Symptom guide
Battery Keeps Dying
Identify by pattern
Dies overnight after working day
Points to parasitic drain, where something keeps drawing current with the key off. An old battery losing capacity does the same thing.
Dies during drive
That's the charging system. The alternator is failing.
Dies after sitting 2-3 days
Fairly normal for an older battery. Replace it.
Dies after specific event
Usually parasitic drain triggered by a faulty accessory or module.
Common causes
1. Old or failed battery (~40%). Typically past the 4-year mark. Clue: the load test fails at the parts store.
2. Parasitic drain (~30%). Current flow stays above 50 mA with the key off. Clue: a multimeter on the ammeter setting between the battery and terminal shows excess draw.
3. Failed alternator (~20%). It isn't charging. Clue: voltage at the battery with the engine running falls below 13.6V.
4. Corroded/loose terminals (~5%). Usually visible on inspection.
5. Bad charge circuit wiring (~3%). Clue: a voltage drop test reveals resistance.
6. Failed battery temperature sensor (~2%). Shows up on some modern platforms.
How to diagnose
1. Battery test (free at parts stores)
A load test confirms capacity. Replace the battery if it fails.
2. Charging system test
Voltage at battery with engine running:
- Off: 12.4–12.8V (rested).
- Running 1500 rpm: 13.6–14.6V.
- Below: alternator failing.
3. Parasitic drain test
Multimeter on amps setting (10A range) between negative terminal and battery post. Key off, doors closed, all modules asleep (wait 30 minutes).
- Below 50 mA: normal.
- Above 50 mA: parasitic drain. Pull fuses one at a time to find the circuit.
Fixes
| Fix | Cost (USD) |
|---|---|
| Replace battery | $120–$280 |
| Replace alternator | $150–$650 |
| Clean terminals | $0–$15 |
| Find and fix parasitic drain | $20–$300 (varies) |