Glad I found this thread... They actually changed a battery (maybe on on all models, but some changes were made). I saw a "used Prius C battery" that says "Lithium Ion". Inside the battery, it looks slightly different - the "individual cells" have a connector on the side. So, may be it was "third-party upgrade" (not an original Toyota pack), but there are Li-Ion battery packs for Prius C. That's why I'm curious myself, if it's fully compatible with 2013 Prius, both, size-wise and computer-wise...
yes, anything li-on would be aftermarket. there aren't many. check these guys: prius-lithium-replacement-pack
