Latitude 5289 - Not working on known good battery

[Prott]

Hello friends,
I am currently troubleshooting a problem with Dell Latitude 5289's motherboard.

First the laptop had a short on the 5V rail (fixed by replacing a shorted cap). However after that it randomly didn't want to start on the battery power - this got worse to the point that now the laptop won't start when powered by battery at all.

- The battery is checked and ok
- If I start the laptop with a plugged charger it shows valid battery data (charge, cycles, etc.).
- The laptop doesn't have problems charging the battery
- If I unplug the charger while it is on it immediately shuts down (even if battery is shown as full)

Recently I also started to experience random shutdowns while powered with a charger.

Does anybody have an experience with similar type of fault? What could cause it?

I have a spare board that I can transfer stuff from for troubleshooting, but I would like to first know where to start looking. :-) My first instinct would be to replace the two main mosfets and (if it doessn't help) the charging IC, but the fact that battery is still charging normally is kinda throwing me off.

Thank you SO much.

[mon2]

Re: Latitude 5289 - Not working on known good battery

Remove all power. Meter in resistance mode. Check the resistance of each and every mosfet in the power path for the adapter; then again for the battery.

Measure:

source & drain
source & gate
gate & drain

Post each measurement. Checking for a low resistance value for the mosfet = defective.

Are you using the official Dell power adapter with the proper rating ?

[Prott]

Re: Latitude 5289 - Not working on known good battery

Thank you so much mon2!

I measured some weird readings on one of the two "main" mosfets (that PWR_SRC and CHARGER_SRC). So I decided two just replace both of them - now the laptop is working perfectly.

The readings were:
VIN -> Bg = 150+ kohm
VIN <-> Vsw = 60+ kohm
Bg -> VIN = inf
Vsw <-> Bg = inf


VIN -> Tg = 50+ kohm on the CHARGER mosfet, BATTERY mosfet inf.