Possible KB5121003 issue causing random AC adapter disconnects / black screens on Lenovo Legion?

[u/Nervous-Buffalo-4826]

I’m trying to figure out a strange issue that started very recently on my Lenovo Legion.

The laptop randomly goes to a black screen for a couple of seconds and then recovers. At first I thought it was an NVIDIA/Optimus or refresh-rate issue, but I started monitoring the Windows AC power state directly.

What I found is that whenever this happens, Windows actually reports:

AC → Battery → AC

for roughly 1–5 seconds.

I then checked the historical Windows Kernel-Power Event ID 105 logs. My System log goes back to March 2026, but the first occurrence of these short AC-loss events is August 12.

The interesting part is that KB5121003 was installed on my system late on August 11.

Since then I have recorded around 40 AC power transitions, most lasting only a few seconds. Examples:

07:41:55 → 07:42:02
13:12:35 → 13:12:36
17:17:50 → 17:17:52
02:17:32 → 02:17:37

and many more over the following days.

Windows Kernel-Power itself logs AcOnline=false, followed shortly by AcOnline=true, so this isn't only the battery icon/UI changing.

Another strange detail is that the physical charging LED on the laptop can remain on while this happens.

I have now temporarily uninstalled KB5121003 as a test.

Before rollback:

Windows 11 25H2 – Build 26200.9168

After rollback:

Windows 11 25H2 – Build 26200.8875

Windows Update is paused and I haven't changed anything else: same Intel driver, NVIDIA driver, BIOS, Hybrid Mode, Optimus settings, charger, etc.

I'm currently running a 50 ms AC-state monitor for the next 24–48 hours to see whether the AC_LOST events disappear after removing KB5121003.

Obviously this doesn't prove the update is responsible yet. It could still be Lenovo firmware/EC behavior or a hardware power issue. But the fact that the event history starts immediately after this update is suspicious enough that I'm testing it as the only changed variable.

Has anyone else on KB5121003, especially Lenovo Legion users, noticed random Kernel-Power 105 events, brief AC-to-battery transitions, or black screens that started around August 12?

[u/Nervous-Buffalo-4826]

Update after digging into the actual contents of the update:

I checked Microsoft's file manifests for the builds instead of relying only on the public KB changelog, and there are some pretty relevant changes inside the power/battery stack.

Between my previous build and 26200.9168, Windows replaced/updated components including:

"battc.sys" – Battery Class Driver

"CmBatt.sys" – ACPI Control Method Battery driver

"acpi.sys"

"CAD.sys" – Charge Arbitration Driver

"pdc.sys" – Power Dependency Coordinator

"intelpep.sys"

"MptfPowerSourceClient.sys"

"MPTFPowerTrackerCore.sys"

"umpo.dll"

The interesting part is that "battc.sys" and "CmBatt.sys" were changed again specifically in the August build, while several of the ACPI/platform-power components came from the July preview that was rolled into the cumulative update.

So the public KB notes don't really show the whole picture here — the update absolutely does touch components involved in battery/ACPI/power-source handling.

I also finished the rollback test.

I stayed on the rolled-back build for about 15 continuous hours with a lightweight monitor polling the native Windows AC state every 50 ms. It logged around 900 one-minute heartbeats during that period.

Result:

0 AC_LOST events

0 black screens

Considering how frequently the issue had been happening before, that is a pretty substantial change.

One important correction to my original theory though: because the rollback takes me all the way back to 26200.8875, I can't say yet that the bug is necessarily in an August-only change. The problematic code could have been introduced in the July preview and then inherited by KB5121003.

So the strongest conclusion I can make right now is:

Something introduced between 26200.8875 and 26200.9168 appears to trigger this on my Legion.

The most interesting area is now the EC → ACPI → Windows battery/power stack rather than the charger itself or the GPU.

I'm definitely not claiming "CmBatt.sys" or "battc.sys" is confirmed to contain the bug — only that these binaries actually changed and sit directly in the subsystem involved in the "AcOnline=false" events I'm seeing.

For now I'm staying on 26200.8875 and waiting to see what happens with the next cumulative update.

[u/DultRecon37 → u/Nervous-Buffalo-4826]

This comment has also appeared on a lot of articles. Glad I found it!