The Ultimate Guide To Testing Parasitic Battery Drain
Checking parasitic battery drain is a straightforward process that needs to be done when a car’s battery keeps dying over a short period of time while the vehicle is sitting. After reading this article, you will understand what a parasitic battery drain is, and how to pinpoint the cause.
What is parasitic battery drain?
A parasitic battery drain is an electrical draw that slowly kills a car’s battery while the vehicle is not in use.
If you experience a dead battery after your car sits for an extended period, such as overnight you most likely have a parasitic battery drain.
What are some things that cause battery drain?
All of the electrical components in a car rely on the battery to provide enough voltage to keep the car running and the accessories working.
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A parasitic battery drain happens when the car is off, but a component stays on that shouldn’t (the parasite.) Without the engine running and the altenator charging the battery back up, you end up with a dead battery after a while. Depending on how large the draw is and the overall condition of the battery, it can take a week or hours to totally kill a battery.
There are many things that can cause a parasitic battery drain
- Interior lights
- Glove box lights
- CAN network issues
- Faulty ECU
- Shorted altenators
Anything from a glove box light staying on, to CAN networks preventing cars from “going to sleep” will cause issues. Keep in mind that diagnosing battery draw can be expensive and time consuming when you bring it to a pro, that’s why in this article I’ll show you the basics so you can tackle this problem yourself, before spending big money at the shop.
Let’s get started…
Rule out the basics first
Two things need to be confirmed before chasing anything else, the battery and the alternator condition. A load test will determine if the battery still has good life and cranking capacity and testing the charging system will let you know the alternator is functioning properly.
Chances are if you are reading this you don’t have access to a load tester, don’t worry, there are many chain stores such as Autozone, Pepboys, or O’Reilly’s that will usually test batteries and charging systems for free. If not, this load tester is a very affordable option that will work perfectly.
For the purpose of this article, all of our tests came back OK. The battery is good and our alternator is charging properly, let’s move on.
How to test for parasitic battery drain
To check the parasitic battery drain we need to put a multimeter inline between the battery negative terminal and the negative battery cable end on the vehicle.
Putting the meter in line forces any amperage leaving the battery to flow through the meter so it can be accurately measured, any amperage that flows from the battery to ground will have to run through the meter where it can be measured accurately.
Check out the video below …
What scale is parasitic battery drain measured in?
Battery drain is typically measured in the milliamps scale, if there is something staying “awake” or turned on when it should be off, it will show itself by the amperage draw shown on the meter. This is how we can determine how large the draw is, and when we acually fix the issue.
What is an acceptable parasitic battery drain?
20 to 80 milliamps is an acceptable battery draw, 80 being a little on the top side of acceptable.
An important note about multimeters
Always plug your red lead into the AMPS scale on the meter, not the MA, or milliamps scale. There is an important reason for this. Even though we are reading the draw in milliamps the AMP scale will protect the meter from overloading.
The meter itself has fuses, these are to protect against overload. On the amp scale, you are fused up to ten amps. The Milliamp scale is only fused to 400 Milliamps.
Why is this important?
If your meter is hooked up in the milliamps port and there’s a big spike in amperage draw, possibly a door opened, or any other large unexpected draw occurs, the milliamp side of your meter will blow the internal fuse inside of the meter to protect it. You have just smoked your meter fuse, if your meter is a cheaper model, you have smoked your meter.
Take a look at the picture below of the Fluke 88, notice how there is a MA port and an AMP port? You always want to use the AMP port and then select the correct range on the meter.
By hooking our lead into the amp port, we are protected to 10 AMPS. We can then use the range button to step down to Milliamps for our reading.
What is a CAN network?
CAN stands for “controller area network” In an attempt to use less wiring and make things faster and more efficient, vehicle manufacturers developed this network.
In a nutshell, the modules communicate with each other over this network. In essence, the massive amount of wires needed to operate older vehcles are now gone. The electrical circuits are now located within the modules themselves.
Each module “talks” to the next in line over the CAN network. This means if one module has an issue it may not shut down certain circuits in the vehicle after a shutdown, leading to a parasitic battery drain.
Can networks is another topic in itself, I will go into depth on in a different article about CAN networks.
For now, we need to think like a technician, we need to prepare and plan. Realize anytime you connect or disconnect that battery you are going to “wake up” this network.
If you open a door or disturb the vehicle while working this network line wakes back up and your initial startup draw will be very high because all of the modules will come online and start communicating with each other.
Depending on the vehicle you will have to wait up to twenty minutes for the network to shut down.
This can make finding a circuit draw very difficult. Open the door to get to a fuse box, or pull a fuse from a circuit, it’s gonna wake up. Then we have to wait for the entire line to shut back down, (which can take 20 minutes and up) It will be almost impossible to find the draw this way without going nuts.
How to get set up for a battery drain test the right way.
To set up a parasitic battery drain test properly start by opening all the vehicle’s doors and trunk, then simply roll all of the latches closed, and do the same for the hood.
Make sure you can get in the car and test circuits without “waking” up the CAN network. Hook up the meter to see the live draw reading.
After the vehicle has been sitting for a period of time (up to 20 minutes) press the min-max button on the Fluke meter, this will start recording the draw to give you the information you need.
It is entirely possible that the can network is not going to sleep due to a faulty component, this can be determined by letting the vehicle sit for about a half hour, (give or take depending on the vehicle.) If you come back and your draw is very high this could very well be the case, but we have to rule out any other possible causes.
An example of how to test for battery draw.
Our meter is inline, let’s take a look …
After sitting long enough this vehicle’s battery draw has dropped from 1448 to 158 MA, this is a good indication that the network has gone “to sleep.”
This is a big drop but remember, we are looking for 20 to 80 milliamps. If this vehicle sits for a few days the battery will most likely be dead, this of course depends on the condition of the battery. This is why it is so important to check for these draws.
If you just replace a battery because the old one tested weak, the new battery will most likely take a little longer to discharge, but it will with a battery draw that high, it will kill a new one, it will just take longer.
Moving on…
In our example, we have let the car go to sleep long enough and we can clearly see the main consumers are shut down. If you were to watch the meter the whole time you would actually see the network drop-down in stages during this period. Now imagine we record a 158 milliamp draw, we are still drawing a little too much power out of the battery.
Checking the basics.
Do a quick visual inspection, remember how we opened up the doors, hood, and trunk and then rolled all of the latches closed? Now go into the vehicle and look around without waking anything up. Avoid pulling on a door handle or opening the hood, anything that will disturb the network must be avoided.
Make sure obvious things are not staying on, overhead lights, car chargers, glove box lights …
Nothing obvious? What’s next, what’s eating that power?
There are a few ways to deal with this. The first way is to remove fuses one at a time and see when the draw drops down on your meter.
Then you would find the wiring schematic and trace everything on that particular circuit until you find the greedy component. It’s good and it works, the problem is with newer cars every fuse you pull will wake the network up again. You will then have to wait for it all to go down again before you see if you even got the right fuse. If you have forever this is good, but not very practical.
Next, we can use the amp hound, with this tool we are able to test each individual circuit via fuses, without removing any fuses. This allows us to pinpoint the fused circuit that is feeding our parasite.
Here is how it looks in action.
The tool will tell us when it has made contact with the fuse, if it beeps steady move on. When it gives you multiple beeps it’s telling you that the fuse you are in contact with is feeding something, there is amperage flowing through the fuse indicating a component is pulling power through it.
This starts to narrow things down a bit. You can see exactly how much power that fuse is feeding out.
The Amp hound is reading a draw at the 10 amp fuse I’m testing in the picture above.
“The amp hound is not a Fluke 88 so really I only look for the amp hound to sense amperage draw, I always take the reading on the Fluke as the actual draw.”
Think of the amp hound as the whistle-blower here, not a meter.
Running down every fuse you will find some that have a slight draw and some that have more draw, this is where I print out a schematic for the vehicle’s power distribution and start making notes on all of the fuses.
Write “OK” next to no draw fuses and write “check” on the ones that give a reading. Remember we can’t just start yanking out the fuses that have a slight draw yet, we have to lay down the strategy first. There are many much more complex issues with electrical diagnoses. I’m trying to keep this very basic. I will do a post on more advanced diagnoses for electrical later on. This includes lab scopes and more complex setups.
How to test many circuits at once for draw.
One quick, more advanced trick is you can do is, wrap an amp meter clamp around a wiring harness coming out of a complete fuse box. This will narrow down what fuse box is feeding our battery drain. This helps in targeting vehicles with multiple fuse boxes, then you know you are in the right spot to start looking.
It’s all about breaking it down into smaller pieces until there is only one circuit left.
I checked all of the other fuses. This ten-amp fuse is the highest draw on our amp hound, it’s location is number 15 in the engine compartment fuse box. Let’s see what it powers…..
Pull out that fuse, and make sure you mark the location so we don’t mix anything up if we end up having to remove more than one fuse. When the fuse is pulled the draw has dropped down to acceptable limits, all that is left to do is look at our wiring and trace out all points that are fed by that fuse.
In this case, the glove box door was warped, causing the door to shut but not fully in contact with the switch, leaving the glove box light on.
