What Happens If You Hook Up A Car Battery Backwards [2024

Connecting a car battery backward is a mistake I once made, and it can cause severe damage to the vehicle’s electrical system. This error, known as reversed polarity, happens when the positive and negative terminals are incorrectly connected.

When a battery is connected with reverse polarity, it can wreak all sorts of havoc with the car’s electrical system—you may be looking at a blown PCM (computer), BCM (another computer), ABS system (another computer), fried audio system, and other problems.

In this article, we will explore in detail what happens when you hook up a car battery backwards, the potential damages it can cause, and how to prevent and address such issues effectively.

What Happens If You Connect A Car Battery The Wrong Way?

What Happens If You Hook Up A 12 Volt Battery Backwards? If you connect a car battery the wrong way, reversing the polarity, it can cause a surge of electrical current in the opposite direction than intended. This surge can potentially damage sensitive electronic components in the vehicle’s electrical system. Common consequences include blown fuses, damage to the alternator, ECM (Engine Control Module), and other electronic control units, as well as potential fire hazards.

Jumper Cables

Will A Car Battery Work If Installed Backwards?

No, a car battery will not work if installed backwards. Connecting the battery with reversed polarity disrupts the flow of electrical current and can lead to various issues, as mentioned above.

In addition to potential damage to the vehicle’s electrical system, the battery itself may become damaged and unable to hold a charge or deliver power to the vehicle’s systems.

Also Read: Can A Car Battery Recharge Itself Overnight

What Happens When You Reverse Polarity On A Car Battery?

Accidentally Connected Positive To Negative Battery? Reversing polarity on a car battery causes the electrical current to flow in the opposite direction than intended. This can result in damage to sensitive electronic components, including the alternator, ECM, fuses, and wiring. The severity of the damage depends on factors such as the duration of the reversed connection and the specific components affected.

 Damage To The Alternator

What Happens When You Put Positive To Negative On A Car Battery?

When you put the positive terminal of a car battery to the negative terminal, and vice versa, it reverses the polarity of the electrical system. This can lead to a surge of electrical current that flows in the wrong direction, potentially causing damage to various components.

As a result, the vehicle may experience electrical malfunctions, blown fuses, and other issues that can affect its operation and safety.

What Happens When You Connect Wrong Battery Terminals?

  • Similar to hooking up a battery backwards, this scenario involves connecting the positive terminal to the negative terminal, and vice versa.
  • The consequences include reversed polarity and the potential for electrical damage to various components.
  • Blown fuses, malfunctioning electronic devices, and potential fire hazards are common outcomes.
Damage To The Battery

Also Read: What Happens When You Throw Car Batteries In The Ocean

Car battery installed incorrectly:

  • Incorrect installation of a car battery can occur if the positive and negative terminals are swapped during installation.
  • This mistake results in reversed polarity and subsequent electrical issues.
  • Damage may include blown fuses, malfunctioning electronic components, and potential damage to the battery itself.

Damage Caused by Connecting Battery Backward:

Connecting the car battery backward, as I learned through personal experience, can lead to severe damage.

  1. Alternator: The alternator, responsible for recharging the battery and providing power to the vehicle’s electrical system, can be damaged by a surge of electricity caused by connecting the battery backward.
  2. Electronic Control Module (ECM): The ECM, also known as the Engine Control Unit, controls the engine and other systems in the vehicle. An electrical surge from reverse polarity can cause damage to the ECM.
  3. Fuses: Fuses protect the electrical system by breaking the circuit when the current becomes too high. Connecting the battery backward can blow fuses, disrupting the vehicle’s electrical system.
  4. Sensors: Various sensors in the vehicle monitor different systems and report back to the ECM. Damage to these sensors from reverse polarity can disrupt their functioning.
  5. Wiring: The wiring in the vehicle’s electrical system can also be damaged by the surge of electricity caused by connecting the battery backward.
 Starter Motor

Solutions To Fix The Hook Up A Car Battery Backwards:

After realizing my error in hooking up the car battery backward, I took immediate action to rectify the situation.

Check High Ampere Fuses:

Most vehicles have a large fuse designed to blow and prevent damage to sensitive electronic components like the ECM. Look for fuses in the range of 40 to 80 amperes, as they may not be listed in the owner’s manual.

Blown Fuse

Also Read: Car Battery Smoking When Jumping

Inspect Smaller Fuses:

If the car still doesn’t start after replacing the blown high ampere fuse, check all smaller fuses. Ensure that fuses for the ignition system, ECM, fuel pump, and immobilizer are still intact.

Check for Spark and Fuel:

If the engine turns over but won’t start, check for spark at the spark plugs and verify fuel pressure. Additionally, ensure that direct injectors are receiving power.

Fried Alternator

Check Starter and Immobilizer:

If the car won’t turn over or crank, inspect the immobilizer system, ensure the ECM is receiving power, and verify if the starter is getting power.

Run System Scan:

If the vehicle powers up but doesn’t start, consider running a full system scan with a scanner to diagnose any potential issues. Keep in mind that a damaged ECM may not communicate with the scanner.

Damaged ECU

How To Put Car Battery In?

  • Take the wires off the battery connections.
  • Remove the screws or other attachments that hold the battery in place. Get rid of the battery.
  • Check the box where the old battery was sitting. Place the new battery on the tray.
  • Put the new battery in place with the screws or fixings back on it.
  • Put your battery cables back on like you took them off.
  • Get rid of your old battery the right way.

Also Read: Car Battery Bubbling When Charging

Hooked Battery Up Backwards Now Car Won’t Start: What To Do?

Hooking up a car battery backwards can damage electrical components and cause the car not to start. To resolve the issue, disconnect the battery, check for blown fuses, inspect for damage, reconnect the battery correctly, and seek professional assistance if needed.

1. Do You Hook Up Red Or Black First?

Start by connecting the red jumper cables first. Attach one end to the positive terminal of the dead battery and the other end to the positive terminal of the working battery. Then, connect one end of the black cable to the negative terminal of the working battery.

2. Will the Car Start If the Battery is Connected Wrong?

No, if the battery is connected incorrectly, the car may not start. Connecting the battery wrong can cause electrical issues that prevent the car from starting.

3. What Happens If You Connect Battery Terminals Together?

If you connect battery terminals together, it creates a short circuit, which can lead to sparks, overheating, and potentially damage the battery or other electrical components.

4. Do Car Batteries Explode If You Connect Them Backward?

No, car batteries are unlikely to explode if connected backward. However, connecting them incorrectly can cause harm to the electrical system and other car components.

5. What Happens If You Connect Two Positive Battery Terminals Together?

Connecting two positive battery terminals creates a short circuit, causing electrical current to flow between them. This can lead to overheating and potential damage to the battery or other components.

6. What Happens If You Charge A Battery Backwards?

Jump-starting with the wires reversed can aggravate the situation if the battery is in bad condition or has not been used in a long time. A faulty battery may even explode, causing injury to anyone nearby.

7. Why Is My Battery Sparking When I Hook It Up?

When you connect a battery, it sparks because there is a difference in potential between the two touch points. If something in the car needs energy before the battery is attached, it will make the potential difference bigger, leading to a spark.

8. What Fuse Blows When Jumping A Car Backwards?

When jumping a car backwards, the safety fuse protecting vehicle electronics typically blows. If absent, electrical current may flow incorrectly through car systems like the ECU and transmission control unit.

9. What Happens If You Reverse Polarity When Jump Starting?

Reversing the polarity when jump-starting results in a drastic increase in electrical current through the jumper cables. This can cause them to melt or even catch fire.

10. What Happens If I Remove The Positive Terminal First?

Removing the positive terminal first can cause sparks, indicating a short circuit and potential damage to the battery. Disconnecting the ground cable first is recommended to prevent this.


In conclusion, the most common issues include blown fuses, damaged fusible links, and a fried alternator. Rarely, the ECU itself may sustain damage, which would need costly repair or replacement. To avoid any potential harm, it is crucial to constantly double-check the battery connections before installing a new battery.

If you accidentally connect the battery backward, it is essential to take immediate action and have the problem diagnosed and repaired by a professional mechanic.

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