Originally Posted by mmystere68
I wanted to know whether the i777 battery is compatible with the the i9100 and vice versa.
You'll lose NFC, as the 9100 doesn't support it, and you may have random reboots from time to time... but it will sort-of work. In the "old" days (I'm sure it still persists), charging the 9100 battery inside an i777 sometimes wouldn't work, so just be aware of these potential issues.
These were my experiences with it 2 years ago (on the i777 side, can't speak to the 9100), and while there are some around here who will say they ARE compatible, they're not really.
One other thing, please search next time before posting - these threads are all over this forum and have been answered/debated for 2 1/2 years.