I realize I've never noted it here, but sometime in the middle of last year I picked Pokemon Go up again. The amount of time I've been able to play has fluctuated a lot over the months, but it's proven itself a good excuse to get outside and do some walking on day when I'd just as well hide inside. There are 3rd party devices that interact with the game (they actually emulate the official peripheral but add several useful features, notably attempting to auto-catch pokemon that spawn near you without any outward fanfare and button-pressing). I understand that some players regard these devices negatively, since they automate a core gameplay loop, but since I derive most of my enjoyment out of the cooperative raiding portion of the game, the idea of scraping up a little extra resource while walking around doesn't feel particularly game-breaking.
With all that context out of the way - I'd helped a friend acquire a Gotcha Ranger, which had worked quite well for him over several months before it suddenly began acting up. After running through the standard debugging steps without success, he handed it off to me to see if it could be salvaged. I was able to pop it open by hand and take a look.
The problem was apparent pretty immediately, the lithium-polymer battery had ruptured and some telltale corrosion was apparent on both the swollen battery and the housing. After some googling around I was able to locate a similar-sized 3.7V li-po battery with a shade less capacity, and was able to get it next-day.
With the new battery in-hand I removed the two screws holding the board in the housing and examined the back face of it. There are a couple other posts online describing the connections, but I hadn't yet seen a labelled photo, so here we go. It's worth noting that the convention appears to be blue wires are always ground, the copper or red sleeved ones are positive.
Noticing the damage to the phone-charging wires, my friend asked me to disconnect that bit entirely as he never used it. Fair enough. I desoldered those connections, as well as the ones for the battery. After the unnecessary connector was snipped off the new battery, it was soldered to the board. Similarly, the wires coming off the front panel (display and controller) were soldered to the other two points near the battery-end of the board. The battery shipped discharged, but once I provided power via USB I was able to confirm everything was connected correctly. It took about 30 minutes for the light indicating active charging to settle down, and afterwards I confirmed function while disconnected from the USB.
With that I closed it up and made sure it could connect to my phone. A $6 battery and maybe 15 minutes of soldering to prevent the replacement of a $45 device - not too shabby.