Excessive Battery Discharge Rate
How to measure resting battery current draw on a 924/944/968 and systematically identify which circuit or component is causing excessive discharge.
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General Information
All 924s, 944s, and 968s use a BCI Group 41 battery (11-3/16" L × 6-7/8" W × 6-7/8" H). Batteries for these cars are typically rated for 50–70 amp-hours (AH). A 50 AH battery can discharge at 1 A for 50 hours before dropping below the ~10.5 VDC minimum cranking voltage.
The normal resting discharge rate — ignition off, key removed — should be less than 60 mA. A reading above 65 mA indicates a problem worth investigating.
As a reference: a 50 AH battery with a 60 mA draw will sustain cranking voltage for approximately 30 days. This varies with battery age and ambient temperature.
Tools
- Digital multimeter
- Test leads
Procedure
- Disconnect the battery positive lead.
- Connect the digital multimeter between the battery positive lead and the battery positive terminal.
- Set the multimeter to the ammeter function and turn it on.
- Read the discharge rate. It should be less than 65 mA (0.065 A).
- If the draw exceeds 65 mA, pull fuses one at a time — reinstalling each before pulling the next. Note how much the discharge rate drops when each fuse is removed.
- Repeat with relays, noting the drop for each.
- Identify the circuits where removing the fuse or relay produced a significant drop. Using the factory wiring diagrams as needed, determine all components on those circuits. Disconnect components one at a time until the ammeter identifies the specific source of the excessive draw.