At the time being, the only battery that fits Marui MP7A1 is the Micro EX. Certainly the fans are not happy about the constrained power. Hopefully, here comes a solution from Nine Ball: External Battery Adapter for MP7A1! It greatly enhances both the discharge cycle and duration. Upon the successful conversion, 8.4V Mini/Small type battery can be connected through the kit's mini-male connector.

Note that 8.4V large type or above battery should be avoided as damage is possible.