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How-To Find Parasitic Draw.


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Old 02-12-2008, 11:56 AM   #1
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How to Find a Parasitic Battery Drain

When your car battery goes dead overnight, usually either the battery is at the end of its life span, or you left something on, such as a light. Occasionally something is drawing power that’s not of your doing. This is a parasitic draw, and it can cause the same result as leaving the headlights on: a dead battery in the morning.




  1. Remove the negative side battery cable from the negative battery terminal.
  2. Attach an ammeter(this measures amperage) between the negative cable and the negative battery post. wait a few seconds to several minutes for the car to go into sleep mode. i.e. when you make the contact with the test light the cars computer systems "wake up" after a bit of time they will go to "sleep".
  3. If the ammeter is reading over 25-50 milliamps, something is using too much battery power.
  4. Go to the fuse panel(s) and remove fuses, one at a time. Pull the main fuses (higher amp ratings)last.
  5. Watch for the ammeter to drop to acceptable drain. The fuse that reduces the drain is the draw. Consult the owners' manual or service manual to find what circuits are on that fuse.
  6. Check each device (circuit) on that fuse. Stop each lamp, heater, etc. to find the drain.
  7. Repeat steps 1&2 to test your repair. The ammeter will tell you exact numbers.
1. An ammeter meaures current (in amps), and that is what you are fighting against if you have a parasitic drain problem.

2. Harbor freight sells a $3 digital multimeter (dmm) that has an amps setting. I use a Fluke, but it's whatever your trying to spend your dollars on.

Tips:
A parasitic drain is when an electrical device is using battery power when the car is closed, and the ignition key removed. Therefore, when doing this test make sure that the dome light, under hood light, trunk light, etc. are OFF!

Don't forget to check inside cigarette lighter and power sockets. Sometimes coins can fall in and cause shorts.

Some after-market alarm systems may make this test too long or loud to be worth the effort. If that's the case, seek professional help.

Be careful working around the battery in a car.

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