When the original FNB-72 1700 mAh battery in my FT817 finally died I decided to replace it with eight Ikea brand Ladda rechargeable AA NiMH batteries. At less than $10 for a pack of 4 it was quite reasonable and they boast a 2450 mAh capacity. Testing would indicate (http://budgetlightforum.com/node/47233) that these are nearly identical to the Eneloop Pro AA batteries of similar capacity. Both are rated for 500 recharge cycles and have a low temperature limit of down to -20 C (-4 F).
I had the opportunity to try them out today on VE7/SL-115 (and what a wet, windy day it was) and they performed quite well. The battery tray even fits more snugly than the old battery did, and I clipped the green wire to allow for internal charging. After an hour of use on SSB they only dropped from 11v to 9.7v, then bounced back to 10v after they warmed up again. The energy density isn’t as good as a lithium battery, but for a no-fuss replacement they work just fine. The best news is that I can operate at 5 watts again!