Parasitic Draw Calculator
Measure the current your vehicle draws while parked and find out how many days it takes to flatten the battery.
Your measurement
- Measure with the meter in series on the negative cable, doors shut and the vehicle fully asleep.
- Do not crank the engine with the meter in circuit. Starter current will destroy the meter instantly.
- Opening a door to reach the fuse box wakes the modules again. Use a jumper lead across the meter or pull fuses through an open window.
What a normal draw looks like
A modern vehicle never fully switches off. Keyless entry receivers, alarm modules, the clock and telematics all sip current. Under 50 milliamps is normal for anything recent. An older vehicle with less electronics should be under 25.
Above 100mA something is staying awake that should not be. Above 500mA you have a fault that will flatten a good battery inside a week.
Wait for it to go to sleep
This is where most measurements go wrong. Modules stay awake for anywhere from 20 to 45 minutes after you shut the door, and during that time the draw can be several amps quite legitimately. Measure too early and you will chase a fault that does not exist.
Worse, opening a door to get at the fuse box wakes everything up again and restarts the timer. Either work through an open window, or fit a jumper lead across the meter so you can break the circuit without disturbing anything.
Finding the circuit
With the meter connected and a stable reading, pull fuses one at a time and watch the number. When it drops, the circuit you just disconnected contains the fault. Put the fuse back and work through what is on that circuit. Glovebox and trunk lamps, aftermarket stereos and alarms, and failing door lock actuators are the usual suspects.
Frequently asked questions
What is an acceptable parasitic draw?
Under 50 milliamps for a modern vehicle, under 25 for something older and simpler. Anything above 100mA is worth investigating, and above 500mA will flatten a healthy battery within a few days.
How long should I wait before measuring?
At least 45 minutes with everything shut and undisturbed. Body control modules, infotainment and telematics all power down on their own schedules, and readings taken before they sleep are meaningless.
Can I measure parasitic draw with a clamp meter?
Only with a good DC clamp meter with milliamp resolution, and most inexpensive clamp meters cannot resolve tens of milliamps. The advantage is not having to break the circuit, which means the modules stay asleep. An ammeter in series is more accurate.
Why does my battery go flat after a week but test fine?
That combination points at a parasitic draw rather than a bad battery. A battery can pass a load test and still be flattened overnight by a circuit that stays live. Measure the draw before spending money on a replacement.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.