Ok so I just had a bit of a eureka moment. Conventional battery flywheels are not made to be portable (gyroscopic forces make this quite difficult). This can be offset with gimbaled systems, negating the gyroscopic effects that occur in a fast-moving flywheel. However, these types of systems are often bulky and require a lot of space. My solution: build the flywheel into the gimbals! Picture this: a tube, bent to form a ring. This ring has a series of electromagnet arrays spaced out along its length. Within this ring, we put another ring, this one solid. This new "inner-ring" is our flywheel. Any comments/suggestions?