# Checking Parasitic Car Battery Drain - Multimeter Question



## HayesFJ (Sep 25, 2020)

I have 2006 Lexus SC430 that has not had a lot of use lately. After the car has sat for a week or so, the battery died twice (three month old battery). I thought there might be a parasitic drain on the battery, so I purchased a multimeter to check it. I have no experience with this so I was depending on YouTube and Google for instructions. I disconnected the negative terminal from the battery and connected the black probe from the multimeter to the negative battery terminal and the red probe to the negative battery cable. I put clamps on the door plunge buttons to simulate the door being closed. I set the multimeter to the 10A setting and plugged the black probe to the COM jack and the red probe to the red 10A jack. After a few seconds, the meter settled at 0.15A. I disconnected the red probe from the jack, changed the setting to 200 mA, and then plugged the red probe into the mA jack. The meter showed a value of 0 (See the photo of the Multimeter below). Shouldn't the meter have shown a value near 150? Am I doing something wrong?

I started pulling fuses until I located the source of the drain. It was the one that controls the receiver for the key fob to unlock the car and pop the trunk. With this disconnected, the draw was down to 0.04A. I am not sure if this is normal or not. This seems to be a common issue with the SC430 series.

Anyway, I thought this was a good way to learn about Multimeter usage, and given Covid-19 and Retirement, it gave me something to do.


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## iamrfixit (Jan 30, 2011)

Your meter has internal fuses on both the amp and ma terminals and my bet would be that you have blown the fuse that protects the ma terminal. Says 200 ma max, does not take much to exceed that, even a momentary surge can be enough to zap the fuse.


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## In Ontario (Oct 5, 2018)

If you don't drive that car only infrequently you might consider putting it on smart car batter charger/maintainer, it will automatically keep it charged when it needs it. Then your car is always ready for you. Car batteries don't do well to deep cycle. 

Some of them are meant to be installed, not just clipped on the batter so that you just plug it in, sort of like a block heater:

NOCO GENIUS2D, 2-Amp Direct-Mount Onboard Charger, 12V Battery Charger, Battery Maintainer, And Battery Desulfator With Temperature Compensation, Battery Chargers - Amazon Canada


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## HayesFJ (Sep 25, 2020)

Thank you for the reply. I think you are correct. I pulled the fuse and the cylinder between the metal ends was cardboard, not glass, so I could not tell if it was blown or not. So after a quick google search, I learned that I could check the fuse with the continuity feature of the multimeter. The fuse was definitely blown. I will get a replacement at the Auto Parts store. And I got to learn another cool feature of my multimeter! Thank you.

I think the issue is that when I first make the connection to the multimeter, the amps start at 1.8 or so as the capacitors in the car reload (assumption on my part) and then quickly fall to the 0.14 range. The fuse is rated at 500 mA so I am way above that at the start. I think the way to deal with this would be to use a jumper cable along side of (and before plugging in) the multimeter and then release the jumper after the amps have normalized.

I appreciate the help.


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## DexterII (Jul 14, 2010)

I like to start with the basics, so the first thing I would do is load test the battery; not check it with a meter, but actually load test it. If you do not have a load tester, that's okay, most auto parts stores have them and will do it for free. You said the battery is only three months old, but it wouldn't be the first one that was bad out of the gate or failed prematurely. Concurrently, I would remove, inspect, and clean the cable connections, definitely at the battery, as well as at the other ends of the cables if you can gain reasonable access, such as at the frame and fuse block. It may seem like wasted time, but virtually any vehicle today has something running 24/7, clocks, alarm systems, etc., and a little bit of "normal" drain can become significant through poor connections.


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## HayesFJ (Sep 25, 2020)

DexterII said:


> I like to start with the basics, so the first thing I would do is load test the battery; not check it with a meter, but actually load test it. If you do not have a load tester, that's okay, most auto parts stores have them and will do it for free. You said the battery is only three months old, but it wouldn't be the first one that was bad out of the gate or failed prematurely. Concurrently, I would remove, inspect, and clean the cable connections, definitely at the battery, as well as at the other ends of the cables if you can gain reasonable access, such as at the frame and fuse block. It may seem like wasted time, but virtually any vehicle today has something running 24/7, clocks, alarm systems, etc., and a little bit of "normal" drain can become significant through poor connections.


Good advice. Thank you.


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## LawrenceS (Oct 21, 2020)

So I'm a Honda master tech but the basics of a parasitic draw should be roughly the same.

In our cars anywhere between a .030a and .050a draw is normal which is probably pretty standard for most vehicles, aftermarket accessories like remote starts or alarm systems will typically add to that but they shouldn't be more then .030a-050a more. But the vehicle has to be asleep as we call it to get an accurate draw reading. Typically that's everything closed and locked. The basic reason is certain things may be on timers like interior lights. Say you were parking and you shut your car off but you are sitting in it for a minute so you turn the overhead light on manually to look for something in your center console, then you open your door and the interior lights turn on because of the door function and you forget you had it on manually so you close the door and don't think anything of it figuring it will go out on it's own. The timers are designed to prevent that light from staying on for an extended period if you are probably not in the vehicle using it. There can also be triggers to change those timers like most people lock their vehicles after parking them which serves multiple purposes such as shortening the timers, securing the vehicle and activating a factory alarm/security system.

Most of our vehicles have a hood latch switch which is also part of the factory security system and if the hood is open you cannot arm the alarm to put the vehicle to sleep faster, it will eventually go to sleep on it's own to prevent excessive battery draw but it will take longer. Now obviously to test a draw on a vehicle with an underhood battery, the hood has to be open. If it has a latch based switch, the easy way to get around that is use a screwdriver or similar to close the latch with the hood open just don't forget to pop the latch again before attempting to close your hood. If there is a button based switch using something to depress it like you did your door switch would work.

As for draw testing I typically only use the 10a setting on my meter as it will still show decimals. To avoid accidentally blowing meter fuses I will typically close all the doors, push the hood latch closed and loosen the negative terminal then lock the vehicle until I get the horn or chirp sound notifying me the security system has been armed. This should start the process of putting the vehicle to sleep quickly, then I will lift the negative terminal without breaking contact to the top of the negative battery post. If you lose contact and the systems lose power, when they get power back that will wake everything up. While holding the terminal to the post with 1 hand I will attach the black lead to the now exposed negative battery post, then the red lead to the negative cable, then remove the terminal from the post and your meter will be in series with the circuitry of the vehicle without ever interrupting power flow, depending on your meter you might be able to hook the leads up opposite and it may just give you a negative amp reading. Typically within a couple minutes everything should be asleep and you will have your resting draw. Versus not locking the vehicle/arming the security system, in Honda's it could take 20-30 minutes for everything to shut down. To be safe since I don't have the specific knowledge of lexus's you could still give it that 20-30 minutes.

As for troubleshooting the draw if there is one, I don't know if I can be much help. Pulling fuses is the way we isolate it but without knowing exactly how the systems operate I don't want to offer bad advice. The only points I will mention is pulling a fuse for a control unit doesn't always condemn that unit as there may be an input to that unit causing it to draw versus a failed unit. Like I have seen rusted stuck tailgate handles causing a door release input to a control module even though the the tailgate is latched or an aftermarket alarm system tapped into a door lock switch circuit causing a constant lock command input. Also I'm assuming there is probably some sort of canbus network and when 1 module is awake it may be communicating on that network causing other modules to stay awake.


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## HayesFJ (Sep 25, 2020)

LawrenceS said:


> So I'm a Honda master tech but the basics of a parasitic draw should be roughly the same.
> 
> In our cars anywhere between a .030a and .050a draw is normal which is probably pretty standard for most vehicles, aftermarket accessories like remote starts or alarm systems will typically add to that but they shouldn't be more then .030a-050a more. But the vehicle has to be asleep as we call it to get an accurate draw reading. Typically that's everything closed and locked. The basic reason is certain things may be on timers like interior lights. Say you were parking and you shut your car off but you are sitting in it for a minute so you turn the overhead light on manually to look for something in your center console, then you open your door and the interior lights turn on because of the door function and you forget you had it on manually so you close the door and don't think anything of it figuring it will go out on it's own. The timers are designed to prevent that light from staying on for an extended period if you are probably not in the vehicle using it. There can also be triggers to change those timers like most people lock their vehicles after parking them which serves multiple purposes such as shortening the timers, securing the vehicle and activating a factory alarm/security system.
> 
> ...


Thank you for taking the time to post such a thorough and informative reply. I learned a lot!


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