UPDATE: Laptop CMOS battery drain

[orbanp]

Hello Everyone,

My son gave me his old laptop as it got water damaged. It was in a backpack while raining, and in the bottom part of the backpack there was a bit of standing water.
The laptop is a Lenovo T400.

It works OK except the CMOS battery drains really fast. Measuring the current draw, it is 0.66mA where it should be in the uA range. The backup battery is a  CR2032 type lithium battery with wires.

I did find the circuit diagram and the maintenance manual of the laptop.
Here are the circuits relating to the backup battery.

It would be really easy to diagnose the problem if the parts were a bit bigger.
My guess would be either C287, 1uF 10V elco, or D6.

Any words of wisdom, or guess, and ideas how to proceed?

Thanks, Peter

[james_s]

That's probably a good place to start, especially if there is any corrosion on the parts in question.

Years ago I repaired a motherboard with that same symptom, it turned out a dual diode that isolates the battery had failed shorted, on your board it looks like two separate diodes are used.

[Armadillo]

You should change the CMOS battery, won't last forever.  Could that be the fault itself.?

Edit: Measure Ohms of cathode of D6 to ground. Remove C287 and measure again. [Edit: with power off and CMOS battery out of circuit].
At the same time, suspect that the circuit board is leaky by measure D6 to the surface of the board. It should read infinity.

[james_s]

It is implied from his post that he already changed the battery and found that it drains fast.

[Armadillo]

That had been a venial troubleshooting habit over the years, to cover that ground, just in case and normally as a precursor to a more detailed troubleshooting actions.
  thanks nevertheless.

[orbanp]

Thanks for everyone for the comments!

I guess the challenge will be to find the components in question on the mother board, and replace them if necessary.

Thanks, Peter

[james_s]

It may not be all that hard, there may be visible signs of corrosion where it got wet. One other thing worth trying is thoroughly dry it out for several days in a warm place, moisture can hide underneath chips for a surprisingly long time.

[amyk]

Wash the board thoroughly with something water-displacing like isopropanol and then bake dry at a slightly elevated temperature. There's probably some conductive residue that's causing more leakage than usual.

[orbanp]

Hello Everyone,

Thanks for the comments, please keep them coming!

Well, some progress, now the current draw is only about 110uA, better, but still too much!

I did disassemble the laptop, and most of the backup battery circuits were at place on the motherboard that did not get water damaged.
There was only a narrow "band" (~1/2") at the bottom of the PCB where I could see that it was under water. I cleaned it with IPA.
BTW, the water damage happened about two years ago, so I think the innards were as dry as they could get.

Well it was not that simple.

Measuring resistances, at the common point of the two Shottky diodes (D3, D6) the resistance to the ground was about 10 kOhm before cleaning. I did remove D6 and C287 (big mistake) as I suspected those components, but nothing changed.
That was when I searched in the pdf schematics for the word "RTCVCC", and it went to another IC that was not installed on the mobo, but was located in the water damaged area. And there was crud.
I cleaned it, and now the same resistance reads about 1.1 MOhm.
Removing those components was a mistake in hind-sight, the diode disappeared, the cap got damaged.
I did get new components and installed them, it was quite a pain!
Resistance is still 1.1 MOhm, but the current draw is still 110uA, which is about 100 days for a 230mAh back-up battery.
This was measured on the bare board, as removed from the PC, everything disconnected from it.
The real time clock is in an Intel hub-chip (ICH9M), and as the mobo is still disconnected and unpowered, none of the registers are initialized.
A quick look at the hub datasheet (close to 900 pages!) lists the real time clock as using 6uA!

I am still trying to reconcile all this info, and wondering if the board being "bare" has any bearing on the backup battery current draw.
I am wondering if I should put the laptop together, or still look for some other things as well.

Any comment would be appreciated!

Thanks, Peter

[Armadillo]

1.1Mohm equate to about 2.7uA, plus 6uA is at most 8.7uA.

So, I will suspect 3 areas;
1. the RTC.  Wash and scrub it or change another one for easy plug in test.
2. The board still need to be further scrub with brush, detergent and distilled water [entire board] and rinse then dry.
3. I would dismantle D3 from the circuit for external resistance testing, just in case.

But to avoid a "catch-you" situation, the current you are measuring is considered very low current, so can you cross check it with a simple calibration measurement?

[orbanp]

Good news, the current draw now is only 3.1uA! That should be fine for that coin battery!

I did a bit more scrubbing of the mother board and then just assembled the laptop. It still worked!
Having booted up, initialized the BIOS settings, turned it off while measuring the current draw.
Took out the main battery, and it still only draws that 3.1uA, even after about an hour off the main battery, and it still remembered the settings.

Again, thanks for everybody for the help!

Regards, Peter