The cord to our original Wii bar was damaged by an unknown culprit, so we went out to buy a new one. All we could find were cordless models, which seemed fine until we discovered how much they eat batteries. So, I endeavored to make it corded, using an AC adapter from my stash, until I realized, "Hey, why not USB?" When it was nearly finished, my daughter said, "Dad, make an Instructable," so here it is. I know that I left out the soldering in wires stuff, but really, it's very easy.
Here we have the victim. It uses 4 AA cells, thus somewhere around or less than 6V. USB is 5V or just a little more, close enough. At least it wont be overpowered. I dug into my stash for a spare cheap USB cable and some other wire and stuff.
Like I said in the beginning, I didn't get pics for the whole procedure. Prior to what you see here, I noticed that the connector on one of my spare wires fit right onto the negative terminal, after it had been bent upward. (Don't let the color of the wire fool you, it is a negative.) I Dremeled some notches in the plastic dividers which separate each battery section, so that the wire will go across without interfering with the battery door. The positive battery terminal was desoldered from its wire, which was then soldered to the positive USB wire, inside the housing to the left. I had removed the screws inside the battery compartment and under the rubber feet in order to access the wire. I also Dremeled a notch for the cable to exit, and crimped a piece of solid wire around the cable to keep it from pulling out.
This is the first connection I made. I happened to have the red wire, with the connector already on it, in my electrical toolkit. Looking at the battery terminal, it occurred to me that if I lifted it up, the connector would fit right on. All it needed was to tighten it up a bit with a pair of pliers. At this point, I hadn't even unscrewed and split the case yet.
Here's where you missed part, but not much, really. I have removed the screws and opened the case. I removed the positive terminal, which used to live where the red wire goes into the housing. After unsoldering the teminal from the wire, I soldered the usb wires (black and red, easy-peasy) to the red negative wire and to the positive wire which used to be attached to the positive terminal. Then I just covered them with some electrical tape and crimped a stiff solid wire, about half-an-inch or so, around the cable so that it wouldn't pull out of the case. Its hole is just a Dremeled notch in the edge of the bottom half of the case.
Replace the battery cover, put the bar on top of the TV, and plug the USB cord into the Wii. Viola, no more batteries! These things come in different brands, constructed differently, and perhaps with different power requirements. YMMV. This is only a guide, but I hope that some of my work here, or at least the idea, can help someone else.