In the circuit you describe above, the voltage across the resistor and LED will vary between 8.4V and 6V. With the right value of resistor, that could vary the brightness of the led but the value of the resistor will have to be picked by trial and error. Even then, the results will vary if you change the LED for one of the same type because of the spread of its characteristics. The value you are asking us for will probably be somewhere between 5Kohms and 100Kohms depending on exactly which led you are using. They don't all give the same amount of light for the same current!voltage variation charged to flat 26.4v to 24v, less 2x 9 volt zenners, less 3v LED, less a resistor voltage. as battery volts fall, so does current thru LED. pls answer question. i know an led/resistor alone would not do it. general indication only req'd.
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by Ikimi .O