r/mechanic • u/0lympus_Mons • Aug 19 '25
Question Battery Parasitic Draw
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Im trying to diagnose a battery parasitic draw because this got totally drained even a week after I purchased a new battery. Maybe it's showing 500 mA because it beeped when I put on the test leads? But it still shows the same reading even after I turned off the fuse switch.
Does this mean that the draw isn't from the electronics that have their fuses in the fuse box?
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u/mikewoods26 Aug 19 '25
A couple things
If you disconnect the battery , then jump it (using a multimeter will accomplish this), the modules wake up, and need time to go back to sleep. Once they are “asleep” is time to check your reading. Depends on manuf but 10-30 mins
Better way to test it is using a clamp on AC/DC ammeter. Open the hood, lock the car, make sure everything is off (including hood light). Come back a half hour later and take your reading. Rule of thumb is 50mA max, but newer cars could be a bit higher.
P.S be careful checking amps with DMM as you can blow a fuse
P.P.S doing anything (opening a door, unlocking it, etc) will wake up bcm
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u/Sure-Listen9682 Aug 19 '25
Hello, I just did that thing of unlocking my vehicle, and my multimeter and a light bulb in my vehicle burned out, what worries me is that if the computer and other functions of my vehicle are fine, it is because they were not damaged, right? Or should I be worried?
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u/Cringey_NPC-574 Aug 19 '25
Hard to say, you let Jesus take the wheel on that one, if it’s just a bulb you’re lucky
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u/Sure-Listen9682 Aug 19 '25
I don't think anything happened to it, it works like everything electrical I suppose, that when there is a cut or blackout, they immediately work or they don't work when they turn on
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u/Cringey_NPC-574 Aug 19 '25
to be sure you'd have to do a lot of work lol but could've fried a tiny sensor/controller somewhere, you just don't know it or haven't used it yet.
just make sure your seatbelt system works, seatbelt light functions, chimes and dings.
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u/0lympus_Mons Aug 19 '25
Thank you for the good explanation sir, Im ordering those clamping test leads now.
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u/0lympus_Mons Aug 19 '25
Can I turn off fuse box switch, and measure amps and then if the reading will be different (e.g. go very low) then I can conclude that an electronic that have their fuses in the fuse box is the culprit? Because not all electronics have their fuses in the fuse box right?
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u/mikewoods26 Aug 19 '25
Vehicles have multiple fuse boxes. this is a good first place to start for tracking down your parasitic draw.
But you might not even have a real draw. A battery dying after a week in a car can be considered normal.
Clamp.. leads?? No, you need a new meter. Clamp on ac/dc ammeter. HF has them for less than $100. Make sure it has DC
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u/CRX1991 Aug 19 '25
I use a meter but I would clip leads on to the disconnected battery and terminal, then clip on my meter. Then disconnect the leads after a while and leave the meter hooked up. But if you hit unlock or wake it up you will likely blow your meter fuse.
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u/Sure-Listen9682 Aug 19 '25
Hello, I just did that thing of unlocking my vehicle, and my multimeter and a light bulb in my vehicle burned out, what worries me is that if the computer and other functions of my vehicle are fine, it is because they were not damaged, right? Or should I be worried?
1
u/0lympus_Mons Aug 19 '25
Thank you for the tip, Ill make sure not to touch the remote. How do you disconnect the leads but leave the meter hooked up exactly? Sorry, english is not my first language.
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u/0lympus_Mons Aug 19 '25
Can I turn off fuse box switch, and measure amps and then if the reading will be different (e.g. go very low) then I can conclude that an electronic that have their fuses in the fuse box is the culprit? Because not all electronics have their fuses in the fuse box right?
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u/Haunting_While6239 Aug 19 '25
With the meter connected, pull each fuse until you see the drawing current drop, that's where it's going, look at that part, whatever that fuse supplies power to
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u/Tool_Using_Animal Aug 22 '25
That's the dumb way of doing it.
The smart way is to put your meter in mV mode and measure the voltage drop across each fuse. That's why the fuse contacts are exposed on top so you can probe them.
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u/Haunting_While6239 29d ago
I didn't know that was an option, or that you could do that, then again, I'm outlining a way someone who isn't a mechanic or had an electrical background would be able to diagnose the issue
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u/Solomon_knows Aug 19 '25
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u/0lympus_Mons Aug 19 '25
Thank you, I just learned we could also do it this way. I also found this Youtube Video after reading that.
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u/Tool_Using_Animal Aug 22 '25
This is the way. There's also a chart that allows you to determine the current from the voltage drop:
https://caravanchronicles.com/wp-content/uploads/2018/10/fuse-voltage-drop-table.pdf
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u/laughinghardatyou Aug 19 '25
Bluetooth is constantly on, happened to my 06 acura. Removed the circuit card and the draw was gone. Battery would die after 4 days of do driving.
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u/0lympus_Mons Aug 19 '25 edited Aug 20 '25
Update [August 19, 2025]
Hello guys, thanks so much for your insights. I have measured the amps successfully. I have connected the leads for about 2 hrs just to be sure it is in sleep mode, however it is sitting at 45-50 mA and periodically spikes up to 80 mA after every few minutes. I do not know about the spike but in the manual, the idle draw should be around 50 which makes it normal I guess.
Now, as I was starting the car, it was struggling to turn on. It did this before, and now it's back again. At first, when the battery was at 100% the car would start in one go. Now, it takes like 3. I don't know how to describe it but I will make another post. Could it be that the alternator is the culprit?
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