Causes of excessive current drain from a car battery include things like lights that remain on (trunk and hood lights, interior lights, brake lights, etc.) and also relays that may be stuck on, or modules that are not going to sleep or powering down.
A fuel pump relay that sticks on may keep the fuel pump running after the engine is shut off. A switch or relay that powers a rear window defogger can stick on, pulling current from the battery after the ignition is off. An electronic suspension module, ABS module or keyless entry module may remain active long after it should have powered down.
An accessory such as a DVD player, game console or cell phone charger left plugged into a rear seat power receptacle may be pulling power from the battery. So before you spend a lot of time trying to figure out where the amps are disappearing, check all of the vehicle’s power receptacles to see if something is plugged in that may be using power.
KEY-OFF CAR BATTERY DRAIN
If your car battery is less than six years old and tests okay, and the charging system is working normally, your battery may be running down because of a key-off current drain.
When you shut the engine off, turn off the lights and remove the key from the ignition switch, the electrical load on the battery should drop to almost nothing. On older cars that do not have computers or other onboard electronic modules, the key-off load on the battery is typically only a few milliamps ( if it has a clock or a digital radio), or it may drop to zero (no load at all). But on late model vehicles with computers and numerous electronic modules, the key-off drain on the battery can range from 20 to 50 milliamps, to as much as 300 to 400 milliamps. On some Fords, the electrical system will continue to pull as much as 850 milliamps (almost 1 amp) for up to 20 minutes or more after the ignition has been turned off.
The key-off current drain is used by the modules to keep their memories alive. The chips that store certain settings and learned information are called the “Keep Alive Memory” (KAM).
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 on the battery after a predetermined length of time when the key is turned off. Some of these modules power down in steps and time out at different rates.
If the battery runs down (or is disconnected), KAM information can be lost. Computers and modules may reset to a “default” setting after the battery is recharged or reconnected, but you may notice some changes in the way your vehicle runs and drives, and you may have to reset the radio channels, clock and other memory presets. Worse yet, on some vehicles the climate control system, power windows or radio may not function at all until they are reset using a special “relearn” procedure or a dealer scan tool. On many BMW, Audi, Acura and VW models, for example, the radio won’t work if the battery has been disconnected or run dead until the factory security code has been reentered. Read This before you disconnect or replace a battery on a late model vehicle.
When disconnecting or replacing a car battery on a late mode vehicle, therefore, it is important to keep a steady voltage supply to the modules to preserve the KAM settings. This can be done by plugging a backup battery into the cigarette lighter, or attaching a backup battery or battery charger to the car’s battery cables BEFORE the cables are disconnected.
MEASURING CAR BATTERY CURRENT DRAIN
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 want to check the parasitic draw on the battery, leave the hood open (or the trunk if the battery is located in the back).
Current drain can be measured with a digital multimeter (DMM) using the ammeter function one of two ways: by measuring current flow between a battery post and one of the cables, or by using a clamp-on inductive amp probe. The latter requires a somewhat expensive amp probe, but is faster and easier because you don’t have to disconnect a battery cable or use a battery backup to maintain the Keep Alive Memory in the vehicle’s computers and electronics.
How To Measure Current Drain Without an Inductive Amp Probe
1. Set the DMM to read milliamps (mA). If the meter is not auto-ranging, select the 100 or 1000 mA scale.
NOTE: the DMM should have a 10 amp fuse in it to protect the meter from possible damage if there is a large current drain on the battery.
2. Plug a battery backup into the cigarette lighter (or attach a backup battery or power supply to the battery cables) to protect KAM while disconnecting the battery cable.
3. Disconnect one of the battery cables and connect the DMM test leads to the cable and battery post.
4. Unplug the battery backup and note the current reading on the DMM.
NOTE: Do NOT remove the DMM test leads until you have plugged the battery backup in the cigarette lighter or reconnected the battery backup to the battery cables, otherwise you will lose the KAM if power is disrupted.
To measure current drain with an inductive amp probe:
1. Set the DMM to read millivolts (mV). If the meter is not auto-ranging, select the 100 or 1000 mV scale. Set the switch on the amp probe to either 10 mV or 100 mV per amp (10mV per amp is the best setting for reading kay-off drains).
2. Clamp the amp probe around one of the battery cables.
3. Note the current reading on the meter.
Also note: Some amp probes can read directly without the need for a DMM.
FINDING THE CAUSE OF A KEY OFF CURRENT DRAIN
If the key-off current reading is above specifications (typically, more than 50 mA one hour after the vehicle has been shut off), the current drain is too high. It’s time to start pulling fuses and relays to find the fault.
checking fuses and relays Remove fuses and relays one at a time to find circuit that is draining the battery.
Refer to your vehicle owner’s manual or a wiring diagram to identify the fuses and relays. Then pull the fuses and relays one at a time until the current reading drops. Avoid pulling the fuses for the PCM or other KAM-sensitive modules until you have checked all of the other circuits.
Once you’ve found the circuit that is causing the excessive current drain, check the relay, switch, module or other components in the circuit and replace as needed.
Sometimes a bad alternator diode can cause a car battery to run down. A good diode should only pass current in one direction. If it leaks current in the opposite direction, it may keep the charging circuit on when the engine is not running, causing the battery to run down. This kind of problem can be diagnosed several ways. If your voltmeter has an AC (alternating current) scale, switch to that scale and observe the charging voltage with the engine running. If the meter shows any AC voltage, one or more diodes are leaking and the alternator needs to be replaced. The alternator’s output can also be observed as a waveform on a digital storage oscilloscope (DSO) or an alternator tester that measures ripple voltage to detect this kind of problem. Or, just disconnect the alternator overnight and see if the battery stills run down. If the battery drain stops, you have found the problem.