This problem started within the last month or so. The battery discharges pretty quickly, within 12 hours while before it used to last for almost a week. I don't have data plan, I only use my Galaxy S2 as a phone, clock alarm, camera, small things like that. When I check the battery status it says that Android OS and Screen use the highest percentage of battery power. I've also noticed that it discharge irregularly. At times the battery drops sharply by about 20-30% with the span of half hour and the phone feels warmer, while at other times it doesn't drop that much and phone is much cooler. As if there is a sudden burst of activity. The Android version on the phone is 4.1.2. I have also reduced the screen brightness, removed apps that I don't need and that can be removed and even bought a new battery. Non of this helped.
Is this a known problem with Galaxy S2? I have had my phone for many years and this is the first time I experience this. Is there anything I can do to find out what is causing the rapid battery discharge, apart from buying a new phone that doesn't have known battery problems?