How to Safely Disconnect or Replace Your Car Battery

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 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.
battery disconnected
If using a plug-in battery saver, use a new 9v alkaline battery.
These devices can keep memory alive while a battery is disconnected, but not for long
(maybe only 15 to 30 minutes depending on the current drain on the 9v battery).

The Schumacher OBD Memory Saver Cable is another handy device for preserving
learned data, stored codes and programmed electronic settings such as
clocks, radios, seat positions, keyless entry systems, alarms and more.
The cable supplies voltage to the PCM by plugging one end into a 12-volt power receptacle on another vehicle,
and the other end of the cable into the OBD connector on your vehicle.
Just make sure the 12-volt receptacle on the other vehicle is ON and is supplying voltage
before you disconnect your battery.

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.

KEY-OFF Current Drain on 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.

How to Install a 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!).

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