Why Does My Classic Car Battery Keep Dying Sitting in the Garage?
Learn why your classic car battery dies while sitting and how to trace a parasitic drain with a simple multimeter test in your own driveway.
A classic car battery usually dies sitting in the garage because of a parasitic draw pulling current when the ignition is off, a failing alternator diode, or a dirty battery ground.
Testing total millivolt draw across fuse terminals with a multimeter identifies the exact circuit without pulling fuses randomly.
Q. How much battery drain is normal for a cl. assic car?
A. An older carbureted car with no electronic control unit should pull almost zero milliamps, usually under 20 milliamps for an aftermarket radio clock. Anything over 50 milliamps will flatten your battery within a week of sitting.
Q. Can a bad alternator drain my battery while the car is turned off?
A. Yes, a shorted internal diode inside the alternator allows current to drain backwards through the charging circuit directly to ground even with the key out.
Q. Should I just disconnect the battery terminal every time I park my project car?
A. A battery disconnect switch is a great safety backup, but fixing the actual parasitic draw ensures your electrical system stays reliable on track days or family cruises.
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