After a good initial 12 hour charge when the
battery pack is new, it doesn't always need
the full duration to charge. When the battery
area of the transmitter gets warm the pack is
probably fully charged.

Whether or not it gets overcharged depends
on the charger output and the battery capacity.
If the charger puts out around 1/10 the mA
of the battery's mAh, it won't overcharge.
For example a 75 mA charger on a 700 mAh
battery will not overcharge. The batteries will
get warm once fully charged, but not hot
enough to do damage.

A 100 mA charger on a 600 mAh battery will
make the batteries get hot once they are fully
charged and may cause the cells to vent which
reduces their life.

Your transmitter will probably tell you when
the battery is too low to continue flying, so
you don't need to worry about undercharging
too much. If it does tell you this, do the full
12-16 hour charge next time. Unless you
hog the airfield all day, you probably won't
run your batteries down that low.

Jenny