Battery Full Charge Indicator Circuit using Two Transistors

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.

Main Feature

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.

Now let's see how the persent circuit is designed to function and how it can be set to perform the required battery warning indication.

We will study two simple designs, the first one will switch ON an LED at the full charge level of the battery while the second one can be used to do just the opposite, that is switch it OFF at the set preset value.

LED Switching ON When Battery becomes Full

The circuit diagram shown below is intended to illuminate the LED indicator as soon as the connected battery reaches its full charge level.

How to Fix the Presets

To set up the circuit the user has to feed the desired upper charge level to the circuit, and adjust the preset such that the LED just begins to illuminate brightly at that level.

Video Clip:

LED Switching OFF at Full Battery

The following circuit is configured to force or to turn off the LED when the battery reaches its upper charge level.

For users who wish to see the LED switch OFF at the upper threshold can use the above shown design, the working may be understood wit the following points:

As per the requirement the LED illumination is supposed to begin diminishing as soon as the battery reaches approximately close to the set full charge 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 just shut down at the desired level..

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 shutting down at around 14V.

PCB Design