iPhone has a new charged battery but displays empty icon
I've just replaced my battery on my 3GS. It measures 4.20V, so it's fully charged.
However, when I turn it on, the iPhone displays the 'empty battery'/'plug the charger' icon. Plugging the charger makes it show the 'empty battery'/'charging' icon, but it never gets to charge, which is expected as the battery is already charged.
Sometimes, after two or three hard resets (Power+Home) it displays the Apple logo for some seconds, then it displays an empty black screen for some seconds, then the default homescreen appears for 2 seconds, unresponsive, and the iPhone dies instantly.
All the "fuses" are working (closed). I've checked the PMIC for high temperatures with my finger while plugged in but it seems normal. I've also checked the mosfets circuitry and it's ok.