My RunCam HD died after 5 months in a drawer. Reason: battery destroyed. It had enough energy for approx. 2 seconds of operation. So I had a choice: buy original battery from RunCam for $9.99 plus shipment, or fix it DIY style. I’ve choces the second option.
Luckily for us, RunCam used generic 803035 LiPo 1S, 3,7V battery that can be purchased in specialized stores. I’ve paid around $6 for mine, but if you look hard enough, you should find something even cheaper. Also, luckily (or not) there is a battery plug: 2 pin JST 1.25mm.
Unfortunately, my new battery came without any plug, so I just resoldered old plug into new battery. It took like 4 minutes of work.
After that, new battery could just be plugged into PCB.
Remember, that RunCam HD stores settings in RAM memory, not flash. If you disconnect internal battery, all custom settings will be lost. The same goes for clock.