Red or Black First? How to Disconnect Car Battery Terminals Safely

Let’s cut to the chase: Which battery terminal do you disconnect first?

Disconnect the black, negative terminal first.

Then disconnect the red, positive terminal.

Also wrap the red terminal with a cloth or cover it with a plastic cup to make sure it doesn’t touch the battery or any metal in the engine on accident.

Knowing which battery cable to disconnect first can be the most puzzling part of removing a car battery. It’s also a safety issue. You don’t want to touch the wrong battery terminal. But which one is the wrong one? If you’re not disconnecting car batteries every day, it’s easy to forget which one to remove.

Interstate Batteries strongly recommends you visit your nearest repair shop or Interstate All Battery Center before attempting to disconnect your battery.

It’s also easy to confuse it with the instructions on how to connect jumper cables. Connecting jumper cables starts with the positive; disconnecting a car battery starts with the negative. Whether you’re changing a car battery or jump starting one, stay safe and protected. Make sure you put on safety glasses and take off any jewelry.

When you disconnect a car battery, always use a memory saver.

You may need to disconnect your car battery for plenty of reasons.

• You might need to replace your car battery. (Get it tested first.)
• You might want to put a car into long-term storage. (Use a battery maintainer instead.)
• You might be taking a vacation and want to save the battery from getting drained.
• You might want to use it to help jump a car if you don’t have room for another car to pull up beside it. (This jumper battery works better.)

Whatever the reason, you need a backup plan.

As in, a plan for backup power.

Disconnecting a car battery will erase vital data from your car’s computer system. There are power modules throughout the engine and car body that store data, like the adaptive fuel calculations, system diagnostics tracking, sound system settings and even the idle-air-control settings.

Some data isn’t critical. You can reprogram your favorite radio stations and Bluetooth connections. Other data, like the IAC settings or system diagnostics, ensures a smooth drive. If you disconnect a car battery without a backup plan, your car may have a few problems when you get it going again:

• Uneven engine turning when idling
• Rough transmission shifting at high speeds
• Restarted diagnostic checks leading to a check engine light
• Poor gas mileage
• Inaccurate fuel emissions
• An unpredictable problem that could take days to diagnose

You should always connect backup power to a car before you disconnect its battery.

You also buy memory savers that connect via the cigarette lighter or by clamping onto the battery terminals. Plenty of repair shops use a backup battery that plugs into the OBD II diagnostic port whenever they replace a battery. (Otherwise, their customers would keep getting new issues after a simple battery replacement.)

Save the car’s memory, and you’ll save yourself from a headache down the road.

Step-by-step: How to Disconnect a Car Battery

1. Safety step: Safety glasses and gloves on. Jewelry off. Car ignition off.
2. Connect your backup power.
3. Loosen the bolt on the black, negative clamp and lift it away from the battery. (You’ll usually need a 10-millimeter open-end wrench and some twisting to loosen it. Be sure to never let your wrench touch the other terminal when working on a battery.)
4. Loosen the bolt on the red, positive clamp. Wrap it in a rag and lift it away from the battery and any metal.
5. Loosen and remove any battery tie-downs, the long bolts securing a rod over the battery to keep it in place.
6. Lift the car battery up from the engine and away. Don’t tip it when pulling it out. If it’s a flooded battery instead of an AGM battery, you could spill the electrolyte. (Careful! Batteries can weigh 40 lbs. or more.)
7. Clean all corrosion from the battery tray, the clamps and the battery. (Baking soda and distilled water can do the job.)

Now, you disconnected the car battery. Don’t turn on or activate any of the vehicle’s systems. You may drain the backup power. It’s best to leave the car alone until you’re ready to install a battery.

Reconnecting a car battery follows similar steps, except you’ll connect the red, positive clamp first.

How to Tell Positive and Negative on Car Battery Terminals

Red is usually positive, but always refer to the owner’s manual for your car to be sure.

The positive terminal is hot. Electricity comes from the positive post, and that’s why it can be dangerous to handle. Danger, red, positive — it’s easier to remember if you think of red as a warning. Be cautious whenever you handle the red terminal: Electricity comes out of the red, positive side of the battery.

However, the newer the car, the more likely it is that both cables are black.

In that case, here are a few more differences between the positive and negative terminals.

• Look for markings or symbols on the battery or the clamp. The plastic may have a plus or minus sign to mark that side’s polarity. If it’s hard to tell, you may need to get out a shop rag to wipe away grime to read the battery.
• The positive clamp and cable are bigger than the negative ones. The actual clamp on the post might be thicker, the cable might be thicker or include more components on its cable. The thicker cable lets more power out of the battery to the starter.
• The positive side usually has a plastic or rubber cover. This prevents short circuits because 12 volts of electricity are coming out of the car battery, and a spark or a short is possible, if unlikely.
• The positive side connects to an engine part. The negative or ground terminal connects to the vehicle’s frame. Unless you’re under the car, this isn’t exactly helpful, but it’s a difference between the positive and negative terminals.

Ask a Professional to Help You With Your Car Battery

Car batteries do far more than start engines.

Cars need a lot of electricity to run. Sophisticated electronics, high-tech sensors and machine-precise timing to optimize fuel output — the power from the car’s electrical system makes a smooth driving experience possible.

A car battery preserves that system when the engine is off.

So why are you disconnecting your car battery?

Do you have the right tools to protect you and your car while you’re working on it?

Again, Interstate Batteries strongly recommends you visit your nearest repair shop or Interstate All Battery Center before attempting to disconnect your battery.

Whatever you’re trying to do, there may be a safer, better, more efficient solution. There are experts to help you find the right solution for what you’re trying to do.