A parasitic drain is something using power after the car is off. A healthy car draws a very small current once its modules go to sleep, commonly cited as around 50 milliamps or less. A larger draw kills a battery over days.
Key takeaways
- Small draws are normal.
- Aftermarket parts are common culprits.
- A meter and fuse-by-fuse test finds it.
What a parasitic drain is
Computers and clocks use a little power when the car is off. A drain is any draw larger than that, from a stuck relay, a trunk light, an aftermarket stereo or a faulty module.
Signs
A battery that is fine after driving but dead after two or three days parked. A new battery that also dies suggests a drain or a charging problem.
How we test
We connect a meter in line with the battery, wait for the car to go to sleep and then pull fuses one at a time to see which circuit drops the draw.
Common causes
Aftermarket alarms and stereos, door switches, glove box lights, stuck relays and modules that do not sleep.
Fix
Repair or replace the faulty component or wiring, and retest the draw.
Why modules need time to sleep
Modern cars have many computers, and each stays awake for a while after the car is off. A proper drain test waits until all modules have gone to sleep, which can take half an hour or more on some vehicles. Testing too early gives false readings. That is why a drain diagnosis takes longer than a quick battery test, and why a technician may ask to keep the car for a few hours.
Aftermarket parts as suspects
Remote starters, alarms, dash cameras, amplifiers and phone chargers wired in by hand are common drain sources. If the problem began after an accessory was installed, tell the technician. Often the fix is correcting how the accessory was wired or removing it. If you installed a dash camera with parking mode, check that its low-voltage cutoff is set so it does not drain the battery.
Need help in south Bakersfield?
Questions about auto electrical repair? Read our Auto Electrical Repair page, call (661) 599-9098 or request a time. Peña's Auto Electric, Smog & Repair, 303 Taft Hwy, Suite B, Bakersfield, CA 93307. Mon to Fri 9:30 AM to 5:30 PM, Sat 8 AM to 1 PM.
Frequently asked questions
How much drain is normal?
Around 50 milliamps or less after the car sleeps.
Can an alarm cause a drain?
Yes, especially aftermarket.
Does a new battery fix it?
No, it will drain too.
How long does testing take?
Longer than a battery test because modules need time to sleep.