Do you know of good external battery packs for ThinkPads?

[SteveDC]

Are there preferred external battery packs for ThinkPads? I used one for many years manufactured by Valence (the N-Charge model). It was about the size of a magazine and would fit neatly under the laptop, and last for much longer than I ever needed it. That company is no longer in that business.

A Google search turns up several options that I am not quite sure how to sort out. So, I wonder if there are models that people have had better experiences with.