Car Battery – Before You Disconnect It – What Can Go Wrong

Car Battery Disconnect Memory Kit
Car Battery Disconnect Memory Kit

Most people do not realize that disconnecting a car battery for any reason may cause problems.

So, Before you disconnect a car battery, there are things you need to know.

Battery voltage MUST be maintained to the PCM and other modules while the battery is disconnected.

Regardless of the year, make or model of vehicle NEVER disconnect the car battery while the engine is running or the ignition key is on.

Disconnecting the car battery can create a high voltage spike in the electrical system that may damage electronic modules.

So, Loss of memory is where the problems start.

On any vehicle that has computerized engine controls, the loss of voltage to the Powertrain Control Module (PCM) causes it to forget its adaptive memory settings.

Adaptive memory contains the adjustments the PCM has learned over time for the fuel mixture, transmission shift points and other control functions.

The Keep Alive memory also includes all the results for the diagnostic self-tests the PCM runs on itself.

Car Battery Disconnect
Car Battery Disconnect

Disconnecting  Your Car Battery On A Computer-Equipped Vehicle May Cause A Whole List Of Issues:

  1. Starting problems
  2. Driveability issues
  3. Loss of air conditioning
  4. Power accessory functions (door windows, seats, sunroof)
  5. False warning lights
  6. Damage to electronic modules
Disconnecting the battery to reset the PCM on late model vehicles can do more than erase the Keep Alive memory. It can also erase vital learned information that is absolutely necessary for other modules to function normally.

Learn What Can Go Wrong Before You Disconnect Your Car Battery

What Will Happen When The Battery Is Disconnected Depends On The Year, Make And Model Of Your Vehicle.

Battery Testing
Battery Testing
  • This may cause the engine to run poorly because the air/fuel mixture is too rich or too lean for a period of time until the PCM can relearn the fuel trim adjustments.
  • The transmission may not feel the same until the PCM or transmission control module relearns the shift adjustments.
  • The affected module may prevent the ABS or airbag systems from working.
  • It resets the Climate Control module. On some vehicles, That means no A/C until the module is programmed with the correct instructions.
  • It resets the Body Control Module (BCM). This can mess up the operation of power accessories such as power windows, memory seats, power sunroof, or electronic suspension settings.
  • It may reset or disable the anti-theft system. The engine may crank but not start because the anti-theft system thinks somebody is trying to steal the vehicle.
  • Loss of power window and/or power sunroof position settings.
  • Loss of steering angle sensor settings. The steering angle sensor will have to undergo a relearn procedure following battery disconnect.
  • Replacing the car battery on some vehicles requires entering the new battery info into the PCM with a scan tool (type, serial number and CCA rating). This is necessary because the vehicle’s charging system is programmed to gradually increase the charging rate as the battery ages.
  • It causes a loss of channel settings on an electronic radio and clock.

How To Safely Disconnect Your Car Battery

A Handy Tool Everyone Could Use Is A Memory Saver.

Battery Memory Saver
Battery Memory Saver

A memory saver is simply a connector with a small battery that has just enough voltage and current to keep all of the computer memory alive while the main battery is being changed. The connector plugs into the car’s cigarette lighter, or the OBDII connector.

The memory saver incorporates a diode on the positive side. A diode is essentially a one-way valve for electricity. It will stop the electricity from flowing backwards causing damage.

Always follow the instructions that come with your memory saver to avoid any of the above issues.


There are many reasons you may need to disconnect the car battery. One of the most common ones is the replacement of a car parts. So, Next time be aware of what could happen and be prepared !

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