What Happens If You Reverse Jumper Cables & Other Mistakes When Jumpstarting Your Car

Anyone who’s been driving long enough has experienced the inconvenience of a dead battery. You go to start your car, but nothing happens, so you head to the trunk to grab your jumper cables.

But wait—do you really know how to use those things? If not, you might end up making a mistake that could cause personal injury, damage to your car—or both.

What Can Happen When You Hook Up Jumper Cables Backwards?

Hooking the jumper cables up backwards can create disastrous results⁠. Possible outcomes include irreparable damage to the battery, fried electronics, and even serious personal injury. Exactly what will happen when jumper cables are incorrectly connected depends on several factors, including the vehicle, the battery, the quality of your cables, and sheer luck.

Here’s what can happen if you accidentally hook the cables up wrong when jumping a car:

Blown fuses / fusible links

Fuses (and/or fusible links) act as circuit protection devices between the battery and the vehicle’s electrical system. Connecting the jumper cables backwards will often result in one or more blown fuses. The affected circuit will not function correctly until the blown fuse is replaced.

Damaged battery

Reversing the battery cables creates a surge of electrical current that can easily damage the battery.

Damaged jumper cables

When you reverse the polarity of the jumper cables, you create a drastic increase in the amount of electrical current that runs through them. As a result, the cables may melt or even catch fire.

Fried electrical components and wiring

In a perfect world, your car’s fuses would protect all of the sensitive electronics from the surge in electrical energy created by reversed battery cables. But it’s not a perfect world, and all too often, the alternator and other electrical components (i.e., wiring, modules, etc.) can become damaged when the cables get swapped.

Personal injury

Batteries vent flammable hydrogen gas. Reversing the battery cables will create sparks that could ignite the gas, causing an explosion.

6 Common Mistakes When Using Jumper Cables

Although jumpstarting a vehicle may seem like a simple task, there are several ways it can go wrong. Here are the most common:

Hooking the jumper cables up backwards

As was covered above, reversing the polarity of the battery cables can cause a range of problems, from blown fuses to fried electronics.

Touching the jumper cable ends together

What happens if you touch jumper cables together? When the jumper cables are hooked up to a battery, touching the cable ends together can create sparks. In some cases, the sparks can ignite the hydrogen gas escaping from the battery, causing an explosion.

Connecting the negative jumper cable to the negative terminal of the dead battery

When jumpstarting a vehicle, the final connection should be to a good ground on the vehicle with the dead battery. Connecting the jumper cable to the negative terminal of the dead battery can create sparks and potentially cause an explosion.

Attempting to jumpstart a frozen battery

You should never attempt to jumpstart a frozen battery, as doing so can cause it to explode. Remove the dead battery, bring it inside, and allow it to thaw before attempting to jumpstart the vehicle.

Allowing the vehicles to touch

Before you hook up the jumper cables, make sure the two vehicles aren’t touching one another. Excessive current flow passed between the two vehicles can damage ground straps.

Not wearing personal protective equipment

When jumpstarting a vehicle, you should wear protective gloves and goggles. Neglecting to do so can result in severe personal injury.

Can You Get Electrocuted by a Car Battery?

Under normal conditions, a regular, 12-volt car battery does not have enough voltage to kill you (the high-voltage batteries found in hybrid and electric vehicles are a different story). Because your body is not a good conductor of electricity, a relatively low-voltage car battery can’t push enough current through to kill you. Still, there have been accounts of people being shocked by car batteries, so you should always proceed with caution.

The primary threat a car battery poses is the risk of explosion. Sparks created by hooking up the cables wrong or touching the cable ends together can ignite the hydrogen gas from the battery, causing it to explode.

Car batteries also contain acid that can get on your skin, which, as you can imagine, is particularly problematic if the battery explodes.

Furthermore, the battery supplies electricity to a number of underhood components that can give you a nasty shock. For example, tens of thousands of volts are running through secondary ignition system components, such as the spark plug wires.

How to Connect Jumper Cables Correctly

A car battery has a positive and negative terminal. Usually, the terminals are marked with a negative (-) and a positive (+) sign. You must connect the jumper cables to the battery properly to avoid major problems.

Here’s how jumper cables should be connected:

1. One end of the red (positive) jumper cable connects to the positive terminal of the dead battery.
2. The other end of the red cable connects to the positive terminal of the good battery.
3. One end of the black (negative) jumper cable connects to the negative battery terminal of the good battery.
4. The other end of the black jumper cable connects to a good ground on the vehicle that won’t start.

How Do Jumper Cables Work?

Jumper cables make a parallel connection between two batteries. Made of electricity conducting copper material, jumper cables transfer electric power from a charged battery on a donor car to a dead battery, slowly charging it. The pace and amount of electricity that is transferred depends on the width or thickness of the copper wire, also known as the gauge.

The lower the number of the gauge, the thicker the copper wire. Lower gauge rating jumper cables can transfer a greater amount of electricity without generating a lot of heat.

Jumper Cables: Positive and Negative

Typically, jumper cables (also known as jump leads), come in a set of two wires with a copper clamp at each end. The clamps are assigned to either the negative pole or the positive pole on the battery. This is indicated by a “-” and “+”, respectively.

You must not interchange the assignments of the clamps when attaching them to the batteries. Also, you CANNOT let the clamps touch each other once they are attached to the donor battery. Any contact between the clamps will produce a spark which will burn your hands, or worse⁠—ignite flammable liquids in your engine.

Jump starting your car with jumper cables is pretty straight forward. Simply connect like poles together. The positive jumper cable should attach both batteries’ positive poles while the negative jumper cable attaches the two negative battery poles.

Sometimes, you can attach the negative cable to the negative pole of the donor battery, and instead of attaching the other end to the negative pole of the discharged battery, clamp it to a metal piece of the opposite car’s frame to create a ground instead.

In a few minutes, the car with the dead battery should be able to start right up.