Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsBattery Diagnosis and ServiceMedium
A customer complains that their car's battery drains overnight. The technician performs a parasitic draw test. With the ignition off and all accessories off, the multimeter shows a current draw of 150 milliamperes (mA) after the body control module (BCM) has settled. What is the most likely conclusion?
- AThere is an excessive parasitic draw on the battery.
- BThe battery is faulty and needs replacement.
- CThe reading is within normal limits for this vehicle.
- DThe charging system is not functioning correctly.
Show answer & explanationAnswer & explanation
Correct answer: A. There is an excessive parasitic draw on the battery.
A normal parasitic draw for most modern vehicles is typically between 20-50 mA. A draw of 150 mA is significantly higher than normal and indicates an excessive drain on the battery, which would cause it to discharge overnight.
Why the other options are wrong
- B. While a faulty battery could be a secondary issue, the primary cause indicated by the test is excessive draw.
- C. 150 mA is not within normal limits; it's considered an excessive draw.
- D. The charging system is only active when the engine is running; a parasitic draw occurs when the engine is off.
Parasitic Draw
The normal or abnormal continuous electrical current drawn from a vehicle's battery when the ignition is off and all accessories are off. Excessive draw can discharge the battery.
- Normal range: 20-50 mA (varies by vehicle)
- Measured with an ammeter in series with battery
- Requires vehicle to 'go to sleep' before accurate measurement
Memory trick: Measure the silent drain, then find the culprit.