Thinkpad E15 NM-C421 charger not detected after port replacement

[danandrei96]

Hi all!
Usually I like to think my diagnostic skills are quite good but this one has me a bit stumped.

Laptop came in with a damaged very loose USB-C port.
I ordered the correct port and replaced it, all good so far. Now when I plug in the usb-c charger there's a solid connection, I can see that the charger voltage jumps up to 20V, and if I press the power button the laptop turns on.

Unfortunately, the charger LED stays off at all times and in Windows the laptop doesn't detect that it's plugged in, despite running off the charger even with the battery unplugged. Needless to say, this also means that the laptop refuses to charge the battery.

I have replaced the RTS5455 PC controller with one ordered from China with correct firmware, and also replaced the BQ25700A charger IC just in case but this has made no difference so far.

Could someone please point me in the right direction to troubleshoot charger detection on this model? Thanks

Schematic - https://www.badcaps.net/forum/showthread.php?t=94842
.

[Sephir0th]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

The PD-Controller has nothing to do with the issue. The 20V volts wouldn't be requested otherwise and would not reach the charging controller.

The issue should be found around the BQ25700, but from where do we know, the battery was working before the repair attempt?

[danandrei96]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

I see what you mean, but I assumed that the PD controller communicates with the superIO to tell the charger status and wattage. perhaps I was wrong

The battery was flat when the machine came in. If I inject voltage from my bench supply to the battery terminals I can charge it, so I believe the battery is good.

I did have a quick look around the bq25700 area but didn't see any components obviously burnt or missing

[Sephir0th]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

The whole charging task is controlled by the charging controller in exchange with embedded controller _only_, still.

And about the battery, well, for my taste this isn't enough proof for a "known working battery". Personally I never use the words "I believe" or "I assume" on chip-level. Either i know, or i don't know. There is no room for something inbetween.

So, if you can charge it external, then charge it and let the board run from battery for a while. This would be a better test.

Regarding the your "quick look": it would be a simple world if each fault could be spotted by visual inspection, no?
This is rarely the case unfortunately and this is where the real work has to start.

[danandrei96]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

you are right, sorry I was being a bit lazy and vague in my previous messages and that might've caused confusion

I charged the battery to 80% using bench PSU, laptop will boot and run from it, it reads the percentage correctly as well as the BMS data (voltage, wear level, number of cycles etc) which you can see in the photo attached. I think it's safe to say the battery is good.

When I plug in the usb-c charger I can see 20V and current being drawn 0.6-0.9A depending on load, but the laptop still shows as running on battery and not plugged in. However when the charger is plugged in, the machine draws next to no power from the battery resulting in some ridiculous time estimates as seen in photo.

If I boot the machine with charger only and no battery connected, windows will then show it as plugged in with no battery detected, but under linux it still shows as "not plugged in". The charger LED stays off at all times, but I think it should come on based on my experience with previous usb-c thinkpads. In either case, if I then connect the battery while the machine is running, it immediately switches to showing as running off battery, with no charger connected, same as before

I have tried my best to measure the passives around the BQ chip, no capacitors are shorted and none of the resistors are blown open or otherwise have values that I'd consider wrong. Please let me know if there's any particular ones I should double-check.

I also measured a few voltages at the bq25700, with the charger plugged in but no battery:
VSYS 9.2V
VDDA 6V
VBUS 20V
REGN 6.0V
CHRG_OK measures around 0.3V on my multimeter, however I can see from the bar graph that there is some strange pulsing going on here. Oddly enough, looking at the schematic the CHRG_OK line from the BQ chip, which they call ACIN, doesn't connect to the superIO or anything else really as both R8554 and D8501 are "NO STUFF"

Instead, the ACIN_EC line is pulled up by a 10k resistor to the 3.3V supply VCC3_LDO_PD. I've measured this ACIN_EC line and it reads around 2.9V on my multimeter.

I am pretty stuck on what to check next so I really appreciate any advice you have to offer

[mon2]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

Remove all power. Meter in resistance mode.

Measure the resistance of the dual mosfet at PQ0205.

Source & drain
Source & gate
Gate & drain

Note again that there are 2 mosfets inside this package so check each.

Repeat for the battery mosfet at PQ0201.

Post each measurement. Checking for a low resistance which often means a leaky mosfet.

Next, confirm the resistance of the path at the charger IC from SRN to SRP. The resistance should match the schematic values. This path is used to gauge how much current us being consumed in the charging process. A broken trace here will halt charging.

[danandrei96]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

thanks for the tips, but I'm afraid I haven't found anything substantial

I've measured the resistance between terminals of all 3 mosfets PQ0204 PQ0205 PQ0201 every way I could think of, and the values range from ~51kOms into the megaohms, so I don't think there's any significant leak. And yes, for the dual mosfets I've made sure to measure all the terminals including the bottom pads which connect to PL0202. I can write down every value if you think it'll help, but it's a lot of readings, and they also change depending on which way I have the positive and negative multimeter leads connected.

I've also traced the path from SRN to SRP, both 10ohm resistors are present and measure 10ohms, and the PR0210 shunt also has very low resistance, although I can't measure accurately 0.01ohm with my DMM but it's under 0.1ohms and measures the same as the other shunt PR0201.

Again, I feel like the not charging behavior is more likely to be a symptom of the charger detection issue, rather than a fault of its own

[mon2]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

Remove the battery -> power up with only the external power adapter.

What is the voltage to ground of

BAT_PWR15 (battery pack PLUS connector) ?

CELL_BATPRES (Pin #18 on charger IC) ?

[Sephir0th]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

You have indeed ruled out some things properly so far and there is really not much left to check besides what mon2 has suggested above. On HPB platforms the ILIM pin can cause weird behaviour for charging. Should be fine if you can measure >1Volts there and good observation with ACIN. It shouldn't be the issue here since they it is lewt floating by factory. What is left is detailed visual inspection if the circuit for cold solder joints, missing components and cutted tracks.

[danandrei96]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

with only charger connected (no battery) I get the following
BAT_PWR15 is 0v both when the board is off and when it's on
CELL_BATPRES is 3.3V when the board is off, but drops to 0V after I press the power button and the board starts booting

[mon2]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

Now add the battery and also the power adapter.

What is the reading for CELL_BATPRES with this combination?

Measure while the board is OFF and after it is ON.

[danandrei96]

Re: Thinkpad E15 NM-C421 charger not detected after port replacement

with battery and charger connected, the CELL_BATPRES line measures 3.3V both with the machine off and with it on

not sure if it means anything at all, but I've also noticed that, in this scenario, VSYS15 is noticeably higher than VBAT/BAT_PWR15 (ie VBAT is 11.74v but VSYS15 is 11.88V). might not mean anything, but I found it interesting as it shows that the bq chip is generating voltage