I apologize for leaving you stranded, or rather "in the dark" in post #42. This circuit operates by producing a square wave out with a very long time period. Minimum time period of square wave is 3 to 4 seconds. This duration is determined by R4 and adjusted "at the factory" and is constant. During that section of the square wave, heater element is not energized and considered a rest period for the battery. The second part of the square wave is determined by R5 and user adjusted. This time duration varies from virtually 0 seconds to 20 seconds. During that portion of the wave, the heater element is energized and current is limited by only internal resistances of the battery pack, the FET, and the load The load value is nominally 4Ωs. So the overall time period of the square wave varies as user demand for heat changes. At the 555 timer output, pin 3, the set time is high and is the charge period of C2. Again, at the timer output pin, the adjustable time duration, the output to the heater is active and pin 3 is low, the discharge time of C2 which is affected by the low current value flowing thru R4, so the formula T = RC is a bit skewed. During the C2 charge period, discharge element internal to the timer is virtual infinite resistance, theoretically, and negligible. Aout to the heater element is sufficiently stable during the usable portion of the NiMH battery current output curve. Overall user time of the unit varies from 20 minutes, extreme demand, to a nominal valve of 90 to 120 minutes, to a value of 4 hours, minimal demand. Hopefully I haven't skipped over any portions of theory of operation.
Another device which will use 2 - 6 cell batteries in parallel, uses an off the shelf PWM which produces a square wave at a frequency of 25KHz, a time period of 40μs, ( that frequency seems too fast so I will verify it before I post again). The conventional use of this PWM gives a finer control of power out but in actuality Aout is limited only by battery pack internal resistance and FET resistance. I need this high current to achieve fast response from load, in this case, not a heater element.