How to Disconnect a Car Battery

The electrical systems in your car are a tangled web of miles of wiring, sensors, switches, motors, and solenoids. At the heart of it all is a device that stores chemical energy that converts to electricity on demand, providing power in your car. The battery sends high-amperage current to the starter to crank the engine over and it provides backup electricity for 12-volt systems before the engine is running.

There may be times during your vehicle ownership that the battery needs to be disconnected. Sounds simple enough, but if it’s done wrong, it can cause serious damage to your vehicle or even start a fire. If you’re at all unsure about disconnecting your car battery, this brief guide will walk you through it.

Why Would You Need to Disconnect a Battery?

Contrary to popular belief, the battery does not supply your car’s electronics with their main source of energy while the engine is running – that’s the alternator’s job. But when there’s a problem with power delivery, it’s nearly always the battery that needs to be checked first. It has a lifespan of around three to five years before it can’t hold up to the demand required of it, and it needs to be replaced.

To replace the battery requires taking the old one out. Naturally, that means disconnecting it first. But that’s not the only reason you might need to do so.

When the engine is off, most people would think that the electrical draws stop too. Not completely true. The electrical demand is substantially less, but take a look around in your car. See the clock on the radio and the dome lights on? They represent a tiny draw on the system. As well, the body control module, powertrain control module, and other computers inside the car need to maintain memory, so a teeny electrical draw keeps them powered up too, even if they’re “asleep”.

In other cases, the battery needs to be disconnected before repairs commence. For example, an extremely off-chance scenario can occur when repairing the airbag system that an airbag could deploy if the module still has power. As well, fixing something in the engine compartment might seem safe with the engine off, but some cars can still trigger the cooling fan when the key isn’t even in the ignition. It poses a risk to health working on some things with the battery connected.

Whether you’re fixing a problem, storing your car, or replacing the battery, here are the steps to take to disconnect it.

Disconnecting the Battery for Replacement

Imagine that you’ve needed to jump-start your car three times this week. Despite charging the battery between, it doesn’t have the gusto to start on its own. You need to replace the battery. It’s not a difficult process but it needs to be completed accurately or sparks could fly.

Disconnecting the Car Battery for Replacement

1. Locate the Battery

It might seem ridiculous at first to mention finding the battery, but not all car batteries are under the hood. In some vehicles like BMW models and some Chevy’s, the battery is located in the trunk, tucked away under a panel. In even rarer circumstances, the battery could be under the rear seat bottom. Check these places if you don’t see it under the hood.

2. Remove the Negative Battery Cable

Always disconnect the negative battery cable first. The negative post is indicated with a (-) near the terminal. Use a wrench to loosen the clamp, typically a 10mm size. Other options can be 8mm, 11mm, 12mm, or 13mm. When it’s loose enough, wiggle the connection off the battery and secure it away from the post.

3. Remove the Positive Battery Cable

Next, remove the positive cable in the same way as the negative side. There’s not much more to it.

4. Unfasten the Hold-Down

Then to remove your battery, take the battery hold-down off. It cold be a block fastened at the battery’s base that needs to be removed or a bar across the top of the battery case. You can’t get the battery out without taking this off.

Disconnecting a Car Battery for Storage

If you’re storing your car long-term like over the winter, you’ll do the exact same process as if you’re replacing it. Store your battery somewhere warm, and put it on a block or shelf off the ground. 

But imagine that you’re taking a month-long trip to Europe and your car is staying in long-term parking. Since you don’t want a dead battery upon your arrival back home, you can just disconnect it. Instead of taking both cables off the battery, all you need to do is remove the negative cable. Tuck the end off to the side where it won’t contact the post, and consider securing it there with a bungee cord or string. 

If you’re performing a repair on your car that requires the battery be disconnected, it’s the same procedure too. When you’re all done, you may have to program the radio presets, the memory seats, and the like, but your battery will be safe in the meantime.

Risks of Doing It Wrong

Why is it a big deal to disconnect the battery the right way? Simply put, the consequences can be extremely expensive or hazardous to your health if you don’t.

When you’re removing the battery cables, it’s imperative that you never touch the positive and negative battery posts with a wrench, ratchet, or other conductive object at the same time. While it’s only 12 volts, it can send a shower of sparks out or cause a fire. If the battery cables are still connected when that happens, it can blow fuses or relays and melt wiring and fusible links, even shorting computer modules out. There’s no telling the time, aggravation, and cost to fix it.

And again, working on the certain parts of your car like the airbags or seatbelt pretensioners can trigger violent reactions that can all be avoided if you simply disconnect the battery.

Need the right wrench to safely disconnect your battery cables, or are you shopping for a replacement car battery? Maybe you need help with the repair process for a different problem. Gert Trustworthy Advice and quality parts at AutoZone so you can get the job done right. Don’t forget to visit your local AutoZone if you need your battery tested or to replace it completely.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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