The module comes apart easily. Be sure to remove the module from AC power before opening it. The top panel comes off after removing screws on the side, top and back. The bottom panel comes off separately after removing screws on the bottom. This is a view of the battery on the top of the board. It's a CR2032 Lithium 3V battery. These are easy to find, I bought a replacement at Radioshack for about 6 dollars.
The factory fitted battery is soldered in place and attached via spot-welded leads. The leads marked above attach the battery to the board.
I removed the old battery using a desoldering sucker tool. I recommend this method instead of using desoldering braid as it uses less heat. The battery can burst if too much heat applied during removal. Also note that all saved data will be lost once the battery is removed.
Since I don't want to desolder the battery every 5 years, I decided to install a battery holder in place of the spot-welded leaded battery. It's easy to find flat CR2032 battery holders, but the spacing on the board difficult to match. I found a holder with 20mm spacing which I could modify to fit. The positive lead remained straight while the negative lead will be bent an extended to fit. To make room for the bend I removed some of the plastic behind the pin with a Dremel tool. I then bent the pin back into the hole.
A resistor lead was cut and shaped to match the lead spacing of the battery. This was carefully soldered to the pin on the holder. Here are the old battery and the new holder for comparison. I tested the old battery with a multimeter and it registered at only 0.3V. The fresh battery will provide slightly more than 3V.
Some hot glue was used to fill the plastic hole removed earlier and hold the soldered lead in place. This holds the lead in place when soldering the other end in place. A piece of electric tape was cut and fit over the hole on the bottom of the holder. This will keep glue from getting into the holder in the next steps.
Here is the component side of the board with the battery removed. The + sign marks the positive lead of the battery holder. Polarity is very important when replacing the battery.
A generous amount of hot glue was applied to the bottom of the battery holder right before it was placed into the board. Hot glue was also applied to the sides of the holder once it was in place. The holder was held in place until the glue dried to keep the surfaces firmly held together. The leads where then soldered in place on the bottom of the board.
The CR2032 battery was then installed. It was not quite the tight fit I was expecting. In the future, I would be more discerning when selecting the holder. This model cost under 2 dollars from Radioshack. A holder with slightly higher walls would hold the battery more securely.
Electrical tape was used to hold everything in place securely. This is an extra precaution to keep the battery firmly in place. The bottom and top panels were replaced.
A factory reset function is available via a power up button combination. Simply hold both cursor keys down when powering up the unit. The firmware revision will be displayed, then the module will inquire regarding a button test. Press the Yes key and push each button in succession. A factory reset occurs when this test is performed, meaning all saved data will be overwritten with factory patches. This restores the Performance patches as well. Thanks to the the folks at The Yamaha TX81Z Homepage for information about the factory reset.
I did some more testing over a few days and now the module is retaining patches again!