Hints on troubleshooting battery draw when off

[lipadj46]

I'm getting a small draw on my battery with my car off. If I remove the negative battery lead and put a multimeter on the positive terminal and the block I get a small draw that starts of at around 200 mV and slowly decreases to about 10 mV and then stays there as far as I can tell (I had to replace my PAS pump this weekend so I only had an hour or so to fiddle with this). I pulled every relay and fuse one at a time and nothing affected the draw. The 10 or so grounds that I checked seemed tight but I did not get to them all. This is a brand new duralast gold battery. Any helpful hints on where to go next?

I have heard that a diode can go bad in the alternator and cause it to draw off the battery but I would have expected the draw to disappear when I pulled the alternator fuse.

 

[chad8]

have you checked the bcm and pcm?

 

[lipadj46]

No I did not, never had to crack open the computers yet.

 

[pcoxe]

You need to check the current with a ammeter in series with the battery. Some of the clamp-on ammeters will read low DC current also. A mV reading probably won't tell you much. You can probably generate a 10mV reading by holding a probe tip in each hand. The decaying 200 mV reading you detected sounds like something capacitive charging up.

 

[Steve S]

I am not sure of the proper current draw for your car but there should be some to keep the comp etc alive.

 

[tomcat27]

yes, you will see a small drain just to keep the dynamic memory in the computer active - and also for the clock in the radio. I am guessing that you mean milliamps, not millivolts.

 

[03cvpi]

Is the battery going dead and if so, how much time before it goes dead. Every car is different, but a small draw of 10 to 35mA is normal for most cars. This keeps the clock alive and most pcm/bcm units on standby. Some newer cars it also keeps the air bag sensors hot in case you are sitting in the car with the ignition off and get nailed. pcoxe is right, the initial inrush is to charge a cap or two, once charged the apparent draw will decline.

Also if an alternator diode goes bad there can be a large draw, I have seen up to 3.5 amps with three diodes blown.

More information please.

 

[lipadj46]

My battery is draining if is has been sitting for a day (especially in the cold now) so I'm guessing my 2 month old duralast gold is going south.

 

[JTK]

Pulling one fuse at a time to see which makes the draw go away is one troubleshooting method.

Joel

 

[duaneb9729]

JTK, could you do it the other way around, take out all the fuses and then put them back one at a time till you get the amp draw?

 

[Eddie]

100-200 MA is about average in my opinion to keep the radio and clock & possible your alarm system active. Power =AxV .200/12.6= 0.0156 Amperes and appears to be a good nominal current draw with no accessories on. JM opinion. Ed

 

[zrxkawboy]

Originally Posted By: lipadj46
If I remove the negative battery lead and put a multimeter on the positive terminal and the block


If you remove the negative battery terminal, then you need to put one lead of your ammeter on the negative post, and the other on a good ground. This puts the ammeter in series, which is how current is measured.

 

[lipadj46]

I will give it a shot that way. It's been a long time since Physics II, always hated electronics.

 

[mechanicx]

Originally Posted By: zrxkawboy
Originally Posted By: lipadj46
If I remove the negative battery lead and put a multimeter on the positive terminal and the block


If you remove the negative battery terminal, then you need to put one lead of your ammeter on the negative post, and the other on a good ground. This puts the ammeter in series, which is how current is measured.


Good catch. He could just check for a battery drain between the unhooked battery terminal and the negative battery post I think.

 

[ATS-V]

If you pull each fuse one at a time when there's no draw and put your probes across the terminal and read mv. If there's no draw the mv should read zero, if there's a short/draw a voltage will show up.

Most low end multi-meters will read millivolts more accurately/safely than milliamps.

 

[mechtech2]

lipad - Actually, zrxkawboy is wrong.
You want your ammeter in series, but probe the battery terminal and the other end to the disconnected cable. If the neg cable is off, use the neg probe to the neg terminal, and positive probe to the cable end. Do the opposite for the plus cable/ lead, if that one is off.

 

[HyperJinx]

Originally Posted By: mechanicx
Good catch. He could just check for a battery drain between the unhooked battery terminal and the negative battery post I think.


Yes, this does work and is how I determined that my volt reg was defective. My battery would totally drain in about 40 hrs.

 

[zrxkawboy]

Originally Posted By: mechtech2
lipad - Actually, zrxkawboy is wrong.
You want your ammeter in series, but probe the battery terminal and the other end to the disconnected cable. If the neg cable is off, use the neg probe to the neg terminal, and positive probe to the cable end. Do the opposite for the plus cable/ lead, if that one is off.


Actually, zrxkawboy is not wrong. Either method will work. They are accomplishing the same thing. Think about it.

 

[lipadj46]

Well I've been a bit busy lately but I will get some time probably Friday to trouble shoot for a few hours. Thanks for the advice I will report back with what I find.

 

[mechtech2]

No, If you take the negative or positive cable out of the equation, you are eliminating checking them, their ends, and their connections.
Or do you assume they are OK?