Yes. Many ThinkPads had a small internal battery to power it when you were swapping the main battery.

Heh - my TRS-80 Model 100 has that feature:

A small NiCad (though I'll be swapping it with a newer super capacitor soon) keeps the memory contents available when you swap out the four AA batteries if they run low (you get several days of runtime off them).

That is awesome, I had no idea. Dumb question, do you know what the feature is called, just hot-swappable capable or something for when Im shopping?

“ThinkPad Internal Battery” works on Google.