Disconnecting or replacing a battery on a computer-equipped vehicle may cause starting, driveability and a number of other problems, including loss of air conditioning, power accessory functions (door windows, seats, sunroof), false warning lights, even damage to certain electronic modules! So if you are replacing or disconnecting a battery for any reason, proceed with caution.
One of the things I wanted to do with the article was list all of the vehicle applications that have known battery disconnect issues. Unfortunately, I have never found such a list. Most vehicle owners manuals don't offer any precautions about disconnecting or replacing the battery. There may be some cautions in the factory service literature, but it is often hard to find even if you know where to look for it.
On vehicles with known battery disconnect issues, there should be a big red warning label on or near the battery to warn people. But except for shock hazards warnings on high voltage hybrid batteries, no such warnings are posted anywhere. None of the battery manufacturers warn consumers or technicians about the possible risks of disconnecting or replacing a battery on their websites. Consequently, most people do not realize that disconnecting or replacing a battery may cause a problem until after they have done so.
What kind of problems am I talking about? Loss of memory is one. When you disconnect the battery on any vehicle that has computerized engine controls (which is virtually every car and truck that has been built since 1981), the loss of voltage to the Powertrain Control Module (PCM) causes it to forget is adaptive memory settings in its Keep Alive Memory (KAM) chip. 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, its sensors and emission control components, plus any fault codes that may have been set (including history codes and freeze frame data that may be needed for diagnosis).
An old mechanic's trick for turning for temporarily eliminating certain kinds of driveability problems is to disconnect the battery ground cable for 10 seconds to "reset" the computer. Essentially, this trick erases the stored fault codes in the PCM's keep alive memory and turns the Check Engine light off. But this is NOT a fix for a Check Engine light. All it does is turn the light of temporarily and return the PCM to its initial base settings for the air/fuel mixture, idle speed and other control functions. If the vehicle has a problem, sooner or later the same fault code(s) will reset and the same driveability problems will return because the cause has not been diagnosed and repaired. It may take a few days, but the problem will be back.
Warning: Disconnecting the battery to reset the PCM on a growing number of late model (2003 and newer) 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.
Also Note: Disconnecting the battery will NOT allow a vehicle to pass an OBD II plug-in emissions test. It may temporarily turn off the Check Engine light, but this won't fool the computer at the test lane. When the test computer is plugged into the vehicle diagnostic connector, it will check the PCM to see if all of the OBD II self-diagnostic monitors have run. If all of the monitors have not completed, the vehicle will be rejected at the test lane. You will then have to continue driving your vehicle until all of the self-checks have completed (which may take up to several days). If the Check Engine light does not come back on after all of the self-checks have completed, the vehicle will pass the test. But if there is a problem, the Check Engine light will come on again and the vehicle will fail the test.
What happens when the battery is disconnected? It depends on the year, make and model of your vehicle, but any of the following may happen:
Here is a short list of some of the problems that can occur when disconnecting or replacing the battery on the following vehicle applications (refer to the OEM service literature for specific model and year applications and cautions):
WARNING: Regardless of the year, make or model of vehicle NEVER disconnect the battery while the engine is running or the ignition key is on. Doing so can create a high voltage spike in the electrical system that may damage electronic modules and/or the charging system.
If a battery has to be disconnected prior to doing electrical work on a vehicle (which is highly recommended to prevent an accidental short that might damage wiring or modules), or if an old or weak battery is being replaced, voltage MUST be maintained to the PCM and other modules while the battery is disconnected.
Voltage to the PCM and other modules can be provided by connecting a 12v backup power supply to the battery cables, using jumpers to attach another 12v battery to the battery cables, or plugging a small 9 volt battery Keep Alive Memory (KAM) saver into the cigarette lighter or a 12v power receptacle. NOTE: If using a plug-in battery memory saver, make sure the cigarette lighter or a power receptacle is always on and has power when the ignition key is off. A plug-in battery memory saver can be purchased at most auto parts stores, and usually sells for less than $20.
Why not use a battery charger to keep the memory alive? Some professional grade battery chargers have a built-in battery backup mode, and can be used for this purpose. But an ordinary home battery charger is designed to charge batteries, and not to serve as a battery backup. If the battery is disconnected, it may attempt to send too much charging current through the battery cables and damage a module. A low amperage trickle charger (1 to 3 amps) is less apt to overload the circuit than say a 10 or 20 amp charger, but it's still a risk. So unless the charger has a battery backup mode, or the charger manufacturer says it is okay to use their charger for this purpose, do not use an ordinary battery charger as a battery backup when disconnecting a car battery.
The Keep Alive Memory chips in most modules require very little current to retain their settings, anywhere from a few milliamps up to 10 to 15 milliamps depending on the module. Normal current drain can vary from as little as 20 to 50 milliamps, to as much as 300 to 400 milliamps on some late model vehicles. For example, the current drain in some Fords may be as high as 850 milliamps for up to 20 minutes or more after the ignition has been turned off.
Many modules have internal timers that either turn off the module to put it into a "sleep mode," or power down the module to a "standby mode" to reduce the parasitic power drain after a predetermined length of time when the key is turned off or the vehicle's occupants leave the car. Some of these modules power down in steps and time out at different rates. For more information on this subject see Diagnosing A Battery That Runs Down.
As a rule, the parasitic drain on most late model vehicles should be less than 50 milliamps one hour after the vehicle has been shut off and left undisturbed. But this is a rule of thumb only. Always refer to the vehicle manufacturer's key-off electrical drain specifications if available (some vehicle manufacturers have no published specifications).
Keep in mind, though, that opening a door, the trunk or turning anything on can wake up various modules and start the timer countdown all over again. So if you are using a little 9 volt alkaline battery backup for the Keep Alive memory while the battery is disconnected, keep the doors and trunk shut (so the lights don't come on) and work quickly so you don't drain the little backup battery. Professional technicians use a 12v backup battery power source that can provide a steady power source for as long as needed.
On late model vehicles that have an AGM battery mounted inside the passenger compartment or trunk, do NOT replace the AGM battery with a conventional wet cell lead-acid battery. Replace an AGM battery with an equivalent AGM battery.
Use a battery saver, 12-volt power supply or battery charger to maintain voltage to the vehicle's electrical system when disconnecting the battery or when replacing a battery. This will prevent all of the problems described earlier in this article if power is lost when a battery is disconnected or replaced.
See BATTERY REPLACEMENT for more details on how to replace your car battery.
If replacing a battery (new or used), make sure the battery is FULLY CHARGED before installing it. This will assure proper voltage for all of the modules when you start the vehicle, and will reduce the load on the charging system. A low battery can affect the operation of the onboard electronics, including the PCM, anti-theft system, ignition system, fuel injectors and fuel pump.
Connect the POSITIVE (+) battery cable first (which may be color coded RED), then the NEGATIVE (-) GROUND cable last (which may be color coded BLACK). Make sure BOTH cable connections are clean and tight. Clean the battery posts and cable terminals if they are corroded. Replace the battery cables if they are damaged or do not fit the battery posts tightly (and remember to use a battery backup power supply while doing this!).