Key-Off Battery Drain Flashcards

KEY-OFF BATTERY DRAIN

Excessive circuit drain when the vehicle is not running can lead to a run-down battery, even if the battery is in good condition and the charging system properly works.

The main reason for a slight draw on the battery is because some systems in the vehicle need voltage at all times in order to keep information stored. Think of radio presets, for example. Memories and stored diagnostic data must be kept alive in the PCM and other electronic modules whether the vehicle is running or not. The digital clock and other such items also draw a very small current while the vehicle is off. This is not a problem for today’s vehicles as long as the drain is not excessive, typically under 30 to 50 milliamps. These circuits can sometimes stay operative (commonly called “awake”) after the ignition is shut off and the doors are locked for as long as eight hours before shutting down (called “going to sleep”). Remember this when checking for excessive key-off battery draw.

Battery electrical drain check. (Courtesy: GM Corp.)

On the other hand, a defective glove box or trunk light switch would draw more current than this, and easily drain the vehicle battery in just a few days. Always use the manufacturer’s suggested key-off drain specifications when trying to determine the proper amount of current that computer systems and accessories should draw when the vehicle’s engine is not running. The most effective way of checking this condition is to use an ammeter in series between the negative battery cable and the negative battery post. This enables full battery voltage to be supplied to the vehicle during the measurement. Another method that can be used is to measure voltage drop across a 1-ohm resistor connected in series between the negative battery cable and the negative battery post. But the quickest and easiest way to check for KAM and/or excessive current draw is to place a low-amperage clamp meter around the battery’s negative cable and take the reading.

NOTE: Make sure the ignition switch is in the OFF position at all times during testing. Also, all courtesy and accessory lights must be off and any KAM circuits should have powered down. The use of a capable scan tool is often helpful in identifying an electronic control module that refuses to “go to sleep.”

If an excessive drain is displayed, remove the fuses from the fuse block one at a time. When the fuse powering the circuit with the drain is removed, the high amperage will return to a normal reading.

Using a wiring diagram, note the specific circuits that run to the particular fuse that was pulled. Reinstall the fuse and allow the DMM to read the excessive draw. Now, disconnect each circuit to further isolate the current draw. When the draw returns to normal, you’ve found the circuit that’s causing the problem.

If the drain is not found using this procedure, use a wiring diagram and locate all circuits connected to the battery that use a fusible link. Disconnect the circuits, one at a time, to isolate the circuit with the excessive draw.

Using Voltage Drop To Test For Drain

Electronic Control Modules that fail to power down after the key has been turned off are a common cause of parasitic drain on modern automobiles. Using conventional testing methods that require any portion of the effected circuit to be opened can result in the module resetting – and make the fault impossible to find. A popular way to isolate the circuit causing the drain is to measure the voltage drop across the fuses. 

The theory is simple. Every electrical component has some resistance, even if it is very small. Fuses are no exception and if there is current flowing through it, there will be a measurable drop across the fuse. This registers in millivolts, so set your meter accordingly. Any reading other than 0.0 milliamps indicates a circuit is on. Use the Power Distribution diagram in the service information to determine what circuits the fuse you found “hot” is supplying to locate the faulty circuit or component.