TL431 is a cheap ‘n’ cheerful yet reliable programmable precision shunt regulator chip available in 3-pin TO-92 package. This monolithic IC voltage reference operates as a low temperature coefficient zener which is programmable from Vref to 36 V with only two external resistors.
In this article, you can find two simple circuits built around the nifty TL431 chip, designed as add-ons for 1S lithium-Ion (Li-Ion) battery circuits. The circuit at the left side in the image shown below is a low-battery indicator while the right side one is a battery-full indicator. First of all, note that the nominal voltage of today’s single-cell (1S) Li-Ion battery is 3.7V whereas its maximum voltage is around 4.2V, and discharging below 2.8V will kill the battery sooner or later!
Both circuits can be connected in parallel with any 1S Li-Ion battery. However, you should calibrate them before the initial use as indicated below:
- Low-Battery Indicator: Connect the circuit to a lab power supply with correct polarity (VBAT=+ & GND = GND) and adjust the trimpot (RP1) so that its wiper (connected to the reference terminal of the chip) is at 2.5 V when the dc input is 3V. Check that the blue indicator lights up at this stage
- Battery-Full Indicator: Connect the circuit to the lab power supply with correct polarity (VBAT=+ & GND = GND) and adjust the trimpot (RP2) so that its wiper is at 2.5 V when the dc input is 4.2V. Check that the red indicator lights up at this stage
After calibration, ensure that the blue indicator in the low-battery indicator lights up when the battery voltage is at 3.0V (or falls below 2.9V), and the red indicator wakes up when the battery voltage reaches at 4.2V (or goes above 4.1V). Low-current (2-5mA) type 3mm blue/red LEDs are preferred for these circuits. Similarly, multi-turn trimpots are good here for better precision.
I hope that you leave learned something new and have fun with this idea. Very best regards from T.K !