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ThinkPad T520 not charging

T400/T410/T420 and T500/T510/T520 series specific matters only
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ThinkPad T520 not charging

#1 Post by kfzhu1229 » Mon Jun 15, 2020 9:33 pm

I have previously acquired this beaten up T520 missing parts as is, but the motherboard does seem to work for the most part and it has the NVS 4200M graphics, so I wanted to fix this board up fully working.
However I have noticed 3 issues:
1. Time keeps resetting
2. Not detecting AC unplug
3. Not charging battery
For 1 it turned out to be corrosion on a trace that the CMOS battery is connected to. I fixed that up and it worked for like a day or so, but now the time keeps resetting again yet the other settings don't. So when a T520 series have a dead CMOS battery, typically are the BIOS settings also supposed to reset or is there another ROM keeping those settings? I am suspecting the battery connector is faulty and intermittent as well but we will see.
For 2 I think it is entirely possible the BQ24742 battery charger chip is faulty. I measured the ACDET pin and it has 2.6V when the charger is connected and 1.4V on battery power. This is definitely a far cry away from the rated 2.4V threshold for seeing the charger is present or not, yet when I unplug the charger the output EXTPWR pin is still shorted to ground (EXTPWR is supposed to be 3.3V when on battery and short to ground when the charger is connected, and connecting the charger does short that 3.3 to ground). When the battery is fully charged this problem doesn't show up.
I can live with this board if it is not for 3. The board works fine off battery, charger only or charger with fully charged battery, but it just doesn't charge the battery up. The Lenovo Vantage utility in Windows 10 shows that the battery is properly detected but the charging power is like 0.1W, which is probably the trickle charging circuit kicking in so the battery doesn't lose charge when using the laptop.
Now I have not dealt with fixing a charging circuit before, so I am confused where the battery charging takes place on the schematics (W520 schematics).
Is the charging circuit directly from that BQ24742 chip? If so does this problem also get blamed on the BQ24742 chip potentially being bad? Is there any fuses just for the charging circuit on the W/T520 series? Can I also use the main battery into the ultrabay to see if the ultrabay charging works fine? If I need to replace that BQ24742 chip, is there any programming done into it or can I just replace it with any BQ24742 chip I find from any other motherboard?
I think it is entirely possible that chip is dying because I did see corrosion on the main battery connector.
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