Galaxy S2, 3 year old. Battery went from 55% to 13% in 5 minutes, then phone started restarting
Was the battery life decrease sudden, or has it been getting noticeably worse recently? If so, might be time to get a new battery for your device. Regarding the restart, are there any apps that you've installed recently that might have a bug or something leading to restarts? When a phone reaches that age, I would personally root it and flash something like CM or Slimbean, to get some solid battery life back out of the device (I've personally flashed ChameleonOS on my S2 to get the best of both customization and battery).