A little info about rechargeable LI-ON batteries
Li-Ion rechargeable batteries have a special circuit in them that allows the battery and the charging circuitry in the said device to talk to each other. The circuit in the battery communicates its current state to the charging circuits both when charging and being used. This circuit in turn determines how much charging current to send to the battery. To prevent overcharging the charging process goes at full speed until the battery reaches 80% of it's charge. At this point, the unit switches to a "trickle" charging mode. This is where most of the charging time is used.