The quick field charger uses a POT to control the charge rate, as do a few others such as the Polycharger.

As such it is very inaccurate, and the pot will vary quite a bit. It is all too easy to set the control to too high of a rate.

I have a couple of these, though I use them only for occasional on field charging.

The Triton, ICE (Tower also sells this rebranded for more $$$ too) or any of the other better digitally controlled chargers are worth the investment. While more money up front, they will help to properly charge, condition and check your batteries.

The ICE features a nice historical charge/discharge graph for testing how your batteries are doing over time.