Thanks for the explanation. It makes perfect sense, if the battery is dropping to 2.6V or something and the mosfet is not turned on very hard it will be dropping voltage as well as the sense resistor so the ld will be left with less voltage. (at least that's how it sounds in my head
)
How would you tell if a mosfet is a logic level mosfet? I have looked at the IRLXXXX datasheets and find that the gate threshold voltage and graphs that plot drain current vs gate to source voltage seem to be very similar to other mosfets that don't seem to mention that they are a logic level mosfet. Like the 2SK1112 for instance. The datasheet claims a max RDS(on) of 0.3ohms with a Vgs of 4V. To my (inexperienced) mind, that kind of seems like it would be perfect to switching decent current at a logic level of 5V. So why would it not be considered a "logic level" fet?
Sorry for all the question. Maybe I should start a new thread with questions like this. :-?