Fixing parasitic battery drain is a pretty straightforward process once you know the basics. After reading this article, you will understand what a parasitic battery drain is and how to pinpoint the cause.
What is parasitic battery drain?
“Parasitic battery drain refers to a vehicle’s battery slowly losing its charge when the engine is turned off and the car is sitting for extended periods.”
Battery drain can be caused by various factors such as faulty electrical components, wiring issues, or accessories that continue to draw power when they shouldn’t.
What causes parasitic drain?
A parasitic battery drain happens when the car is off, but a component stays on that shouldn’t (the parasite.)
Depending on how large the draw is and the overall condition of the battery, it can take a week or hours to kill the battery.
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A good battery maintainer can prevent this for cars that sit over the winter. For your everyday transportation, however, this can be a nightmare.
Common causes of battery drain
- Interior lights
- Glove box lights
- CAN network issues
- Bad ECU
- Shorted alternator
- Internal battery failure
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.
Diagnosing battery draw can be expensive, 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…
Start with the basics first
Battery and the alternator condition.
A battery load test will determine if the battery still has life and cranking capacity.
Testing the charging system will let you know the alternator is functioning properly.
These two things need to be pinned down first, before anything else to avoid a lot of wasted time and money.
If you don’t have access to a load tester, don’t worry, many chain stores such as Autozone, Pepboys, or O’Reilly’s will test batteries and charging systems for free.
If not, this load tester is a very affordable option that will work perfectly.
After the integrity of the battery and charging system are determined, we can 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 the ground will have to run through the meter where it can be measured accurately.
Check out the video below …
Battery drain is typically measured in the milliamps scale.
A power draw will show itself on the digital multimeter. This is how we can determine how large the draw is, and also tell us when we fix the issue.
Additionally, digital multimeters are handy tools for diagnosing battery drain, as they can indicate whether there is too much draw on the batteries.
This information is crucial in diagnosing and solving the issue of parasitic battery drain.
What is an acceptable parasitic battery drain?
20 to 80 milliamps is an acceptable battery draw.
Anything higher than 80 milliamps can be the cause of concern, however, the draw will need to be higher than this to cause issues in a car that is driven daily.
How to set up the multimeter properly
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, your meter will blow the internal fuse inside.
You have just smoked your meter fuse, if your meter is a cheapo, you have smoked your meter.
Take a look at the picture below of the Fluke 88, notice how there is an MA port and an AMP port. You always want to use the AMP port and then select the correct range on the meter to avoid overloading the tool.
What is a CAN network?
CAN stands for “controller area network”
In a nutshell, modules communicate with each other over this network.
In essence, the massive amount of wires needed to operate older vehicles are now gone. The electrical circuits are now located within the modules themselves.
This is an attempt to use less wiring and make things faster and more efficient, vehicle manufacturers developed this network and it works well.
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.
How to work around the CAN network properly
You need to think like a technician, prepare, and plan.
Anytime you connect or disconnect the battery, or disturb the car 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. Obviously, this can make finding a battery drain very difficult.
Opening a door to get to a fuse box, or pulling a fuse from a circuit is going to wake it up. If this happens you will 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 needing to open doors or the trunk.
- Hook up the meter to see the live draw reading.
After the vehicle has been sitting for some 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.
Here is a basic example of how to find a draw once the meter is hooked up and the car is ready.
Here’s the scenario…
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 1290 milliamp drop, but we are looking for a normal reading of 20 to 80 milliamps. If this vehicle sits for a few days, the battery will most likely be dead.
This is why it is so important to check for battery draw, including any parasitic draw occurring within your vehicle’s electrical system. A normal reading typically ranges between 20-150 milliamps; anything higher may indicate an issue with excessive drain affecting your 12V battery’s lifespan.
Moving on…
In our example, we have let the car go to sleep long enough and we can see the main consumers are shut down.
If you were to watch the meter the whole time you would see the network drop-down in stages during this period.
We record a 158-milliamp draw. The parasite is still drawing a little too much power out of the battery.
Check 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 you can get in the car 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 …
Find anything? … For this example let’s say NO
There are a few ways to deal with this.
- Remove fuses one at a time and see when the draw drops down on your meter.
- Use the amp hound
Option#1 Remove the fuses one at a time
This is just as it sounds, you pull fuses one at a time to see what fuse makes the draw drop down to acceptable levels on the meter.
Then find a wiring schematic and trace everything on that particular circuit the fuse feeds until you find the greedy component.
It’s good and it works, in the old days this was the standard way, the problem is newer cars.
Every fuse you pull will wake the network up. 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 a good option, but not very practical.
Option #2 The amp hound
The amp hound is a game changer when it comes to diagnosing electrical draws. It allows you to see how much current is flowing through any fuse WITHOUT needing to remove it!
This is an unbelievable time saver, every mechanic needs one.
The tool will tell us when it has made contact with the fuse first.
If it beeps steadily we move on to the next fuse. 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.
Our Pick
Cal-Van Amp Hound
This tool is a game changer when it comes to finding electrical draws.
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. I only look for the amp hound to sense amperage draw, I always take the reading on the Fluke as the actual draw.”
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.
How to test many circuits at once for battery drain
One quick, more advanced trick is, wrapping an amp meter clamp around a wiring harness coming out of a complete fuse box.
This will narrow down what fuse box is feeding the battery drain. By using the meter reading from the amp meter clamp, you can target vehicles with multiple fuse boxes and determine if you are in the right spot to start looking for DC battery drain.
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.
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Wrapping up
This article should have shined some good light on parasitic battery drain. Knowing the fundamentals and the procedures professional mechanics use will help you find and fix battery draw like a pro!
