The 3400s can use either NiMH or LiON batteries although AFAIK all were shipped with LiON. Originally 5300s were also shipped with LiON but were recalled due to battery fires- firmware was changed to not allow LiON, only NiMH. So 5300 NiMH packs will be around and there were definitely 3rd party ones (BTI?). The ones I have now are not leaking at least externally, but are good and truly dead. From what I have been able to find, they can be rebuilt. So I am hoping to use them in my 3400's and live with their lower capacity. I am not sure if rebuild is hard but I am having a battery repair shop do them. I recently rebuilt a couple of Newton 2100 NiMH batteries... Now that was a nightmare as the clearance are SO tight that after what seemed to perfect seam gluing, I had to sand the cases almost to the batteries to get them to slide it.