The charger that Highflight has recommended is the only one I ever had, and it has kept my battery OK for four years.

It cost me $15 at Wal-Mart.

There are only two things that kill this type of batteries: strong vibrations and deep discharges.

Each of the six cells will not accept charge much more than what it needs at 2.4 volts each; it is just a chemical reaction inside.
I believe, they cannot be overcharged, since I have never seen any at more than 14.4 volts.

Check this tutorial out:
http://www.batterystuff.com/tutorial_battery.html