Hooked Battery Up Backwards Now Car Won’t Start [5 Easy Fixes]

The main reasons behind hooked battery up backwards now car won’t start are a dead battery, blown fuse, blown fusible link, fried alternator, and damaged ECU.

Keep reading this article to find out more about these issues in detail and how you can fix them to start your car once again. Let’s get your cranking and running again!

Hooked Battery Up Backwards Now Car Won’t Start [5 Easy Fixes]

What happens if battery is charged with reverse polarity? You can mess up the connection by hooking the battery backward and putting the positive side on the negative side and vice versa.

Let’s look at the most common reasons why your car won’t start if you connect the battery backwards:

Note: You can also read why Car tries to start when i connect the battery.

1. Dead Battery

What happens if you put battery in backwards? Putting in the battery in the wrong way and starting your car can kill your battery. In most cases, the damage to your battery will be visible as the stuff inside the battery will spill and create a mess.


The first thing to do if your car won’t start because of a dead battery is to replace the battery. The reverse polarity batteries are marked with an R somewhere in the battery.

If you’ve connected a battery of reverse polarity that got damaged, connect a normal battery in place of it.

2. Blown Fuse

If changing the battery doesn’t solve the issue on its own, the next thing you should look at is the fuse. A blown fuse is the most common reason that keeps the car from starting if you’ve accidentally connected positive to negative battery to protect the other parts from severe damage.

The fuse that connects to the battery can get easily blown when the battery dies. Even if you try to jumpstart the car with jumper cables, your car won’t run if there’s a blown fuse. Most fuses that are connected to the battery are either 80 amps or 100 amps.


Pop the cover of the fuse in question and check if you have a blown fuse first. You must replace a blown fuse with a new one while making sure that it has the same amperage.

You can find the fuse of the same amperage but the shape of the fuses won’t be identical in shape and size. If you can’t find the fuse of the same size, get a new fuse that’s bigger than the old fuse so that you can cut it to match the shape of the old one.

You’d need access to some tools so that the new fuse can go where the old fuse used to be. Follow these steps to replace the fuse of a car:

  • Take the old fuse by unscrewing the two screws surrounding it.
  • Get the new fuse.
  • Compare the dimensions of the two fuses by placing one on top of the other.
  • Mark where you need to cut the new fuse with a tungsten pen.
  • Use scissors to cut the edges of the new fuse to match the old one.
  • Drill the hole in the new fuse.
  • Compare the two fuses again and see if they match each other.
  • Put the new fuse in the fuse box.
  • If it doesn’t sit because it’s too wide, adjust the width with a pair of pliers and put it back in.
  • Tighten the two screws around the fuse.

Now you’ve successfully replaced the fuse in the car. You can save yourself from all this trouble if you find the fuse of the right shape and size that’s in your car. But more often than not, you’d have to adjust the shape of the new fuse as it won’t match the previous one.

It’s also important to note that if the current generated was too high, you’ll have more blown fuses along with the main fuse.

If your engine cranks and runs after changing the main fuse, check what other parts aren’t working properly. Locate the fuses by looking at a wiring diagram and see what parts are affected by the battery. If the fuses of those parts are blown, replace them and your car will be fully back in action again.

Many older cars have fusible links to protect the car from damage. It’s a thin wire with thick insulation that’s designed to break when there’s too much current passing through the system.

The fusible links are thin wires that are placed between layers where there is a high amount of electricity. But if it goes beyond the limit, it’ll cut the power supply and the thick insulation is meant to protect the car from catching fire.


If you have a blown fusible link, you need to replace it. You can replace the fusible link by following these steps:

  • Disconnect the battery terminal on the negative side.
  • Take a pair of pliers and cut the fusible links on both sides.
  • Get the new fusible links and start connecting them to the inside of the wires to the adjacent wires.
  • Twist the wires well to make sure that they’re well connected.
  • Add a crimp so that the connection stays in place.
  • Heat the attached wire with a soldering iron to make the connection more permanent.
  • Take a heat shrink tube and cover the overlapping wire with it.
  • Set the heat shrink tube with a heat gun.

You’re all set and you’ve replaced the fusible links successfully. If you don’t have access to the necessary tools, you can take it to a mechanic and have this job done by him.

4. Fried Alternator

If the problem is solved by replacing blown fuses or fixing fusible links, you’re in luck as no important engine component has been damaged. But if the current generated is too big, it can fry the alternator even after the fuse is blown. If that happens, your car won’t start.


The alternator needs to be replaced if it has gone bad. You can do it yourself if you have some experience in working with engine parts as it’s an easy part to replace. Otherwise, take it to a mechanic and he’ll replace it for you.

5. Damaged ECU

The worst-case scenario of having connected wrong terminals car battery is a damaged ECU. It is rare for the flow of current to be so big that it damages the ECU but it can still happen.


If the ECU is damaged then you need to repair it or replace it. You need to have a mechanic take a look at it and tell you if it’s possible to repair it. If the damage from the electric current wasn’t too much, repairing it can do the trick.

But if it’s not possible to fix it, then you need to replace it. Unfortunately, it’s expensive to replace the ECU as the cost can range from $800 to $1500 depending on your vehicle. But you don’t have any other choice and you must replace a damaged ECU if you want to drive your car again.


What happens if you jumpstart a car with the cables backwards?

If you jumpstart the car by switching the positive with the negative terminal, you’ll see sparks instantly and your car won’t start anymore. You’d have to replace the fuse in most cases to start the car again.

Do car batteries explode if you connect them backwards?

You can cause significant damage to the electrical components if you connect the batteries backward but doing so will not explode the batteries.

Will a car start if the battery is backwards?

Hooking up the batteries in the wrong way can easily blow up the main fuse alongside other small fuses. You won’t be able to start your car if the fuse is blown.

What happens if you hook a starter up backwards?

It’s difficult to hook up a starter backward as you’ll have to make a short circuit to do that. But if you do that, many starters will change their fields to make the starter motor spin in the same direction.


Now you know the most common reasons behind the hooked battery up backwards now car won’t start issue. Inspect the causes and follow them step-by-step to fix the issue.

We hope this guide has helped you figure out why your car won’t start after you’ve connected the battery in the wrong way and how you can fix it. Drop a comment below if anything is unclear or if you have any questions about what happens if you hook up car battery backwards.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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