though you still have to make sure the wrench doesn’t touch the negative post of the battery when it’s also touching the positive cable’s clamp bolt.

After removing the battery, you may want to place it inside something like a plastic grocery bag or at least drape the bag over the top of the battery to prevent the battery from touching your skin or clothes. (Any acid film on the outside of the battery that gets on your clothes can eat through the fabric over time.) Note that you’ll still have to pick the battery up by either its built-in carrying strap or with the clamp or strap you used to take it out, as the bag’s handles shouldn’t be relied upon to hold the battery’s weight.

Make sure the battery clamps are free of corrosion, which typically looks like white, flaky foam; be careful not to get any on you. Special terminal brushes, a wire brush or even a flat-blade screwdriver can be used to scrape off corrosion.

To install the new battery, replace the positive cable first, then the negative cable and reinstall the hold-down clamp. Disconnect the memory saver if you used one.

In most cases, the place from which you bought the new battery will add a “core charge” for the old battery — typically $7 to $12 — to encourage you to return it for recycling, which you most certainly should do rather than just throwing it in the trash.

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