I’ve done a number of replace-batteries with a charger projects (biggest savings was for a child-swing). Having recently purchased an Apple Magic Trackpad I disliked that it required batteries. It makes some sense for a mouse to loose the cord, but a trackpad is stationary and corded (aesthetics aside) power simply makes one less device that I need to recharge.
Original battery powered trackpad.
The trackpad is powered by a couple of AA batteries or 3 volts.
I took an old USB cord and cut off the female connector.
I started with a USB 2.0 cord so I had four wires after removing the insulation. Because you’re just adding power you don’t need the ground or the green wire (which is for data), just the red and black.
I used a 1/2 dowel as a mass replacement for the two AA batteries.
The fit is slightly loose, I tightened it up by adding a few wraps of electrical tape to both ends.
I wanted a really clean replacement with all the guts on the inside. I bored out the center of the dowel to thread the power through it.
Electrical tape increasing the width of the dowel.
The far end of the battery compartment has a slight spring loading, but to improve the fit I bored out a little wider hole on each end in which I could place a spring.
Next I needed an exit for the power. I could have bored into the wall of the battery container but it would have made insertion harder. I opted to drill out screw-top lid.
To keep the lid from spinning as a drilled it out, I placed a washer in my vice and set the lid slot over it.
Wire threaded through the lid.
Fully assembled battery replacement.
I tried the direct power from the USB (5 volts) and it worked. But given that that’s almost twice the original power I decided to add a resistor to bring it closer to 3 volts.
I soldered in a 4.7 mega ohm resistor to the positive end spring, bringing down the power to ~3.4 volts, close enough. After sharing this design I got feedback from some smart people who told me I didn’t need to worry about the resistor and in fact the one I had chosen was likely to make the pad perform unreliably. I subsequently removed the resistor and have had a solid performance from the trackpad.
Finished modification, almost neat enough for Apple.