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 have the same result as leaving the headlights on: a dead battery in the morning.
Steps
- Remove the negative side battery cable from the negative battery terminal.
- Connect the black wire to the com input on a multimeter and the red wire to the 10A or 20A input on the multimeter. The meter needs to be able to read at least 2 or 3 amps for this test to work. Connecting the red wire to the mA input on the multimeter won't work and could damage the meter.
- Attach a multimeter (set the dial on the multimeter to measure Amps as per multimeters instructions) 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 ammeter, the car's computer systems "wake up." After a bit of time they will go back to "sleep."
- If the ammeter is reading over 25–50 milliamps, something is using too much battery power.
- Go to the fuse panel(s) and remove fuses, one at a time. Pull the main fuses (higher amp ratings) last. Perform the same steps on all relays found in the fuse panel. Sometimes relay contacts can fail to release, causing a drain. Be sure to observe the ammeter after pulling each fuse or relay.
- 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.
- Check each device (circuit) on that fuse. Disconnect each lamp, heater, electrical device one at a time to find the drain.
- Repeat steps 1 & 2 to test your repair. The ammeter will tell you exact numbers.
- Try unhooking the big wire from your alternator. The alternator can sometimes have a shorted diode that can cause amps to flow through the alternator's power cable and through the shorted diode and into the case and through the bolts and back to the negative battery terminal. This will drain a battery in a hurry.Make sure to read the ammeter before and after unplugging your Alternator.
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.
- The fuse pulling sequence can be more easily performed with the ammeter resting on the outside of the windshield facing the technician at the inside fuse boxes thru the glass, with the under-hood ammeter probes clamped in place by plastic clamps.
- The fuse pulling sequence may be too complicated without a helper: a quick elimination of the problem might be to reconnect the battery and go through all switches and devices including radio switches, window switches, rear windshield wiper switches, and on/off every electrical switch. Any of them may have an intermittent electrical leak, such as a windshield wiper that has not returned to its home/off position. But when you set the ammeter back up to check for success, you won't know what switch or device or wire was the culprit.
- Don't forget to check inside cigarette lighter and power sockets. Living a phone charger plugged in for a long time can cause parasitic draw. Also, sometimes coins can fall in the sockets and cause shorts.
- 50 milliamps is a good rule of thumb for max allowable parasitic draw. Anything over 50 milliamps would require further investigation into the exact device which is drawing.
Warnings
- Exercise caution when working with your car's electrical system. Protect eyes and skin. Similarly, be certain, without any doubt, that changes made to the electrical system are within the boundaries (fuses in place and w/correct amperage) when adding or changing anything electrical, whether aftermarket or OEM).
- Be careful working around the battery in a car.
- 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.
- In more and more models made after 2003, disconnecting the battery will reset the PCM requiring the modules to relearn. In certain cases this requires a factory scan tool. It is best to take such cars to either the dealer or a professional in auto electrical systems.
Things You'll Need
- Safety goggles
- A digital multimeter or ammeter.
- A fuse puller. (You can also use pliers, but try not to crush the fuse.)
- Any tools needed to access battery and fuse panel(s).
- An owners' manual or maintenance manual showing electrical circuits.
