![]() A battery with an intermittent shorted cell can sometimes mask itself as a phantom battery drain. Many times, the customer complains of a battery drain or “short” when the actual problem is a faulty battery or charging system problem. ALWAYS test the battery and charging system prior to performing a parasitic battery drain test. Some DTCs that return may be good clues especially if they are U-codes (communications related) as we’ll see later. Regardless, retrieve and document prior to clearing. DTCs stored in modules after a battery drain are often “effect” DTCs rather than “cause” DTCs. ![]() The following steps will help you unmask your next phantom battery drain. Ouch: this test still burns your fingers too. The old trick of opening a trunk or glove box and grabbing the bulb to see if it’s “two hours hot” or “two seconds warm” still works today just as it did 40 years ago. Faulty door latch/door lock switches can cause drains as can glove box and trunk light switches. Even a brand-new vehicle (non HEV/EV) has an alternator that can suffer from a battery draining diode leak. The most complex modern vehicles, however, can still have old school drains that can run a battery down in a few hours or few days. Parasitic drains on newer vintage vehicles seem to be intermittent more times than not due to the complexities of data buses and modules staying awake when they should be asleep. Intermittent battery drain – the phantom! Factory battery saver devices at the battery or in the BCM along with automatic module power down algorithms during extended storage times have also contributed to aiding today’s mobile electronic monstrosities in keeping their 12-volt starting batteries alive for storage periods of months at a time. This has helped to lower the normal parasitic on many newer vehicles to as low as 5-10 mA in some cases. For this reason, many OEMs have moved toward greater use of EEPROM to provide more permanent memory storage and improved power management to decrease battery drain. The number of ECUs has increased in recent decades making the overall vehicle normal parasitic drain tally up to as much as 30 mA to 50 mA – the numbers that seem to be the unofficial max specs. KAM is necessary for keeping both long term and short-term memory alive for functions such as the vehicle’s clock, DTC storage, telematics and module adapts to name just a few. The KAM (Keep Alive Memory) current draw for individual ECUs is typically around 1-3 mA per device. A nominal amount of battery drain is present on every vehicle in the road once the vehicle is switched off and had enough time for its various modules to complete their “go to sleep” processes.
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