Parasitic Battery Drain

Symptom: Battery dead after parking for hours or overnight. Battery tests good when charged.

Normal parasitic draw: 20–50 mA. Modern vehicles with many computers may draw up to 80 mA for several minutes after key-off, then settle to standby current.

Measuring parasitic draw:

1. DMM set to DC amps (use 10A or 20A range first)
2. Key off, all doors closed, wait 10 minutes (modules sleep)
3. Connect DMM in series between battery negative terminal and negative cable
4. Read current
5. If >80 mA: parasitic drain present

Isolating the circuit:

With DMM connected, pull fuses one at a time from the fuse box. When current drops significantly: that circuit has the drain.

Common culprits: amplifier with always-on power (should be on switched circuit), alarm/remote start module, aftermarket accessories installed improperly.

Amplifier drain:

Amplifiers must receive their power from a fused circuit at the battery (always hot — this is correct). The amplifier turns on via the remote wire from head unit. If remote wire fails to go to 0V when head unit is off: amplifier stays on, drains battery.

Test: With key off and system off, is there voltage on amplifier remote terminal? Should be 0V. If 12V: head unit remote output is stuck on or remote wire is connected to a constant 12V source.