Theres an instance in Legion of the Damned where one of the Excoriators power packs is damaged and is slowly shutting down so the armor was getting heavier as time passed. He ended up staying behind and dying as a result because he wouldn't have been able to keep up. I can't remember if he said whether or not he'd be able to move when the power pack completely shut off though. I can recall reading a few other instances like that, though I can't remember which books they were from.
I think i've also read once or twice that a marine was still able to move around in their unpowered armor, but they were very much hindered by it as you might expect. I think, but I'm not entirely positive.
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