This little circuit will alert the user regarding a battery reaching its full-charge level (over charge) while it's being charged, by illuminating an LED. The circuit uses just a couple of transistors as the main active components.
The main feature of this design is not only its mini design but also its supply voltage specs which can be as low as 2V, meaning it can be used for all batteries ranging from 2V to probably 60V with minor changes
I have already discussed a similar concept which is designed for exactly the opposite function, that is to indicate the lower discharge threshold of a battery.
Simulation and Working
Both the designs are exactly similar, just having opposite polarities and indication specs.
Referring to the proposed battery full charge indicator circuit we can see that it is made using just a couple of transistors and some resistors and a single preset for setting up the desired upper threshold indication.
The LEd is supposed to begin illuminating as soon as the battery reaches approximately close to the set threshold.
The setting up procedure of the preset is actually very simple.
The user must feed a supply voltage that may be equal to desired high charge level of the battery, and then gently adjust the preset with a screw driver to force the LED to illuminate marginally.
For example suppose the indicator circuit is been installed for monitoring a 12V battery over charge level at 14.3V, then the preset may be tweaked to make sure that the LED just begins illuminating dimly at around 14V, that would automatically enable the illumination to grow at its optimal brightness until the charging level has reached the 14.4 or 14.5V limit.