How to perform a parasitic battery drain test (2024)

Step-by-Step Guide to Locating the Cause of a Parasitic Battery Drain

As a retired ASE Master Technician, I know firsthand that few things are more frustrating than dealing with a dead car battery, especially when it seems to happen without any warning. One common reason for this issue is parasitic battery drain, which occurs when an electrical component or system continues to draw power even after the vehicle is turned off. Left unchecked, this persistent power draw can deplete your battery, leaving you stranded. Let me walk you through the causes, symptoms, and most effective solutions for diagnosing and addressing parasitic battery drain.

There are two ways to locate the cause of parasitic battery drain:

Current draw testing— This test is done by connecting a current measuring device on the negative battery cable and removing fuses one at a time until the current draw drops, thereby identifying the problem circuit. I prefer to use a low-amp clamp probe to measure current, however, you can also use a multimeter connected in series with the negative battery cable.

Voltage drop testing— I use a multimeter to check the current draw on each fuse to locate the fuse that’s drawing power when the engine is off. Voltage drop testing is just as effective as current draw testing and it’s easier to perform because I don’t have to remove the fuses. Scroll down to see how to conduct parasitic battery draw using a voltage drop testing method.

How to Perform Current Draw Testing for parasitic battery drain

In a current draw test, I connect a low-amp probe to the negative battery terminal. Then I wait until all the modules have entered low-current sleep mode. Then I remove fuses, one at a time while watching the current draw. I look for a large drop in current to identify the circuit drawing the current. Once I identify the problem circuit, I refer to the wiring diagram for that circuit to further test each module on that circuit.

I’ve found two big problems with current draw testing

#1 Pulling fuses cause problems

The biggest problem with current draw testing is that once I’ve pulled a fuse, I can’t reinstall it until I’m completely done with my testing. Here’s why. If I reinstall the fuse, that’s wake up any modules on that circuit, causing me to start the time-out period all over again.

To avoid this, I take a picture of the fuse box so I’ll know where each fuse goes. Then, I pull the fuses one at a time and leave them out until I’ve tested all circuits and identified the circuit causing the draw.

#2 Using a multimeter connected in series can be an issue

To do a current draw test with just a multimeter, I install the meter in series with the negative battery terminal. The instant I make the last connection, the reconnected power wakes up all the modules in the vehicle and that can draw more than 10-amps; which will blow the 10-amp fuse in my multimeter.

In addition, some vehicles will activate the alarm system as soon as I disconnect the main battery. So the alarm will be blaring as I try and connect your meter in series. When I finally get it connected, the current draw can be more than 10-amps, which still blows the fuse in my multimeter.

For all those reasons, I prefer using an amp clamp for current draw testing.

The best way to conduct a current draw test is with a low amp clamp. You can get a free-standing probe or an accessory probe that connects to your multimeter. Make sure it’s capable of reading milliamps. Low amp clamps are available as free-standing units or add-on devices that connect to your existing multimeter

You can also use the amp reading feature on your multimeter. But there’s a downside to this approach. The multimeter amp fuse is usually only 10-amps and it can blow if you exceed that current draw. If you accidentally wake up a module during the test, you’ll probably blow the fuse. If you choose this method, buy extra fuses to keep going.

How to set up a multimeter for current draw testing

• Disconnect the negative battery cable
• Set the multimeter to the lowest AMP setting
• Move one lead to the AMP socket on your meter
• Connect the other lead to the COM socket on your meter
• Connect one lead to the battery NEGATIVE post
• Connect the other lead to the negative battery terminal

How to conduct current draw testing after your multimeter is all set up

• With the key out of the ignition, let the vehicle sit for 15-45 minutes to allow time for all computers to enter “sleep” mode.
• Note current draw. If the draw is above 50Ma, you have something drawing power.
• Using your cell camera, take a picture of the fuse box so you can reinsert the fuses back into the proper slots.
• Remove one fuse at a time until the current draw falls below 50Ma. Do NOT put the fuse back in until you’ve completed all the testing. Inserting the fuse can cause that circuit to power up the modules on that circuit. That would force you to wait until the circuit times out before proceeding to test the other fuses.

The fuse that causes the drop is the circuit causing the problem

How to use an amp probe to conduct current draw testing

• Leave battery terminals connected to the battery.
• Connect the low current amp clamp to the negative battery cable.
• With the key out of the ignition, let the vehicle sit for 15-45-mins to allow time for all computers to enter “sleep” mode.
• Note current draw. If the draw is above 50Ma, you have something drawing power.
• Using your cell camera, take a picture of the fuse box so you can reinsert the fuses back into the proper slots.
• Remove one fuse at a time until the current draw falls below 50Ma. The fuse that causes the drop in the circuit causing the problem. Remember, if you’re doing a current draw test, you CANNOT put the fuse back in or you will wake up the modules on that circuit.

So the best way to do current draw testing is with a low amp probe.

How to locate a parasitic battery drain using voltage drop testing

In this method, I use a multimeter to check for voltage drop across each fuse. See the testing routine below.

Prepare the vehicle before testing

The computer modules must be in sleep mode before you can conduct a parasitic draw test. But since you’ll need access to the cabin fuse box during the test, I open the door to wake up the body control module. Then I prepare the vehicle using the steps below.

1) Open all the doors, hatch, trunk lid, and hood.

Since many carmakers mount the door light switch in the latch, I trip the latch on each door by sliding a screwdriver into it to simulate the latch hitting the strike. This tricks the body control module into thinking all the doors are closed.

How to use voltage drop testing to locate the parasitic draw

I like this method because I don’t have to pull any fuses or risk waking up any modules.

I use my regular multimeter to test the voltage drop across every fuse. I look for the fuse with the highest voltage drop. That tells me which circuit is causing the parasitic battery drain.

1) I set my multimeter to the millivolt (mV) scale
2) Starting with the megafuses, I connect the multimeter leads to the test point on each fuse. The mega-fuse with the largest voltage drop is the circuit that’s drawing the most power. I then test the smaller fuses in that circuit to find the one with the highest voltage drop. Keep in mind that some fuses have a constant voltage drop because they power keep-alive circuits (radio memory for example).
3) If there is no current flowing through the fuse, the meter will read 0-volts
4) If current is flowing through the fuse, I will see some reading. So I refer to a voltage drop to amp conversion chart to determine current draw
5) Once I determine which circuit is drawing the most power, I don’t pull the fuse to that circuit. Instead, I locate all the devices on that circuit and disconnect the devices one at a time until I see the change reflected in my readings.

©, 2020 Rick Muscoplat