How to diagnose a discharged car battery

Content

Part 1 of 3: Battery Check
Part 2 of 3: So You Have a Parasitic Battery Draw

It's safe to say that every car owner reading this has probably experienced the fact that when you left your house or walked to your seated car, only to find that the battery in your car was dead. This scenario is very common, but this case is actually different because the same thing happened the day before. You may have had an AAA or a certified mechanic check the charging system and find that the battery and alternator are working properly. Well, there is something electrical in your car that is draining the battery and this is what we call parasitic battery discharge.

So how do we know if you have parasitic draw or if it's really just a misdiagnosed bad battery? If it's a spurious prank, then how do we figure out what's draining your battery?

Part 1 of 3: Battery Check

Necessary materials

DMM with 20 amp fuse set to 200 mA.
Eye protection
Gloves

Step 1: Start with a fully charged battery. Disable or disconnect all accessories installed in your vehicle. This will include things like a GPS or a phone charger.

Even if your phone is not connected to a charger, if the charger is still connected to a 12V outlet (cigarette lighter), it can still draw current from the car battery, preventing it from fully charging.

If you have a modified stereo system that uses additional amplifiers for the speakers and/or subwoofer, it would be a good idea to remove the main fuses for those as they too can draw current even when the car is turned off. Make sure all lights are off and all doors are closed and the key is off and out of the ignition. This will allow you to start with a fully charged battery.

If your car needs a radio or GPS code, now is the time to find it; it should be in the owner's manual. We will need to disconnect the battery, so with this code handy you should be able to take control of your GPS and/or radio once the battery is reconnected.

Step 2 Attach the ammeter to the battery..

You will then need to connect the correct series ammeter to your electrical system. This is done by disconnecting the negative battery terminal from the negative battery terminal and using the positive and negative probes on the ammeter to complete the circuit between the battery terminal and the battery terminal.

Functions: This test can be done on either the positive or negative side, however it is safer to test on the ground side. The reason for this is that if you accidentally create a short circuit to the power supply (positive to positive), it will create a spark and can melt and/or burn wires or components.
Functions: It is important that you do not try to turn on the headlights or start the car when connecting the ammeter in series. The ammeter is only rated for 20 amps and turning on any accessories that draw more than 20 amps will blow the fuse in your ammeter.

Step 3: Reading the AMP Meter. There are several different readings you can choose from on the multimeter when reading amps.

For testing purposes, we will select 2A or 200mA in the amplifier section of the meter. Here we can see parasitic battery consumption.

Readings for a typical car with no parasitic draw can range from 10mA to 50mA, depending on the manufacturer and the number of computers and features the car is equipped with.

Part 2 of 3: So You Have a Parasitic Battery Draw

Now that we've verified that the battery is experiencing parasitic discharge, we can move on to learning about the various causes and parts that may be draining your car's battery.

Reason 1: Light. Electrical devices such as dome lights with a timer and dimming may remain 'awake' and drain the battery excessively for up to 10 minutes. If the ammeter reads high after a few minutes, then you can know for sure that it's time to start looking for the component causing parasitic draft. The usual places you want to look at are areas we can't really see too well, like the glove box light or the trunk light.

Glove box: Sometimes you can look into the opening of the glove box and see if the light is shining through, or if you're feeling brave, open the glove box and quickly touch the bulb to see if it's hot. This may contribute to the drain.
Trunk: If you have a friend on hand, ask them to climb into the trunk. Shut it down, have them check the trunk light and let you know if it's still on. Don't forget to open the trunk to let them out!

Reason 2: new car keys. Many new cars have proximity keys, keys that wake up your car's computer when they are a few feet away from it. If your car has a computer that listens for your key, it emits a frequency that allows you to walk up to the car and unlock and open the door without having to physically insert the key.

This takes a lot of energy over time, and if you park next to a busy footpath, in a crowded parking lot, or next to a running elevator, anyone with a proximity key who accidentally walks past your car will wake up your car's eavesdropping computer. After waking up, it will usually go back to sleep within a few minutes, however, in a high traffic area, your vehicle may experience battery parasitic discharge throughout the day. If you think this applies to you, most vehicles have a way to disable the proximity sensor in the owner's manual.

Reason 3: Other Common Culprits. Other spurious prank culprits that need to be checked include alarms and stereo systems. Bad or poor quality wiring can lead to a leak, which will also require a mechanic to inspect. Even if these components have been installed safely and correctly beforehand, the components themselves can fail and drain the battery.

As you can see, the problem is not always obvious. You may need to find the fuse box and start removing the fuses one at a time to see which circuit is draining the battery excessively. However, this can be a lengthy process, and we highly recommend that you enlist the help of a certified mobile mechanic, such as one from AvtoTachki.com, who can properly diagnose your car's battery parasitic discharge and fix the culprit that's causing it.