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Disconnecting or replacing car batteries may look like an easy task but in reality it's not. You may not know but the loss of power accessory functions like door windows or loss of air conditioning is also related to car battery disconnect or replacement.

We tend to underestimate the danger of battery disconnect issues. There are not enough warning signs for customers in manuals or from car dealers about the possible dangers of replacing or disconnecting a battery. We only realize it after the task is performed and issues arise. 

The risks always depend on the make, model, type and year of the vehicle.; but here are the most common issues that car owners will face when replacing or disconnecting car batteries. 

1. Loss of Adaptive Memory

One of the biggest issues when disconnecting the car battery that has computerized engine controls is loss of memory. The car will lose the voltage to the PCM (Powertrain Control Module), causing it to forget the adaptive memory setting in the KAM (Keep Alive Memory) chip. 

When this happens, the engine will run poorly because the air and fuel mixture is too rich or too lean for a period of time until the PCM can relearn to adjust the fuel trim. This may take around several days and 50 to 100 miles of driving until engine operation is back to normal. 

2. Loss of Steering Angle Sensor Settings 

Another issue that we might face when disconnecting or replacing the car battery is the loss of steering angle sensor settings. This is a potential risk that could affect our driving experience. The only workaround is to let the steering angle sensor undergo a relearn procedure. 

3. Loss of Power Window and Power Sunroof Position Settings

If you lose the power to the car's electrical system during battery replacement, the power windows or sunroof may not work properly. To solve this issue, you need to reset the position values using the vehicle manufacturer's relearn procedure. 

4. Affect Transmission Shifts and Feels

The way the transmission shifts and feels may also be affected by erasing the PCM’s (Powertrain Control Module) adaptive memory. This usually happens when you disconnect the car battery. 

The transmission may not feel the same until the PCM or transmission control module relearns the shift adjustments. This may take 50 to 75 miles of driving for the system to learn again. 

5. Reset or Disable the Anti-Theft System

Replacing or disconnecting the car battery can reset a lot of modules and may reset or disable the anti-theft system. The engine may not work because the anti-theft system will think that somebody is trying to steal the vehicle. 

It may require a special reprogramming procedure to the anti-theft system using a factory scan tool to solve the problem. 

6. Reset the ABS and SIR

Sometimes disconnecting and replacing a car battery also resets the ABS (Anti Lock Brake System) and SIR (Supplemental Inflation Restraint) as well as airbag modules. When this happens, the affected module may prevent the ABS or airbag systems from working properly. 

These modules need a special relearn or reprogramming procedure after power has been lost. Otherwise, it will affect your safety and the car's working system. 

7. Reset the FMEM

A reset on the FMEM, also known as Failure Mode Effects Management is also possible when you disconnect or replace a car battery especially on late model Ford vehicles. 

This module provides adaptive fail-safe strategy which automatically fixes data or substitutes for missing sensor data. 

It will become a huge problem if the vehicle has a bad sensor or has been substituting data from the FMEM module for a missing input. 

8. Reset the Climate Control Module 

Disconnecting the car battery can also reset the Climate Control module. Most of the time, the module won't work until a special relearn procedure or reprogram is performed with a factory scan tool. 

This means no AC until the module gets reprogrammed with the right instruction. 

9. Reset the BCM

A reset on the Body Control Module (BCM) can also happen when disconnecting or replacing the car battery. The same with the Climate Control Module, this program will not function until a special relearn or reprogrammed session is done. 

Power accessories such as power windows, memory seats, power sunroof, or electronic suspension settings will be messed up. What's worse is that the BCM is referred to as the "gatekeeper" module on many 2003 and newer vehicles. There's a big risk of ruining the whole module system in your vehicle if it doesn't function properly. 

10. Entering the New Battery Info Into the PCM with a Scan Tool 

Replacing the battery on some vehicles requires a scan tool. You also need to enter the new battery info into the PCM. The type of battery, battery serial number and CCA (Cold Cranking Amps) rating. 

This is important because the vehicle's charging system is programmed to gradually raise the charging rate as the battery ages. If the charging rate is not reset back to that, the battery may overcharge and fail.

11. Loss of Channel Settings on An Electronic Radio and Clock

Lastly, replacing or disconnecting the car battery may cause a loss of channel settings on an electronic clock and radio. 

This can be fixed though by manually resetting the time and radio channels. 

Conclusion

Irrespective of the year, make or model of vehicle, NEVER disconnect the battery while the engine is running or the ignition key is on. If you do, you are likely creating a high voltage spike in the electrical system that may damage electronic modules.

That's why it's very important to know the problems your car might get into when you disconnect and reconnect the battery. This will help you gauge whether you are ready to go along with the problem or if you need to seek professional assistance.