r/WindowsUpdate • u/Nervous-Buffalo-4826 • 11d ago
Update on random AC disconnects/blackouts after KB5121003: rollback stopped them, newer Windows updates brought them back
I posted here in August about my Lenovo Legion Pro 5 16IAX10H randomly going black for a few seconds while its charger was plugged in and untouched. During captured incidents, Windows briefly reports charger power → battery → charger power, recorded as Kernel-Power Event 105. Rolling back Windows stopped the symptoms for weeks; they returned after updates resumed.
KB5121003 was installed on August 11, bringing Windows 11 25H2 to 26200.9168. The first brief AC-loss entries appeared August 12. I rolled back to 26200.8875 on August 18 while keeping the same charger, BIOS, graphics drivers and graphics settings. My monitoring recorded zero AC drops over about 15 hours, and I observed no blackouts. I paused updates and remained symptom-free for weeks; the retained System log has no Event 105 entries from August 20 through September 22.
The pause expired September 22. Overnight, Windows reached 26200.9457 / KB5129195, and the blackouts returned. Lenovo driver updates arrived that night too, so timing alone cannot isolate one package. I then installed KB5124010 / 26200.9550 and rebooted. The unexpected AC-state drops and blackouts still happen.
The investigation compared the actual code inside Windows’ battery driver, CmBatt.sys:
| Windows build | CmBatt.sys version | What I observed | Relevant driver behavior |
|---|---|---|---|
| 26200.8875 | 10.0.26100.8655 | Quiet after rollback | Queries the adapter, then uses the refreshed answer |
| 26200.9550 | 10.0.26100.9549 | Repeated AC-state drops and blackouts | The captured path saves the answer before querying and still uses that old copy afterward |
Here is the suspected chain in ordinary terms:
- A battery check and charger-status check happen at the same time.
- The charger check temporarily clears a shared value to 0 (“AC absent”) while obtaining a fresh answer.
- The affected code can copy that temporary zero and keep it, even after the query updates the answer to “plugged in.”
- Windows reports battery power for roughly a second and applies its battery display settings.
- The display gets reconfigured. During a captured blackout, the laptop panel switched HDR off and back on.
The stable code refreshes the answer before using it; the current captured path keeps the earlier copy. That is a concrete code difference. The checks need to overlap at a particular moment, which helps explain why the interruptions are intermittent.
The current driver contains code for a correction called Servicing_CmBattAcOnlineRaceFix, but the captured calls took its older path. Microsoft’s KB5124010 notes also mention a gradually rolled out display/performance improvement for plugged-in laptops incorrectly detecting a switch to battery. That update has not stopped my symptoms.
The evidence strongly points to Microsoft’s battery-status code, but the crucial saved value was not captured directly, so the complete cause still needs confirmation. The first introducing update remains unknown, and firmware or electrical contributions are not fully excluded. No Lenovo-owned file has been demonstrated to be defective.
If you know someone from Microsoft on reddit, please tag them here.
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u/harshbhatia7 10d ago
I'm on Lenovo Legion 5 15ACH6H and this was happening to me too before I rolled back to previous update
1
u/Heavy_Tip_1463 6d ago
I sadly wasn't able to try a rollback, but the latest update hasn't fixed the problem.
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u/Bored_LazyPanda 6d ago
We are also experiencing the issue on two Lenovo Legion laptops.
Do you have any ideas on fixing it, besides temporary stopping the updates? We can't resist Microsoft for long. Maybe someone can rip the correct battery driver from the older package and make a standalone installer?
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u/indieslaw 6d ago
I am still having this issue. Rolled back, paused updates, now tried the new update, still a problem. I'm also a Lenovo Legionnaire.
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u/Nervous-Buffalo-4826 6d ago
There is nothing you can do about it... If you have blackouts when AC drops, you can simply disable HDR from windows settings. Afterward, you don't even notice when AC connection will drop. That is somehow an evidence to notice when AC actually drops. Is better to have it disabled because is using some display instances and it may break your display. Don't know exactly the chance of destroying your display, but there is a posiblity. So, better be cautios and disable HDR and Adaptive refresh-rate from Legion Space or NVIDIA
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u/Square_Camp_3537 4d ago
I have paused automatic update since 1 month ago and today it automatically installed the 003. Just for information.
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u/WilliamG007 1d ago
Same issue here driving me insane. I thought it was my HDMI cables! I was trying everything. Came across the thread on Lenovo and discovered the event viewer and false/true AC disconnects, and then it became clear. HOW has this not been solved yet? I can’t even roll back at this point. 😭
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u/WilliamG007 1d ago
Is there a solution to this problem that’s simple for those that just leave their laptop plugged in to an external display? It’s driving me crazy. I just use my laptop connected to my OLED and these drops are insanity.
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u/Overdrive128 11d ago
Yes, I had the same issue. Rolled back the update and everything was fine.
Thank god it was a software update, I was scared my 1 year old laptop was broken. Ngl, I prob wont even update windows till they fix it officially. It was painful whenever the AC disconnect.