Galaxy S2, 3 year old. Battery went from 55% to 13% in 5 minutes, then phone started restarting this morning going to work I had my battery at 55% was listening to music and chatting in skype. All of the sudden skype started lagging really hard and then phone restarted. When it turned back on, the battery had went down to 13% and phone restarted quite a few times ever since

what could cause this?
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).