Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsBattery Diagnosis and ServiceHard
A customer reports that their car's battery drains overnight, even when the vehicle is turned off. After confirming the battery is fully charged and healthy, the technician uses an ammeter to measure current draw with the ignition off. The ammeter reads 150 milliamperes (mA) after all systems have 'gone to sleep'. What does this reading indicate?
- AA normal parasitic draw for a modern vehicle.
- BAn acceptable draw for a vehicle with multiple electronic modules.
- CAn excessive parasitic draw, requiring further investigation.
- DA healthy battery that will not discharge overnight.
Show answer & explanationAnswer & explanation
Correct answer: C. An excessive parasitic draw, requiring further investigation.
A normal parasitic draw for most modern vehicles is typically between 20-50 mA (0.02-0.05 amps) after all modules have gone to sleep. A draw of 150 mA (0.15 amps) is significantly higher than acceptable and will cause the battery to discharge relatively quickly, especially overnight.
Why the other options are wrong
- A. 150mA is much higher than the normal parasitic draw (20-50mA).
- B. Even with multiple modules, 150mA is considered excessive and indicates a problem.
- D. A 150mA draw will indeed discharge a battery overnight, leading to starting issues.
Parasitic Draw
Parasitic draw is the continuous current draw from the battery when the ignition is off and all systems are 'asleep'. An excessive draw (typically over 50mA) indicates a fault that will drain the battery.
- Normal draw: 20-50 mA (0.02-0.05 Amps).
- Measured after all modules 'sleep' (15-60 min).
- Excessive draw drains battery over time.
- Requires systematic circuit isolation to find the source.
Memory trick: Fifty Milli, Not a Lot; Over That, You've Got a Spot!