Lenovo T14 Gen2 dies when battery goes empty

[huzo]

THe board is NM-D353

The situation is that the laptop came to me dead...i plugged the charger, there was only 5V present on the usb meter thingy...i tested the battery ,it was dead, i plugged the other battery and laptop turned on,charged ,everything was ok...i've sold it and the customer returned it after two weeks ,says it's dead...i tested again it was dead,only 5V present on the charger...I injected 10V to the battery and it started again and everything was fine,charged up and was working...

So the probem is ,when the battery goes empty the charger won't start charging again , no 20V ,only 5V on the charger, But as is remember that board should power on without the battery..there is something missing ...when there is power on the battery terminal it starts,when there is no voltage it doesn't start or turn on 20V on the board...What could be the problem, i've already measured all the mosfets ...

[mon2]

Review the dcin mosfets. Post the resistance of each mosfet.

source / drain
source / gate
gate / drain

no adapter and no battery during these tests.

Review the charger bible sticky above by piernov

[huzo]

This is a different system, and i already said i've checked them.....It's USB-C input , and i get only 5V when the battery is empty,no20V ...i have to inject the voltage to the battery terminal to activate 20V from USB-c...First time i thought it's the battery ,when i put the new one it worked normally,but then again when the battery was left to deplete it wouldn't start neither on the charger nor the battery.....So the issue is ,it doesn't turn on when there's no battery or when the battery is empty..only when there is at least 10V on the battery connector ... So the laptop works normally ,but the battery must not get empty,you have to keep her charged above 10V or 5% state of charge...If it goes under ,the whole laptop is dead...I hope i've explained it well!

[mon2]

Attach the power adapter only. Disconnect the battery. Measure the voltage to ground of:

VREGIN20_OVP
VREGIN20

[huzo]

i don't have the boardview...but i am sure i don't have 20V anywhere on the board,exept when the battery is working or i inject 10+V on the battery terminal,then i have 20V on the board....

[mon2]

Just posted the boardview files for this board here:

https://www.badcaps.net/forum/docume...ews-collection

[huzo]

You never dissapoint

I have to find what turns the 20V line when there is no battery...Since when the battery is charged ,everything is ok,i get 20V through and everything is working, it just must not get drained under 5%...then it dies and wont charge.

[mon2]

Review post #4. Suspecting the power path from the pd controllers.

[huzo]

i have 5V without battery, and 20V when battery is plugged and has more than 10V in her...The PD is BQ25710, i will try to find one and replace it

[mon2]

The PD controller is TPS65994 (using NM-D351 schematic which is linked to the T14 Gen2 chassis) @ U5901.

The BQ25710 is the battery charger IC that follows after the PD controller. Respectively, for your case, when the battery is connected to the board, the PD controller is able to negotiate a 20v power delivery contract.

Need to study the link between the battery power rail of BAT_PWR15 and U5901. At this time, the BQ25710 is not a suspect.

For giggles, remove the power adapter. Connect the battery pack (if it has some voltage). Test if there is a voltage @ PL0201. Test also @ PL0202. We are testing if the battery voltage is back feeding in the power adapter direction due to leakage of the mosfets.