ASUS X54H laptop thinks it's always charging even with charger unplugged

[flat-earther]

The charging indicator light on my ASUS X54H laptop is always on even when the charger is disconnected.

Windows also shows ''Plugged in'' when charger is not connected.

This problem started year ago gradually, when unplugging the charger the charging light took several seconds to disappear, then minutes, and now almost always on even when unplugged for an entire day.
Strangely, before sometimes immediately when plugging in the charger the charging light went away, and later came back with it connected.
If I let battery run out and say it has 3% Windows will warn me to plug in the charger, and even if I do, the computer shuts down as if the charger isn't connected, even though the charge light is on!
Sometimes the charging light flashes green and red alternatively which I think means some fault, but replugging the battery fixes it.
As you see very weird behavior.

The problem is the same whether the computer is turned on or off the charging light stays on.

Some things I tried:
In Device Manager uninstalling Microsoft ACPI-Compliant Control Method Battery and restarting.
I measured RTC battery and it was a bit low 2.96V so I replaced it and it didn't help.
I have two laptop batteries and the problem is the same on both.

I think the problem is on the motherboard.
How to troubleshoot it?

Label on laptop bottom says
MODEL: X54H
MB Ver. X54HR
but printed on motherboard K54LY REV 2.1

[m1ch43lzm]

Boardview here
https://www.badcaps.net/forum/troubl...-2-1-boardview

From the datasheet of BQ24740 there's a voltage divider connected to pin 5, the output of it should be around 2.6V with charger plugged in

ACDET Adapter detected voltage set input. Program the adapter detect threshold by connecting a resistor divider from
adapter input to ACDET pin to AGND pin. Adapter voltage is detected if ACDET-pin voltage is greater than 2.4 V

Measure both resistors circled in red (with power removed), the one in parallel with the cap goes to GND, the leftmost one the other end goes to the charger input, maybe one of them is out of spec
Measure voltage at ACDET, marked with a blue line on the picture:
1. Only battery connected
2. Charger connected

Unfortunately there's no schematic available and the "repair guide" is useless for this, you have to calculate the voltage divider with Voltage=19V and the measured resistor values
The values from the typical application circuit of the BQ24740 may not match what ASUS used
Keep one of the resistors, then calculate the bottom one with Out=2.6v, then match it to the closest 1% standard value

EDIT: Actually the boardview has the values:
PR8803: 330K
PR8802 (parallel to the cap): 53.6K

[mon2]

Review the dual common cathode diode that is likely feeding the VCC of the BQ charger IC. Remove all power. Meter in diode mode. Confirm that the dual diodes are working properly.

Red meter lead on anode of one diode; black meter lead to Vcc pin. Meter should conduct. Reverse the leads. Meter should block.

Repeat with the next anode of this dual diode and the VCC pin. Have seen where this part is leaking / defective and raises the same issue.

[flat-earther]

Thanks guys.

Measure both resistors circled in red (with power removed), the one in parallel with the cap goes to GND, the leftmost one the other end goes to the charger input, maybe one of them is out of spec

I measured the two resistors you circled in red with power removed
PR8802 the one parallel beside the cap 46.16 kohm
PR8803 the one on left which connects to charger input 57.55 kohm

Measure voltage at ACDET, marked with a blue line on the picture:
1. Only battery connected
2. Charger connected

measured voltage on ACDET
only battery connected 0V
battery and charger connected 2.685V

Review the dual common cathode diode that is likely feeding the VCC of the BQ charger IC. Remove all power. Meter in diode mode. Confirm that the dual diodes are working properly.

Red meter lead on anode of one diode; black meter lead to Vcc pin. Meter should conduct. Reverse the leads. Meter should block.

Repeat with the next anode of this dual diode and the VCC pin. Have seen where this part is leaking / defective and raises the same issue.

I tested the diode in diode mode with all power off

red probe on pin 1 black on pin 3 gives 0.259V, reversed OL
red probe on pin 2 black on pin 3 gives 0.258V, reversed OL
one probe on pin 1 and other on 2, both ways 0V, expected as anodes are on same pad.

[mon2]

Remove all power. Meter in resistance mode. Measure the resistance across each dcin mosfet.

source & drain
source & gate
gate & drain

post each measurement. Test both dcin mosfets.

[flat-earther]

Remove all power. Meter in resistance mode. Measure the resistance across each dcin mosfet.

source & drain
source & gate
gate & drain

post each measurement. Test both dcin mosfets.

Near the DC in jack I only see one mosfet PQ8802. Is there another I need to test?

I tested PQ8802
red probe on source black on drain 60kohm and rising, reversed OL
red on source black on gate 140kohm, reversed 197kohm
red on gate black on drain 230kohm and rising, reversed OL

[mon2]

red on source black on gate 140kohm, reversed 197kohm

This is a low measurement to me. Flux and carefully remove this part. Then test the resistance again of this part while it is on your workbench after it has cooled. Are the measurements similar? Each measurement must be hundreds of k ohms on a mosfet else may be defective.

[flat-earther]

This is a low measurement to me. Flux and carefully remove this part. Then test the resistance again of this part while it is on your workbench after it has cooled. Are the measurements similar? Each measurement must be hundreds of k ohms on a mosfet else may be defective.

I removed the mosfet with two soldering irons because that's what I have.

now with it on the bench I repeated the measurements

red probe on source black on drain OL, reversed 60kohm
red on source black on gate OL, reversed OL
red on gate black on drain OL, reversed OL

[mon2]

Yes, looks ok.

[flat-earther]

Yes, looks ok.

Okay I returned the mosfet, was a challenge.

I probably should've plugged the battery in before returning it just in case to check if the problem existed without the mosfet but I already returned it.

BTW with the battery plugged in without charger the mosfet source is at 11.85V, gate 11.62V, drain 0V

[flat-earther]

Anothere example of the problem, only battery plugged into motherboard and charging light is on:

[flat-earther]

After removing and returning the mosfet there is change in behavior.
Now sometimes when I plug the battery in without charger the charging light doesn't come on, and sometimes it does.
And when I unplug the charger sometimes the light goes off as if the problem went away and other times not.
and sometimes when the charging light is on without charger connected and then I plug in the charger, the charging light blinks off and immediately comes on again, and sometimes it just stays on without blinking.

And few times with only battery connected the charging light randomly turned off and on.

[mon2]

Replace this mosfet. To me, it is flaky. If you are not comfortable with the process, suggest to work with a local cell phone tech with hot air tools and flux. That is the secret.

[flat-earther]

Replace this mosfet. To me, it is flaky. If you are not comfortable with the process, suggest to work with a local cell phone tech with hot air tools and flux. That is the secret.

Okay then I think I need to buy it from aliexpress. do I need exactly same brand and model mosfet or what do you suggest?
BTW I have a PCB from my old laptop battery which has two mosfets the same size, would those be compatible?

I used a lot of flux and added 63/37 solder, although it was challenging I think I can manage.

[m1ch43lzm]

On MOSFETs you have to match at least:
- Type (N or P channel)
- Package
- Max Drain-Source voltage
- Power rating (same or higher)
- Current rating Id (similar or higher)
- Vgs(th), gate threshold voltage (similar)
- Rds(on), drain-source on resistance (same or lower)
- Maybe some other parameters

The MOSFET you have is EMB20P03G (marking B20 P03), it's P channel, -30V, -10A, 20mOhm Rds(on)
A suitable replacement could be AO4435
Probably you can find a suitable replacement on another laptop motherboard, check the datasheet and compare it to yours

The one from your battery is TPC8028, it's N channel so it won't work here, can't read the other one

[flat-earther]

On MOSFETs you have to match at least:
- Type (N or P channel)
- Package
- Max Drain-Source voltage
- Power rating (same or higher)
- Current rating Id (similar or higher)
- Vgs(th), gate threshold voltage (similar)
- Rds(on), drain-source on resistance (same or lower)
- Maybe some other parameters

The MOSFET you have is EMB20P03G (marking B20 P03), it's P channel, -30V, -10A, 20mOhm Rds(on)
A suitable replacement could be AO4435
Probably you can find a suitable replacement on another laptop motherboard, check the datasheet and compare it to yours

The one from your battery is TPC8028, it's N channel so it won't work here, can't read the other one

Thanks for the info. I already found B20P03 and ordered it. It looks unbranded. https://www.aliexpress.com/item/1005009041221345.html
I don't have other motherboard.
The other mosfet from the battery was the same.