Lenovo Yoga C740-14IML charging issue

[trepachka]

I am working on Lenovo Yoga C740-14IML that was missing 5V rail. I had that fixed and the computer is running. However, computer is not charging, though the orange light comes on whenever I plug in the charger.

It runs off the charger with battery detached. This is fine.
It runs off the battery when the charger is unplugged. This is fine, too.

However, with both the charger and the battery plugged in, the charger's current immediately plunges to 0 and the computer is completely running off the battery.
I had the BQ chip replaced with the same results. The circuit seems to be ignoring presence of charger

Does anyone have a clue why the charging is not working?

[trepachka]

I never had this issue resolved. I just got another computer with identical issue. Computer runs but when both the battery and the charger plugged in, I get orange charging light but the computer only runs on battery. I get 20V/0A on the charger. Unplugging the battery and the computer runs off the charger.

[mon2]

See attached. Review the power path mosfets from the power adapter side and also the battery side.

Remove all power. Meter in resistance mode.

Measure the resistance of each mosfet as follows:

source (1-2-3) & drain (5-6-7-8)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)

Test the highlighted mosfets and post each measurement.

[trepachka]

I had both of these mosfets replaced just in case and that did nothing. Here are the measurements:

PQ4802
source (1-2-3) & drain (5-6-7-8): 8M
ource (1-2-3) & gate (4): 202K
gate (4) & drain (5-6-7-8): 400K

PQ4805
source (1-2-3) & drain (5-6-7-8): 15M
source (1-2-3) & gate (4): 26M
gate (4) & drain (5-6-7-8): 21M

[mon2]

Carefully, measure and post the voltage to ground of each pin on the charger IC. List the measurements with their PIN number and function for a review.

Initial thoughts are to inspect the usual dual common cathode diode used to power (Vcc) of the charger IC. This dual diode feeds power from the adapter and also the battery but cannot be leaky.

[trepachka]

I had the charging IC replaced as well. No change. Interesting that with the charger on (no battery), the gate on PQ4801 is about 10.5V which is correct. As soon as the battery is plugged in, the gate voltage drops to about 5.6V. In a working laptop, it's supposed to rise to over 12V

[mon2]

Not the charger ic but the Vcc pin when both the adapter and battery are plugged in. What is the voltage on each leg of the (assumed to be onboard) dual diode that powers the charger ic?

REGN voltage?
ACDRV voltage?
ACDET voltage?
ACOK voltage?

Review the datasheet of the charger ic to study when and how the charge feature is enabled. Compare the above test points to the working laptop. Be sure the power adapter is suited to power this unit.

[trepachka]

I took the measurements. See attached image.

[mon2]

Something is incorrect with the measurements of pins 20 & 19 on the charger IC. Confirm these again. When both, the adapter & battery are connected, the voltage should not be 0v on one leg of this current sense circuit. Instead, the voltage on SRN should be about the same as SRP voltage.

[trepachka]

Yes, when charger+battery plugged in on pin 20 is 11V, not 0V as previously reported. This is the voltage provided by the battery. The rest of the measurements are correct.