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

Car Battery - Before You Disconnect It - Know What Can Go Wrong
Car Battery - Before You Disconnect It - Know What Can Go Wrong

Disconnecting a car battery for any reason; may cause unexpected things to happen.

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.

NEVER: disconnect the car battery; while the engine is running or the ignition key is turned on.

Automotive Car Battery With Negative Cable Removed
Automotive Car Battery With Negative Cable Removed

Disconnecting the car battery can create a; high voltage spike in the electrical system. Consequently, damaging electronic modules.

So, Loss Of Memory Is Where Things Start To Go Wrong

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. Consequently, adaptive memory contains the adjustments; the (PCM) has learned over a period of time. For example, 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.

Disconnecting A Car Battery, May Cause A Whole List Of Issues:

  • Starting problems
  • Drivability issues
  • Loss of air conditioning
  • Power accessory functions (door windows, seats, sunroof)
  • False warning lights
  • Damage to electronic modules
Removing Cable From Car Battery
Removing Cable From Car Battery

So, disconnecting the car battery to reset the (PCM) on late model vehicles; can do more than just 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

It all depends On; The Year, Make And Model Of Your Vehicle.

This may cause the engine to run poorly. Because, the air/fuel mixture is too rich or too lean; 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 also, resets the climate control module. So, on some vehicles, that means the A/C will not work. Until the module is programmed, with the correct instructions. It also, 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 the battery disconnect. Replacing the car battery on some vehicles requires entering the new; battery info into the (PCM) with a scan tool. This is necessary because; the vehicle’s charging system is programmed to; gradually increase the charging rate, as the battery ages. Finally, it causes a loss of channel settings on an, electronic radio and clock.

Learn How To Safely, Disconnect Your Car Battery

A Handy Tool Everyone Could Use Is A Memory Saver.

Car Battery Disconnect Memory Kit
Car Battery Disconnect Memory Kit

So, a memory saver is simply a connector, with a small battery. It has just enough voltage and current to keep all of the computer memory alive; while the cars main battery is disconnected. In short, the connector plugs into the car’s cigarette lighter or the (OBD-II) connector.

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

The steps to use a memory saver are easy:

  • Always turn off the car; before disconnecting the battery.
  • Then, plug in the memory saver with its small battery attached into the; cigarette lighter or the OBD-II connector.
  • The OBD-II connector is located; under the driver’s side of the dashboard, on the steering column.
  • Next, remove the old battery; making sure that the positive battery wire is not, touching the car’s body.
  • Then, install the new battery and attach the connectors.
  • Finally, unplug the memory saver from the car. That’s all.


It takes approximately 10 cold starts, with a running time of approximately 15-30 minutes; for the PCM/ECM to completely relearn, LTFT values. During this time, the engine will run progressively better; with each cold start.

Thank You !