I used the Rhinos in my MERV for 3S. I think they were 3S 1550mAh 30C packs.(25C and 20C ones should fit even easier) They were tight going in and out but it was possible (little bit more effort than normal) to close the doors. For 4S running I used Turnigy 2S 1600mAh 30C. You can get bigger packs in there but it takes a lot of effort to push them in and pull them out of the compartments. Not to mention the extra weight of the bigger packs kills the handling a lot in these little fellas.