The ZIppy-H 3S 5000mah and 4s 3500mah packs fit perfectly in the e-revo compartments. But yeah, they are too long for a lot of people, but i find their length perfect, only their width is a problem in my emaxx where the inside edge rests on the raised section of the chassis and not the recess made for the battery pack. This however is a small trade off given the price of each pack, and 2x 4S 3500mah packs are still lighter than 2x 6 cell 4200mah packs, which I would have to run in series as well. A couple of well place velcro straps with foam chocks ensure the packs stay in one place.