Just recieved my Joby battery grip this morning...had a quick go with it & took some photos to post on here...looks promising will keep your OA charged all day nice comfortable silicone grip & not heavy at all it even has a screw thread on the bottom for tripod mounting..it has a 2600mAh rechargable battery built in the grip.
i know it's not a new idea it's been around for a while for the gopro's but at £29.99 it was too good to miss cause i don't think you can get more than 40-50mins out of the OA battery in the real world.
Just tried the joby grip out at Mallory park ..soon as the Osmo action battery got low on power i plugged the joby in & it charged it back up again whilst still filming...i'm chuffed with this grip it works superbly.