Symptom guide
Car Won't Start After Sitting
What it means
A modern car draws 30–80 mA with everything off. A healthy 60 Ah battery loses only about 0.5–1% per day at that draw, so even after two weeks it sits well above the ~12.0 V crank threshold. If a few days of sitting flattens it, then either the battery is weak or something is pulling more current than spec allows.
Common causes
1. Aging battery (~30%). A battery 4+ years old loses cold cranking amps. Clue: a load test reads below 50% capacity.
2. Parasitic drain over spec (~25%). A glovebox light, aftermarket amplifier, or sticky module keeps drawing power. Clue: an amp clamp on the negative cable shows >100 mA after 30 min of sleep.
3. Corroded battery terminals (~15%). Voltage drops across the corrosion before it ever reaches the starter. Clue: green or white powder on the post.
4. Bad ground strap (~10%). Resistance climbs as the strap ages. Clue: a voltage drop test reads >0.3 V from engine to chassis.
5. Fuel system bleed-down (~10%). The engine cranks a while before it starts. Clue: it cranks fine but takes 5+ seconds.
6. Failing starter (~5%). A slow crank that improves with a throttle tap or repeated tries.
How to diagnose
- Measure battery voltage. Resting should be >12.4 V.
- Load-test the battery (most parts stores do free).
- With car asleep (locked, alarm armed), amp clamp the negative cable. After 30 min should read <50 mA.
- If drain is high, pull fuses one at a time until current drops.
Fixes
| Fix | Cost (USD) |
|---|---|
| Clean and re-grease terminals | $5–$20 |
| Replace battery | $100–$300 |
| Replace ground strap | $15–$60 |
| Replace starter | $150–$500 |
| Diagnose parasitic drain (shop) | $80–$200 |