S2 is at least 2 years old. Whatever battery (1650 mAh) it came with is probably due to be replaced anyway.

Just get an extended battery such as Qcell [http://www.amazon.com/QCell-Samsung-SGH-i777-Extended-Compat...] (a whooping 3600 mAh!) and have same 4 days charge with all the updates/gps/bluetooth/etc enabled, all the time.