How To Test Parasitic Battery Drain: Find What’s Killing Your Battery Overnight
A dead battery is never a good thing, especially when you have places to go. What’s worse is buying a new battery and finding it’s dead the next day too! When this happens there’s a parasitic battery drain that you need to pinpoint. In this article I will show you how to test parasitic battery drain so you can pinpoint what’s killing your battery and fix the issue yourself.
Checking parasitic battery drain is a straight forward process that should always be done when a battery keeps dying in a short period of time. After reading this article you will have a good understanding of what a parasitic battery drain is, and how to pinpoint the cause.
What is parasitic battery drain?
When driving your car the alternator is constantly keeping your battery charged by supplying amperage to recharge the cells within the battery. A parasitic battery draw occurs when the car is off and sitting, a component stays on that should not be (the parasite) slowly draining the battery until it dies.
There’s many things that can cause a parasitic battery drain, from glove box lights to CAN networks preventing cars from “going to sleep.” Either way if your battery is good and continues to die overnight there is a good chance you have a battery draw. This article will show you how to find it.
Always rule out the basics first
Two things need to be confirmed OK before chasing anything else, the battery and the alternator. A load test will determine if the battery still has good life and cranking capacity, testing the charging system lets us know if the alternator is functioning properly.
If you don’t have access to a load tester there are many chain stores such as Autozone, Pepboys, or O-Reilly’s that will usually test batteries and charging systems for free. For the purpose of this article all of our tests came back OK. The battery is good and our alternator is charging properly.
How to test for parasitic battery drain
To check the parasitic battery drain we need to put a multi meter inline between the battery negative terminal and the negative battery cable end on the vehicle.
Check out the video below …
What we are doing is putting the meter in-line so any amperage that flows from the battery to ground will have to run through the meter where it can be measured accurately.
How is battery draw measured?
A parasitic battery drain is typically measured in milliamps, this reading is the amperage that is being pulled from the battery as the car sits with everything off. As you can imagine, if there is something staying “awake” or turned on in the vehicle when it should be off it will show itself by the amperage draw shown on the meter.
What is acceptable battery drain?
Anywhere from 20 to 80 milliamps is acceptable battery draw, 80 being a little on the top side of acceptable.
A few things here to note, we are inline on the ground side. Always plug your red lead into the AMPS scale on the meter, there is a important reason for this. 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?
Lets say your meter is hooked up in the milliamps port and there’s a big spike in amp draw, possibly a door opened. If you are plugged into the milliamp side of your meter guess what? 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. Before we go any further, let’s talk about a few things. Depending on the vehicle, you may be dealing with a CAN network.
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 before are located within the modules themselves. Each module “talks” to the next in line. This means if one module has an issue it may not shut down certain circuits in the vehicle after a shut down, leading to a parasitic battery drain.
Can networks are another topic in itself that I will go into depth on in a different article. For now we just need to think like a technician, we need to prepare and plan. 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 shut down.
This can make finding a circuit draw very difficult. Every time we open a door to get to a fuse box, or pull a fuse from a circuit it’s gonna wake up. Then we have to wait to see if the draw is lower when things shut back down, this is downtime that we need to avoid. 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, do the same for the hood.
We are just making sure we can get in the car and test circuits without “waking” up the CAN network. Once we have the meter hooked in we want to see the live draw reading. After the vehicle has been sitting for a period of time (up to 20 minutes) is when you want to 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 pretty 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 is depending 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? We can now go into the vehicle and look around without waking any thing up. The point is you don’t want to pull on a door handle so you can start looking for the issue and wake the whole car up again.
Make sure obvious things are not staying on, overhead lights, car chargers, glove box lights …
So we look around and see 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.
The tool will tell us when it has made contact with the fuse, if it beeps steady we 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 on in our example.
“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 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 cant 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, 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 contact the switch, leaving the glove box light on.
This is a reliable and easy way to find parasitic battery drain.
Honestly this was a very simple one to just get the basics out there for you guys. I will definitely get into some real world complex issues on another post.
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Scoot is a Dad, ASE certified mechanic, and blogger. He has been in the automotive business for over 25 Years.
