I just powered off my C-3 a couple hours ago... the final shutdown having migrated the ZWave stick to a C-5.
However, in the 3 years I've had it, I haven't encountered the problem you describe. The hub does 100/Full ONLY and there could be a problem if they aren't negotiating nicely.
There's a discussion (today) about this negotiation question.. maybe there's an answer there??