I use reclaimed laptop cells... I'd say about 75% are keepers, with the odd one being 'dead' and getting damaged by my less-than-subtle attempts to get them out.
The cells are all unprotected, but they are of good quality - I can't say what capacity they have as I don't have the kit to measure them, but they charge up to 4.15v, they hold their charge, and they give decent run times. It would be interesting to have a 'known' quality cell to compare them to... maybe I'll buy one at some point, but a decent cell costs more than a new light!
I now have a C8 XML-U3 and 2 x 501b XM-L2s... that's just over £30 at today's prices.