A6 · General Electrical≈ 7 minThe key-off battery killer
01 The Concept
A dead battery every morning, and nothing left on. Something won't go to sleep.
Parasitic (key-off) draw is the current the vehicle pulls with everything switched off. A little is normal — the clock, radio presets, and module keep-alives sip a few milliamps. But a module, relay, or accessory that won't shut off keeps drinking and flattens the battery overnight. You measure it with an ammeter in series in the battery cable — after the modules go to sleep, which can take 20–45 minutes.
Under ~50 mA
typical key-off, modules asleep
Find the offender
With the draw reading high, pull fuses one at a time and watch the meter. When the reading drops to normal, the circuit you just opened is the culprit. Then chase what's on that circuit.
⚑ Don't wake the car up
Opening a door, or breaking the circuit to insert your meter, wakes the modules and restarts the sleep timer — now everything reads high and you're chasing ghosts. Use a low-current clamp or a meter switch that keeps the circuit live, and give it time to sleep before you trust the number.
02 Pull to Find It
The meter's in series and the car is asleep, but the draw is way over spec. Pull fuses one at a time and watch the reading.
325
mA · key-off
Over 50 mA
03 What If…
Reason first, then reveal.
?You connect the meter and immediately read 800 mA, but it slowly falls over 30 minutes to 35 mA. What happened?
Nothing's wrong — the modules were awake and going to sleep. Connecting the meter (or opening a door) woke them; as each times out and sleeps, the draw settles. Wait for it to stabilize before judging. Final 35 mA is in spec.
?Draw is 320 mA. You pull the "Amp/Radio" fuse and it drops to 34 mA. Fix the amp?
You've found the circuit, not yet the part. Something on that fuse isn't sleeping — the amplifier, the head unit, or a stuck relay/wire on that feed. Inspect what's on that circuit, confirm which device stays powered, and repair that. The fuse just narrowed it down.
?Every fuse is pulled and the meter still reads 60 mA. Where's that coming from?
An unfused feed — something wired directly to the battery ahead of the fuse box: the alternator, starter, an add-on amp or accessory, or a chafed main cable. Fuse-pulling can't isolate it, so disconnect those direct feeds one at a time.
04 Practice
ASE-style. Pick an answer to see why.
05 Recap
Parasitic draw = key-off current. Typical is under ~50 mA once asleep.
Measure with an ammeter in series (or a low-current clamp) after modules sleep — 20–45 min.
Locate it by pulling fuses one at a time; the pull that drops the reading is the circuit.
Don't wake the car — opening the circuit or a door restarts the sleep timer.
Draw with all fuses out = an unfused direct-to-battery feed.