The best battery for you depends on what you want. The 18350 tends to take a little less time to charge, last a little less time in use and is a bit smaller. The 18650 takes a bit longer to charge, lasts a bit longer and is a bit bigger. Technically the MAH rating of the battery determines charge and use time but the bigger batteries usually hold more energy. I would look for something with 20amp+ rating. Because of the safety limits on the device you would be fine with a 10amp limit battery but I like to invest in the future and one day you may want something that can deliver more. A full charged battery uses 20amps at .21ohm which is well in the sub range. Sony VTC4 or 5 where the favorites last time I looked with that kinda rating. Sony may be on the VTC8 by now for all I know but you get the gist.