How To Test Your Car Battery For A Parasitic Battery Drain Leak

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Published: 06th March 2012
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If you wake up in the morning to find your car battery dead, your car might be suffering from a parasitic battery leak. When a car battery goes “dead” overnight, there are typically 2 reasons: the battery is very old and needs to be replaced, or you left something in your car on, like a light. If neither of the previous 2 options are true, then something else is drawing power from the car’s battery. Car professionals call this a “parasitic draw”, and it can cause the same “dead” battery as leaving the car’s headlights on overnight.

“What exactly is parasitic draw?,” you’re probably asking now. All vehicles draw some power from their batteries, even when they are shut off, because some car accessories (clocks, radios remote door openers, alarms) always need a power source. The normal power used when a vehicle is off is called the “parasitic draw.” This is a very low draw, so that it does NOT run the car’s battery down. NOTE: Some car owners install additional car accessories, such as shortwave radios, and plug in things like invertors & portable refrigerators. Then, they operate these too long without the engine running, which can drain a car’s batteries so low it’s engine won't start. An electrical short in the vehicle or a malfunctioning accessory can draw a much higher than normal load, resulting in a drained battery. For example, simply leaving a car door open all night, with the inside lights on, could drain the battery “dead.”

The first step in accessing the cause of your “dead battery” is to test for a major short. To do this, remove the positive and negative cables from the batteries, and put an Ohm-meter across them. If it’s reading is close to 0 Ohms then the battery has a direct short. You MUST trace the nature of the short before you can perform these tests.

Tools You'll Need For This Job:

Safety goggles & gloves
A digital multimeter or ammeter
capable of reading up to 10 amps DC for $15-$50.
A fuse puller or pliers.
All tools needed to access battery and fuse panel(s).
The Owners' Manual showing the car’s electrical circuits.

Steps To Locate & Repair A Parasitic Drain

A “parasitic drain” is when an electrical device uses battery power, even though the car is closed with the ignition removed. So, it’s VERY important, when performing the steps below, to make sure ALL LOADS ARE TURNED OFF: dome lights, under hood light, trunk light, empty cigarette lighter, keys removed, etc.. Once all power sources are off, the 1st thing you must do is PUT ON your SAFETY GOGGLES & GLOVES.

Carefully remove the negative side battery cable from the negative battery terminal. Disconnect the thick positive (Red) cable that goes down to the starter. Connect the black wire to the “com” input, and the red wire to the “10A” or “20A” input, on the multimeter. Your battery must have a reasonable charge (not be completely dead) for this test to work. IE. If your dome light operates, the battery has enough charge in it. If the battery does NOT have enough charge left, put a charger on the batteries overnight. For this test to work, the meter must read at least 2-5 amps . NOTE: DO NOT connect the red wire to the “mA” input on the multimeter, as this can damage the meter.

Using the multimeter’s instructions, set the dial on it to measure Amps. Make sure your meter is set to the 10 amp DC range. Some meters have a special connector for the red probe when you are reading current. If the meter has one jack for high amps and one for low amps, make sure the meter end of the probe is in the right connector. Connect the multimeter between the negative cable and the negative battery post. Connect the negative probe to the red cable that is still connected to the vehicle. Make sure this cable and your probe do not touch ground. Wait for the car to go into sleep mode (a few seconds to several minutes). NOTE: when you make contact with the ammeter, the cars computer systems "wake up.” Within a few minutes, they systems go back to "sleep” again.

Do this next step by holding the meter probes to their respective contact points (there should be no shock from only 12 volts). You can use probes with alligator clips to hold them in place. Connect the positive probe to the battery - either battery is fine, since they are connected together by the black, negative wires. (Polarity doesn't matter on digital meters because they are autosensing.) If the ammeter reads above 25-50 milliamps, something in the car is using too much battery power, resulting in a battery drain. A severe current draw (above 10 amps) will either pop a fuse in your meter or destroy it. So, you MUST TEST FOR A SHORT. Otherwise, your meter should now be reading the current drain on your battery. If the vehicle has an alarm system or remote locks, the power draw may still be around 2-3 amps for a few minutes, even after the doors are all closed. This is normal, but wait at least 20 minutes after the last door is closed before you take a reading. If everything is normal, the meter will read less than 35 milliamps (.035 amps). If the current drain is higher than that, find out what is draining your batteries.

Find the car’s fuse panel (under the dashboard). Remove each fuse, one at a time. The purpose of this is to observe the ammeter after pulling each one. Start by pulling fuses until the load goes away. If doing this does NOT reduce the battery draw, look for a wire that is corroded or frayed. NOTE: Main fuses have higher amp ratings, so pull those last. Repeat this step for all relays in the fuse panel as well. If relay contacts fail to release, this would also result in a battery drain.

Watch the ammeter for a drop to an acceptable drain. The fuse that reduces the drain considerably is the battery draw. Consult the car’s owners' or service manual to locate what circuits are powered by that fuse.

Check each circuit connected to that fuse individually. IE. Turn off each light, heater, etc. until you find the drain. NOTE: If there are several shorts in the vehicle, it may take awhile to do these tests.
Once you find and repair the car battery’s “parasitic drain,” repeat steps 1&2 to test the repair. The ammeter will read the exact numbers.

Final Notes & Warnings For All DIY Car Enthusiasts

Check in the car’s cigarette lighter and power sockets for coins that might fall in and short circuits. After-market alarm systems may make testing too long and LOUD. In that case, you MUST seek professional help to fix the car’s parasitic battery drain. In many models later than 2003, disconnecting the battery resets the PCM, which requires the modules to relearn. Sometimes, this requires a factory scan tool. Take these cars to a dealer or auto electrical systems professional.

To do a parasitic draw test in a dual battery system, when you have an electric winch, rewire the truck temporarily as two separate circuits, one to the winch and one to the truck. Each must then be checked independently for shorts. Always use caution when working with any cars electrical system. You must BE SURE that the changes made to the electrical system of any vehicle are within it’s boundaries. IE. All fuses are back in the right places, and with their correct amperage, when adding or changing anything electrical.

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