If you want to spend the money, the Li-Ion's work great, but they're expensive. The Ni-MH's have been working good for me though this season. I had them out at -34ºC and they worked just fine though. I'll keep using the Ni-MH's this season and keep the Li-Ions as backup if the others fail. The true test is to cold soak them, which I haven't done yet on the Ni-MH's. If you go the Li-Ion route though, you'll have to stick them in a pack or something on the outside of the FX3 and wire up a plug to power it. Not a big deal, just that they won't fit into the battery compartment.