Parasitic Load


Parasitic drain is a phrase that is derived from the word “parasite” which means an animal or plant that lives on and/or dependant on another living organism.

And the study and problems of parasitic drain in today’s automotive market is indeed  “a different kind of animal”.


When you drive your car, the alternator charges the battery according to the load demand during the driving operation. Turn your AC or rear defog on, the voltage regulator senses a drop in voltage and tells the alternator to “kick it up a notch”. And so it goes for the whole time you are operating your vehicle. BUT, what happens when you turn your key off and let your car sleep at night?

All sorts of gadgets and modules are at work the minute you turn your back. What runs these gadgets? Well, the battery of course. These accessories drain your battery when the car is not running, and your alternator cannot replenish the charge. These current drains…when you add them all together, are referred to as parasitic load or parasitic drain and is measured in amps. (Milliamps actually). Cars average between 20 to 50 milliamps on the norm. 

A milliamp, contrary to it’s name is NOT a millionth of an amp. It is 1/1000 of an ampere.

Another way to think of it, and the way you read it on a lot of self scaling multi meters is 

.001 amps or 1 milliamp. This is why it’s “very “important to test and rate a battery for amp hours as well as cold cranking amps. 

All the accessories that are not related to driving the car like engine and tranny are mostly powered by the BCM (Body Control Module). Radio station and power settings memory, clocks and timers, burglar alarms, and  some video systems are the parasitic loads that remain on a battery the entire time a vehicle is parked. Other accessories like computer monitoring systems, interior and exterior lighting, and power seats and locks are parasitic loads that remain on for a short time after the vehicle is shut off, then power down or “time out”.

The BCM is what controls the interior lights on most newer vehicles. Thats how manufactures get them to stay on for a short period of time, after the vehicle has been shut off, then slowly fade out. This enables engineers to design an entire plethora of functions and programs and system monitors, after the vehicle has been shut off.


What do you do when your battery goes dead when it sits all night, or when you don’t drive it for the whole weekend? How do you know if it’s the car draining your battery, or the battery just isn’t strong enough to hold the charge (specifically in cold weather).

Or even worse yet: what if you don’t know you have a parasite, because it’s not a real serious one. Maybe the car still starts up every morning, but the battery is low enough to make the alternator strain and run hot every day as you’re on your way to work, or drop the kids off at school. Alternators batteries and starters fail at very early stages when subjected to this situation. Nobody wants to change an alternator or battery every 6 months.


If your car seems to drive and run fine, but you’re having a dead battery after the car sits for a while, first let’s determine if you have a drain problem or a battery problem :  You’ve driven your car as you normally do, or at least 20 minutes. Shut the car off, un hook the negative battery terminal. Wait 10-12 hours. Hook the battery back up and start the vehicle. If the vehicle won’t start, you have a bad battery. (it won’t hold a charge). 

If the vehicle starts and operates as normal, you have a parasitic load or drain problem.

The only right way to move forward without a professional auto repair facility is with a low scale inductive amp meter (capable of measuring below 1 amp). You can also use a shunt and a voltmeter but you have to have the shunt in place before running and shutting off the car.

1. Run the car 20 minutes, shut it down, take the key out of the ignition. Leave battery cables on.

2. Place the inductive probe over one of the battery cables and lay it so it can be readily  observed. You may observe readings of 2-3 amps or higher. 

3. Open a door, you should see the amps go higher. Close the door, you should see the amps  drop back down.

4. Wait 30 minutes

5. Re calibrate the amp meter. (if applicable) You should read 20-50 milliamps or .02-.05 amps DC. If you have readings higher than that, something has an excessive drain.


1. Take a digital close up picture of the fuse box with the lid off. If you get confused or interrupted, you can always refer to the picture for help getting the fuses back exactly where they came from. Many fuse boxes have diagrams on the inside cover, we still highly recommend a picture in case any aftermarket modifications have been made.

2. Remove ONE fuse. CLOSE THE VEHICLE DOOR AND WAIT 1 MINUTE or however long your dome light typically stays on. Observe the amp probe. Did the drain drop off? If not, re install the fuse and move to the next fuse. Be sure to follow an organized pattern, for example moving from top to bottom and left to right, so all fuses will be systematically removed and replaced ONE at a time. 

3. When you find the fuse that eliminates the drain, that’s the circuit that is causing the problem. You must consult a wiring diagram to make a decision as to what component(s) or circuits(s) to suspect. Many are controlled by the BCM. 

4. This simple process of elimination can be continued within the circuit of the fuse. For example if you find the tail lamps fuse is causing the drain. Go to each tail light and unplug one at a time to determine which lamp or circuit is causing the drain.

5. Also the alternator wires can be disconnected and observed.


Always initially suspect aftermarket add ons like stereo and burglar alarm.

Also note any previous accidents or fender benders as wires or body shop repairs could hinder electrical systems.

Inspect all doors and hatch backs for crimped or damaged wires.

Inspect all trunk light and hood light switches for operation and mounting stability.

Note any vehicle history of blown fuses or electrical accessory malfunction.    

Also remove and suspect flasher relays and accessory relays.