Guru
Joined: 05/10/2019
Location: United KingdomPosts: 8894
| Posted: 08:13am 10 Mar 2025 |
For lithium cells you won't be far off with a resistor, Michael. You are only working over about a 1V range from fully charged to the 3V cut-off point. No good for primary cells.
The above circuit is easy to control though, just short the base to GND to stop the charge. You'd need a transistor or mosfet in series with the resistor anyway so it only adds a resistor and LED. You could use a 3V3 supply for the LED but increasing the voltage and series resistor causes it to behave more like a constant current source for the LED and increases accuracy a bit. 12V would be even better but you would then need a transistor to short out the LED.
Measuring the voltage needs a mosfet to isolate the potential divider if you want best accuracy otherwise the PD forms a non-linear load in parallel with the test current. Easy enough to do if you can be bothered. :)
It's an interesting little circuit. The temperature coefficient of a red LED running in its "sweet spot" (usually 5-10mA but it varies) is similar to that of a silicon P-N junction but opposite so the current through R has at least a degree of temperature compensation. :) If you don't get 1V2 across R then just change the 1V2 "fudge factor" and re-calculate R.