To disconnect the battery did you use a plastic tool (spudger) or a metallic tool ? A metallic tool could have shorted the logic board charging pins in the socket sending 12 volts to the 3.4 volts circuit. If your old battery was holding a charge you could try to connect it and check if you get a normal start up with it. If you have a multimeter you could check the charging pins with battery disconnected and charger connected. Normally you should see + - 11v on the 3 first pins, 3.4 volts on the middle pins and 0v on the last 2 pins (ground).