When I first looked at the Nyko Energy Pack, a rechargeable battery pack designed for the Balance Board, I was dubious. I had no problem believing that the thing could function (as in be charged and make the balance board work), but I thought the form factor indicated a hack job. As it turns out, that was dumb and wrong of me. Though it may look unmistakably kludgey (involving a battery pack to which a cord is attached, which is then attached to another cord, which then goes to the wall), the Energy Pack is smartly designed, unobtrusive, and easy to use.
The main component of the Energy Pack is the, well, energy pack. It's a battery pack that fits flush into the Balance Board battery compartment, replacing the battery door. Conveniently, this battery pack puts a Balance Board sync button on the bottom of your board. A small adapter is plugged into a port on this battery pack, which runs out to the front of the Balance Board and clips into the screw holes next to the power button. This adapter serves two purposes: first, it allows the unit to be plugged in and unplugged easily fom the front of the board; and second, it displays the Energy Pack's charging status light.
The AC cord then plugs into this adapter. For a power cord, it seems like quite the deluxe item: it has a velcro strap for easy cord-winding, and a really bright blue light on the AC adapter part. Since the Balance Board is designed to sit directly in the middle of the room, and thus necessarily distant from walls which would house outlets, the Energy Pack requires a long cord. The cord comes in at ten feet long, meaning you can probably charge the Balance Board from its normal gameplay position in your room.
The only thing I can't speak about regarding this accessory is battery life. I've been using it for a day, and it has yet to die. Nyko promises 30 hours per charge, which sounds like plenty. Like all rechargeable batteries, I expect that this would lose charge capacity over time, but the ability to play while charging amelioirates those issues.
$20 seems like a lot to spend on an accessory for an accessory, but in the long run, rechargeable batteries are always worth it. It's a matter of personal preference whether you want to purchase a specialized unit for your Balance Board or just use rechargeable AAs, but it's certainly nice to be able to plug the Balance Board directly into the wall to charge, especially since you can do this while playing. If you're planning to use the board every day (as every Wii Fit owner is still planning to do, at least), it would be a sound purchase.