06 · Keeps Dying
Car battery keeps dying
When a battery drains overnight, the cause is either parasitic drain (something pulling current with the engine off) or a battery that cannot hold a charge. The 7-step diagnostic below isolates which one, and if it's a drain, exactly which circuit is responsible.
Triage
Two-question split
Before any tools come out, two questions cut the diagnostic in half.
QUESTION 1
How old is the battery?
- ≥ 4 years: Test the battery first. Worn batteries self-discharge fast.
- < 3 years: Skip to the parasitic-drain test below.
QUESTION 2
When does it die?
- Overnight: Parasitic drain.
- While driving: Alternator failure.
- Within minutes of parking: Big draw (stuck relay, trunk light).
Procedure · 7 steps
Parasitic drain test
- 01
Battery age check
How old is the battery? Over 4 years and dying overnight is far more often a worn-out battery than a parasitic drain. Test the battery first.
- 02
When does it die?
Overnight only suggests parasitic drain. While driving suggests alternator failure. Within minutes of parking suggests something massive (stuck relay, trunk light).
- 03
Set up the multimeter
Set to DC amps (10A range to start). Disconnect the negative battery cable. Place the meter in series between the cable end and the post. Wait 30 to 45 minutes for modules to sleep.
- 04
Read the parasitic draw
Under 50 mA is normal. 50 to 100 mA is borderline. Over 100 mA is a problem. If the read is over 1A, do not wait, start pulling fuses.
- 05
Pull fuses one at a time
While watching the meter, pull each fuse one at a time. When the draw drops, you found the circuit. Note the fuse number and pull it again to confirm.
- 06
Identify the component
Cross-reference the fuse with the owner's manual or fuse-box diagram. Disconnect components on that circuit one at a time until the draw normalises with all components reconnected.
- 07
Verify the fix
Reconnect the cable, drive normally for 24 hours, then measure draw again. Some modules wake briefly when reconnected; wait 30 to 45 minutes before declaring the fix successful.
Common culprits
What usually causes a parasitic drain
| Suspect | Typical draw |
|---|---|
| Aftermarket dash cam (parking mode) | 30 to 200 mA |
| Amplifier or remote start module | 50 to 300 mA |
| Trunk or glove-box light stuck on | 200 to 800 mA |
| Failing body control module | 100 to 500 mA |
| GPS or fleet tracker | 20 to 80 mA |
| Faulty alternator diode | 100 to 400 mA |
| Old / failing battery (self-discharge) | Normal at 50 mA but battery cannot hold charge |
When to escalate
If the fuse-pull test fails to isolate it
Some drains do not show up because the offending module sleeps when you open the door, then wakes silently after you close everything up. A diagnostic shop with a current-clamp scope can trace these in 30 to 60 minutes. Expect $50 to $100 for the diagnostic. Cheaper than buying a battery every six weeks.
Continue your research