I found an upgraded battery for my Logitech G900 mouse and it has been working well for the past 2 months. The new battery has a capacity of 1000 mAh vs the original’s 750 mAh; it does seem to last longer on a single charge even though the Logitech Gaming Software still reports the same amount of running time (in my case, 25 hours with my lighting settings). The software is probably hard-coded for the original 750 mAh battery; I’ve noticed when the mouse is down to 20%, the software will continue to report there is 20% remaining for a few more hours before it continues to drop.
Opening the G900 is fairly straight forward; there is one T5 Torx screw by the micro-usb port and six 00 Phillips screws under some of the feet. I highly recommend purchasing a set of replacement feet rather than trying to reuse the ones you pull off. Be very careful when opening the G900 after removing the screws; there is a delicate ribbon cable connecting the two halves of the mouse as you can see below. You’ll want to disconnect the ribbon cable from the bottom half so you don’t accidentally rip it when you work on disconnecting and removing the battery. The battery is secured with double-sided tape so it will take some time to pry it out gently. The replacement battery has much longer leads but with some care it will all fit back inside.