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

[flat-earther]

Disconnect the power adapter. Connect only the battery (with some charge on it).

Measure the voltage to ground on each pin of the mosfet @ PQ8802.

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

source (1-2-3) 11.8V
gate (4) 11.6V
drain (5-6-7-8) 0V

[mon2]

Disconnect the power adapter. Connect only the battery (with some charge on it).

Measure the voltage to ground on each pin of the mosfet @ PQ8802.

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

[flat-earther]

Today a new problem appeared where my battery wasn't being charged with charger plugged in:
I used my laptop on battery for couple hours, then plugged it to charge, and hours later when it should already be full I noticed that the battery is still at 51% and has not charged, and the charging light wasn't even on!
I unplugged and replugged the charger, and Windows recognized when I unplugged and replugged the charger, but the charging light on the motherboard didn't come on.
So the battery just would not charge.
Then I unplugged and replugged the battery after which it started charging normally with the charging light on.

Could this new problem be the battery and not motherboard?
I have two batteries and this particular battery I was using is a high capacity one which has extra cells.
I will now change to my regular capacity battery and see how the behavior is with it.

BTW both of these batteries are aftermarket, but I'm very confident that at least the original problem that started this thread is motherboard related as it happened with both batteries.

[flat-earther]

I have received the new DCIN mosfet B20P03, but the problem of the charge light staying on with charger unplugged has not come back at all, so for now I'm not changing the mosfet.

One related problem that I mentioned initially does still exist though, occasionally the charging light on my laptop starts flashing green and red alternately.
Replugging the battery makes it stop, until it randomly starts again.

[flat-earther]

For me there isn't any indication to replace the Mosfet. P-Channels almost never fails and there was no measurement shared which proves the necessity either.

I seriously suggest to check charging controller requirements and normal Working conditions before proceeding with random Mosfet replacements

Thanks for your insight.

Even if the culprit is the DCIN mosfet, I don't comprehend how can that cause the laptop to think it's charging? because the charging chip AC detect pin didn't have 2.6V unless charger was connected.

I have assembled the laptop after returning the mosfet and the problem isn't happening now. So I feel the mosfet or something near it is the culprit, because the heat from soldering must have somehow caused the problem to go away.
The battery charging chip is far from the DCIN mosfet where I soldered.

Tell me if you want me to check anything specific with the multimeter next time I disassemble the laptop.

[Sephir0th]

For me there isn't any indication to replace the Mosfet. P-Channels almost never fails and there was no measurement shared which proves the necessity either.

I seriously suggest to check charging controller requirements and normal Working conditions before proceeding with random Mosfet replacements

[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.

[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]

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.

[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]

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.

[flat-earther]

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

[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

[mon2]

Yes, looks ok.

[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]

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]

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]

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]

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]

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.