X220: Computer won't boot after momentary CMOS battery disconnect

[WormMan]

Here is a recurring phenomenon I have observed both times I have de-assembled my two X220 to replace the thermal paste and the thermal mat. This requires the CMOS battery to be disconnected for a moment. Both times, after putting the machines back together it won't boot with the regular battery pack plugged in and I need to let it sit for a few hours. Then it will only boot with the regular battery unplugged. The first machine started acting normally after that and I hope this one will also.

1. Is this a sign that I need to replace my CMOS battery?
2. Why can't I boot with the regular battery-pack plugged in, but only with the battery-pack unplugged?
3. Why does the computer need to sit for 1-2 hours after re-assembly before I can boot? (This happened to me twice so there is some kind of logic to it, I suppose).

Clearly there's something about the hardware/battery I have yet to understand, I would be very thankful if anyone could take me to school on the topic.
Best regards, WormMan

[WormMan]

I should add I had a look at the Hardware Maintenance Manual for X220 without seeing anything related to these symptoms. A link or something where I might read up upon these aspects of the machine could bring me a long way.

[RealBlackStuff]

There is absolutely NO reason to disconnect/remove the CMOS when you replace the thermal paste.
By now those CMOS batteries are about 10 years old, time to replace them.
For a cheap how-to see here: viewtopic.php?p=863727#p863727
(The X220 needs CR2032.)

[WormMan]

Hey, thanks a lot for responding RealBlackStuff.

There is absolutely NO reason to disconnect/remove the CMOS when you replace the thermal paste.

You're right. I just got into the habit of doing it.

By now those CMOS batteries are about 10 years old, time to replace them.

What function does the CMOS battery have in booting? What is the possible explanation for this situation where the machine needs to sit for a while before it can boot?

I'll replace them for sure. I'd just really like to learn more than the habit of replacing the CMOS battery from this.

[RealBlackStuff]

CMOS batteries keep the BIOS-settings 'alive', such as date/time, boot-drive preference, too many details to just list here. (look in the BIOS to get an idea).
If CMOS is not working, booting (and other items) will be slowed down, because so much needs to be recalculated from 'scratch'.

[WormMan]

OK. I get it. If anybody else searches for X220 boot problems after disassemble I hope they'll find this post. Thanks RBS for providing some background.