Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsBattery Diagnosis and ServiceMedium
A customer reports that their vehicle's battery is 'always dead' after sitting for a few days, even after being fully charged. The technician suspects a parasitic draw. Which of the following is the most appropriate initial step to diagnose this issue?
- AInspect the battery terminals for corrosion.
- BCheck the charging system voltage output.
- CConnect an ammeter in series with the battery's negative cable.
- DPerform a battery load test.
Show answer & explanationAnswer & explanation
Correct answer: C. Connect an ammeter in series with the battery's negative cable.
To diagnose a parasitic draw, an ammeter must be connected in series with the battery to measure the current flowing when the vehicle is off. This allows the technician to identify excessive current draw.
Why the other options are wrong
- A. Corrosion can impede current flow but doesn't cause an electrical component to draw power when off.
- B. Charging system output addresses charging issues, not a draw when the vehicle is off.
- D. A load test checks the battery's ability to deliver current, not an excessive draw when off.
Parasitic Draw Diagnosis
The process of identifying components that consume electrical current when the vehicle is off, leading to battery discharge.
- Measured with an ammeter in series.
- Vehicle must be in sleep mode for accurate reading.
- Typical acceptable draw is 20-50 milliamps (mA).
Memory trick: To find the 'current' thief, you must insert yourself 'in series' with the money flow.