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I guess I'm a little late to the party, but:The sentence: "Connect a load of your choosing with a resistance of less than 1.4 k to the emitter terminal of the transistor (the nonconnected terminal) and to ground as shown in the circuit diagram." misstates the transistor terminal as being the emitter, when in fact it is the collector. The choice of the op amp is not arbitrary, since the inverting input is held near the V+ rail and for many op amps (esp. older ones) the rails are off limits and can result in destruction at worst and invalid operation at best.
You state NiMH in the intro, but in every case that I have run across, older power tools use NiCad, not NiMH. Their chemistries are slightly similar, their charged Voltage levels are about the same, but their self-discharge rates are way different and NiMH does not suffer from "memory effect". Newer varieties of NiMH have extremely good shelf life of 80% or better after 1 year.I agree with getting rid of the battery level monitor. Most hand drill have a three position switch, and should be stored with the switch in the center position (that also locks my B&D drill shafts) that is completely off, so the only "load" on the battery is its self-discharge.
Your analysis of the op amp based constant current source is incorrect. The FET is not SWITCHED on and off. If it were, this circuit would be an oscillator (which is possible with an unstable circuit). What a constant current should do is BIAS the FET so that the FET is only biased "on" enough to ensure that the Voltage drop across the current sense resistor is sufficient to balance the op amp's reference Voltage input. This is done within constraints of the op amp's open loop bandwidth.Not all Chinese parts are bad. They can't be, since even Japan outsources manufacturing to China. I suggest you repeat your testing on "known good" batteries and compare the results before you make your final judgement.
Nice, but it's way easier to just view your remote on a phone or tablet screen in camera mode or just on a digital camera's screen. Since their IR blocking filters cannot block ALL of the IR, it shows up as white. Push the button and you'll see it blink on the screen. Of course, you need to aim the IR output at the camera lens. Quick and easy.
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