Why does my car battery keep dying overnight?

A parasitic drain — something is drawing power when the car is off. Most common: aftermarket accessories, a module that won't sleep, interior light staying on, or a bad alternator diode. Normal drain is under 50mA.

How do I find a parasitic battery drain?

Use a multimeter in DC amps mode between the negative terminal and cable. Wait for modules to sleep (20-30 min). If draw exceeds 75mA, pull fuses one at a time until the draw drops — that's your circuit.

Parasitic Battery Drain: Why Your Car Battery Keeps Dying

Your battery is dead again — and you know you turned everything off. A parasitic drain means something is drawing power from the battery even when the vehicle is off. Normal draw is 20-50 milliamps. Anything above 75mA will flatten a battery overnight.

What Is It?

Modern vehicles have dozens of electronic modules that stay partially active (door locks, alarm, radio memory, ECM). Normally, these draw minimal power. A parasitic drain occurs when a module fails to enter "sleep mode" or a circuit stays energized.

Common Causes

Aftermarket Accessories (Very Common): Dash cameras, GPS trackers, amplifiers, or LED lights that are hard-wired to a constant-power circuit drain the battery continuously. This is the #1 cause of parasitic drain.
Failed Module (Not Sleeping) (Common): A body control module, radio, or other electronic module that doesn't enter sleep mode draws 100-500mA continuously.
Interior Light Left On (Common): A trunk, glove box, or underhood light with a faulty switch stays on when closed, draining the battery.
Bad Alternator Diode (Moderate): A leaking diode in the alternator allows current to flow backwards through the charging circuit, draining the battery when the engine is off.
Old Battery (Common): Batteries over 3-5 years old lose capacity. A battery that holds charge fine in summer may die overnight in cold weather due to reduced capacity.

How to Diagnose

Charge the battery fully before testing.
Set a multimeter to DC amps (10A or 20A range). Connect in series between the negative battery terminal and cable.
Wait 20-30 minutes for all modules to enter sleep mode. Normal draw: 20-50mA.
If draw exceeds 75mA, pull fuses one at a time while watching the meter. When the draw drops, you've found the circuit.
Trace the identified circuit to find the specific component causing the drain.
Check for aftermarket accessories wired to constant power — this is the most common cause.

When to See a Mechanic

If you can't find the drain with basic fuse pulling, a mechanic with advanced diagnostic equipment can trace current flow through individual circuits and modules.

Typical Cost: $100 (battery) to $300+ (module replacement)

Frequently Asked Questions

Why does my car battery keep dying overnight?: A parasitic drain — something is drawing power when the car is off. Most common: aftermarket accessories, a module that won't sleep, interior light staying on, or a bad alternator diode. Normal drain is under 50mA.
How do I find a parasitic battery drain?: Use a multimeter in DC amps mode between the negative terminal and cable. Wait for modules to sleep (20-30 min). If draw exceeds 75mA, pull fuses one at a time until the draw drops — that's your circuit.