Battery Drain Measurements

[Smallville]

2008 MS3 GT

I'm tracking down battery drain using a low current clamp and everything I'm reading sounds straight forward except for the fact that our batteries have two cables coming off the negative terminal. After letting the car sit for 30 minutes to have the electronics time out, I'm reading ~70mA through the left cable and ~150mA through the right cable.

Do I add these current measurements together or focus on just one particular cable?

As a side note for anybody dealing with the same issue I highly recommend this current probe. Only $41 and has sensitive measurements down to the milliamps range.

https://www.amazon.com/Uni-T-B4Q094-...ct_top?ie=UTF8

[Thor Hammer]

Use a meter with leads for measuring DC amps. I use Fluke at work.

Then, disconnect either the negative or positive post clamp from the battery. Then place the meter probes in series between the battery post and cable.

Or instead of measuring both negatives just put your clamp on meter on the positive cable. That will give you the total parasitic draw. For such low amp measurements I wouldn't trust the reading on any clamp on meter. YMMV.

Parasitic draws of at least 85mA is normal on a newer car. GPS, memory seats, radio presets, etc. all contribute. I don't know whats normal on yours though.

What is your battery voltage with the car off? Should be at least or above 12.4VDC.

To locate the draw you can pull the suspected circuit fuse and put your leads across the fuse terminals and see if you get a reading.

[Smallville]

Originally Posted by Thor Hammer Use a meter with leads for measuring DC amps. I use Fluke at work. Then, disconnect either the negative or positive post clamp from the battery. Then place the meter probes in series between the battery post and cable. Or instead of measuring both negatives just put your clamp on meter on the positive cable. That will give you the total parasitic draw. For such low amp measurements I wouldn't trust the reading on any clamp on meter. YMMV. Parasitic draws of at least 85mA is normal on a newer car. GPS, memory seats, radio presets, etc. all contribute. I don't know whats normal on yours though. What is your battery voltage with the car off? Should be at least or above 12.4VDC. To locate the draw you can pull the suspected circuit fuse and put your leads across the fuse terminals and see if you get a reading.

I didn't even think to try measuring the current off the positive cable - will try that tonight.

The battery reads 12.4 volts at rest and it's probably a little low since I haven't done any long drives to recharge it. FWIW I had autozone check the battery and they said it's good.

[Thor Hammer]

Originally Posted by Smallville I didn't even think to try measuring the current off the positive cable - will try that tonight. The battery reads 12.4 volts at rest and it's probably a little low since I haven't done any long drives to recharge it. FWIW I had autozone check the battery and they said it's good.

If its still an issue when we do your TMM I can help you locate the source of the draw.

[Smallville]

Originally Posted by Thor Hammer If its still an issue when we do your TMM I can help you locate the source of the draw.

[Smallville]

I hooked up a meter in series to my negative battery terminal and its cable and was reading a draw of 240mA even after letting the car sit for 30 minutes. Coincidentally if I added the two cable's current flow using the current clamp method, it adds up to 210mA.

I should've mentioned in the original post my power locks don't work. Anyways, I'm gonna pull fuses this weekend to see which circuit is bad (guessing it's the power locks). I'll report back afterwards.

[5doorsoffury]

I installed a battery matainer because i have the same issue with excessive draw. I spent days and ripped my car apart. Fuse box out unplugged every module in the car and i traced it down to a bad BCM. Bcm controls locks dome light ect.... since your locks are fucked and the battery is draining i would start at the BCM.


Fwiw always do the parasitic load test on the positive cable. If you have 2 ground cables off the battery and they have diffrent currents across each cable there is a bad (ish) ground on one of the cables. There going to the same location so they are essentially in parrallel so they must have the same voltage drop. Same voltage drop means same current unless one cable has more resistance (poor ground) then the other. That cable will show less current then the other.

[Smallville]

Originally Posted by 5doorsoffury I installed a battery matainer because i have the same issue with excessive draw. I spent days and ripped my car apart. Fuse box out unplugged every module in the car and i traced it down to a bad BCM. Bcm controls locks dome light ect.... since your locks are fucked and the battery is draining i would start at the BCM. Fwiw always do the parasitic load test on the positive cable. If you have 2 ground cables off the battery and they have diffrent currents across each cable there is a bad (ish) ground on one of the cables. There going to the same location so they are essentially in parrallel so they must have the same voltage drop. Same voltage drop means same current unless one cable has more resistance (poor ground) then the other. That cable will show less current then the other.

Hmmm, thanks for the lead. How much did a new BCM cost you? Installed yourself or had a shop do it?

[5doorsoffury]

Originally Posted by Smallville Hmmm, thanks for the lead. How much did a new BCM cost you? Installed yourself or had a shop do it?

I never replaced it. I wouldn't pay some one to do it its really not bad.

[Smallville]

I made this thread to ask HOW to measure current draw off a MS3 battery. I have a separate thread with me actually tracking down the source of my battery drain and will update that as I continue to test.

To close out this thread and to summarize my findings.

- There are two ways you can go about measuring current draw off the battery. Hooking amp meter in series with the negative battery cable OR by using a low current amp clamp.
- If using the amp clamp, you can't simply clamp around the negative battery cable because it splits into two lines and both have different current flows (current is directional). To correctly use the clamp you need to measure each line separate and then add them together.
- After trying both methods I prefer hooking up an amp meter in series. Just be sure to not activate any circuits that could blow your meter's fuse.

SIDE NOTE: For my 2008.5 MS3 it takes exactly 30 minutes for the electronics to time out. I couldn't find this info anywhere so hopefully it helps someone out.