ThinkPad T470p – CMOS Checksum Error Persists After Main Battery Removal (even with new CMOS)

[Genghis_Khan]

Hello everyone,

I’m currently troubleshooting an issue on my Lenovo ThinkPad T470p (Type 20J6) and would really appreciate any insights or solutions.

Issue summary:

Whenever I remove the main laptop battery and AC adaptor, then place them back, the system gives a “CMOS checksum error” at next boot, and the BIOS clock resets to its default time/date.

However, the RTC works fine as long as the main battery remains attached — even if the laptop is unplugged for hours. Same as I remove the battery while the AC is attached, RTC remains. Obviously, the CMOS battery does not supply the NVRAM to store the RTC.

What I’ve done so far:

Installed a brand-new CMOS (RTC) battery — polarity confirmed correct.
Measured 3.22 V on the VBAT/RTC rail near the connector (stable).
Performed power drain (removed main battery, held power button 30 sec).
Updated to the latest BIOS version 1.41 (R0FET61W) released 7 Nov 2023 (from Lenovo support site).
Verified that BIOS correctly holds settings during soft reboots, only resets after full battery and AC removal.
Despite all this, I still get the checksum error whenever the main battery is detached — even briefly.

Questions:

Is anyone else with a T470p on BIOS 1.41 experiencing this issue?
Does the latest BIOS package include the most recent Embedded Controller (EC) firmware, or could mine be outdated?
Has anyone found a hardware fix or modification (e.g., cleaning the RTC line or addressing the EC/RTC hand-off issue)?
Is there an unofficial or engineering build of the EC firmware that resolves RTC retention on the T470p?
Any detailed guidance would be appreciated. I’ve already ruled out a weak coin-cell, since I've used a few brand new ones. So I’m wondering, if this is a known EC power hand-off fault on certain board revisions (mine is NM-B071).

And the one that I dread most to ask - Do you think my BIOS corrupted?

Thanks in advance for your help!

—
System details:

Lenovo ThinkPad T470p (Type 20JN-S30G00)
BIOS R0FET61W (1.41, 07-Nov-2023)
EC Version R0FHTxxW (unknown, please advise how to verify)
CMOS battery: 3.22 V measured
Board ID: NM-B071
OS: Windows 11 Pro 64-bit

[reformatt]

With only CMOS battery connected, check for 3V at RTCVCC. If missing, then check the connector JRTC1, R2501 and D2502.

[Genghis_Khan]

With only CMOS battery connected, check for 3V at RTCVCC. If missing, then check the connector JRTC1, R2501 and D2502.

Dear reformatt,

Thanks for the lowdown, sir!

Turns out the JME1 pad was yeeted off the board by the previous owner, breaking the RTC_VCC line right before D2501. So that 3.22V from VBAT never stood a chance of getting there!
You’re truly a Badcaps Legend!

Time for a micro-jumper….and a macro-jump of happiness!

I really appreciate the advice!

Kindest regards,

Genghis_Khan