In this instructable, I'll show you how I modded a Logitech Performance Mouse MX into a wired mouse, eliminating the annoying battery and charging with the rigid cable.
I got rid of the battery compartment and the USB connector, cut an USB extension cord I had lying around to connect both the unifying receiver inside the mouse and power for the mouse itself, added a voltage regulator for clean 1.5V for the battery connection and stuffed it all back in.
Step 1: Disassembly
First of all, remove the gliders on the bottom, the screws are hidden under them. Take care to tear off all of the glider because they tend to rip apart into the gliding part and the sticky part, rendering them useless.
Then, open the case and take care not to pull too hard on the cable connecting the upper shell with the board. Unplug the upper shell and put it aside, we don't mod anything there. Unscrew the battery compartment and remove it from the board. Unplug the sensor board (the flex cable). To do so, pull the dark lid up to unlock and the flex cable goes out without any force.
Pull out the small plastic bolt holding the mouse wheel down (it's near the button locking the endles scrolling function) and remove the wheel. Then, unscrew all screws and lift the main board up. Finally, unscrew the USB port board and remove it, too.
Unsolder the USB connector board, no need for that anymore. In the pictures, I unsoldered the thermal sensor of the battery compartment, too. Not a good idea as the mouse logic relies on this sensor giving valid data, so don't unsolder it, just take it off the battery compartment. I re-soldered it later once I noticed the error.
Step 2: Wiring
Now cut the USB extension cord just like in the photo and re-solder it. Doesn't sound very smart but we need access to the power wires (red and black) for the mouse. The female USB connector is for the unifying receiver and must stay intact. Solder a red and a black wire to the USB red and black wires and insulate all wires against each other (wrap electrical tape around each individual solder joint and finally around everything). I also soldered the shielding together to make sure everything was connected as before I cut the cable.
I first tried directly sourcing 5V to the board where the USB connector used to do that but ignored the small SMD parts on the USB connector board. Seems like they do have a purpose, the mouse blinked red and refused to work. In the next step, we'll connect a voltage regulator to get clean 1.5V battery voltage for the battery connector. That works much better.
Step 3: Voltage Regulation
To get 1.5V battery voltage from a 4.5 to 5.0V supply (i.e. the USB port), I used the good old LM317 linear voltage regulator. It is not super-efficient but it does its job. And it needs virtually no external parts being soldered to it (except from two resistors to adjust voltage).
Each LM317 has three pins: Adjust, In, Out. I added one 200 Ohm resistor from Adjust to Ground (the black wire of the USB cable) and one 1 kOhm resistor from Adjust to Out. This sets the output voltage to 1.5V with an input voltage of everything above about 3V. Then, I soldered the battery connector (cut off from the battery compartment) with the black wire directly to Ground (the black wire of the USB cable) and with the red wire directly to the LM317's Out pin. And I connected the USB cable's red wire directly to the LM317's In pin. A quick check with the multimeter verified clean 1.5V coming out of the battery connector.
Add a little electrical tape again and we can begin with the stuffing.
Step 4: Assembly & Result
Put all pieces back in places in reverse order and screw them tight. The unifying receiver and the USB connector it's plugged into nicely fit where the battery compartment used to be. As the battery compartment now is missing, the three screws that held it and also held the board are now obsolete. But the remaining screws hold the board good enough.
For the cable, I led it under the board on the right-hand side of the mouse (in the pictures: on the bottom). Near the front, there are a few plastic poles I could use to clamp the cable in between before leading it out through the hole where the USB connector used to be. So there is no tension on the cable on the inside if someone pulls on it outside the mouse.
To connect the upper shell, you pull the USB connector with the unifying receiver down through the battery replacement door at the bottom. Then you can easily put the shell back on and screw it closed. Finally, tilt and push the USB connector back in. It's tight but it does fit. At least in my case it did fit.
Voila, now you have a wired Performance Mouse MX with no need to recharge a battery ever again! And it's a little lighter, too.