It needs to be 14500, PCB protected and 3.7v
There are higher mAh ratings than 800, but those are mostly fake or inferior to most genuine Li-Ion batteries.
The max capacity rating of genuine Li-Ion batteries that are suitable for Ultrasabers' battery packs is 800 mAh (mostly from TrustFire or XTAR).
The trustfires US used to ship are 900mAh I still prefer those over the cells they ship now. After 6 years I haven't had any go bad and they still run longer than the newer yellow ones.