My wife's nearly 5-year-old Logitech MX Ergo trackball's battery no longer retains its charge. She has the bad habit of keeping it plugged in for long periods of time. I bought a replacement 500mAh 3.7v battery from Amazon for $13. Very pricey for such a nothing of a battery, but I couldn't find it any cheaper after shipping from Ali-express, Banggoods, ebay, or anywhere else. Whatever. Here I replace it. Very simple overall, but the 2 top T6 screw ports on the back of the MX ergo is deep as a filthy ho. So I had to drill it out to access it. Sucked big time. Anyway, it is otherwise easy as pie to replace the battery.

Here's the one I bought:
https://www.amazon.com/gp/product/B07...