First of all, battery voltage of 4.88 is alarmingly high and can damage your battery.
(Assuming the voltage measurement is correct of course. Double-check with a multimeter.)
Normally, li-ion chargers should stop charging when the battery gets to 4.2 volt, this is especially important since the teensy circuits aren't designed for anything over 5 volts.
So, I have to ask: You didn't use a 2-cell (7.4v) charger, right?