U can test the phone if is he draining power too much whitout buying a new Sga battery. If you can find some battery like Nokia bl-5c(i use it sommetimes) or some battery similar pins (+//-) to fit it in and try it on.
Test old sga battery.
Try to charge your old Sga baterry in a univerzal charger,do not worry abouth overcharging beacuse it got to protect it.
Sent from my GT-S5830 using xda app-developers app