A simple indicator of the discharge of Li-ion batteries

How tightly did Li-ion batteries come into our lives? The fact that they are used in almost all microprocessor electronics is already the norm. So radio amateurs have long taken them into service and used in their homemade products. I contribute to this significant advantages of Li-ion batteries, such as small size, large capacity, a large selection of versions of different capacities and forms.
Simple indicator of the discharge of Li-ion batteries
The most common battery has a mark of 18650, its voltage is 3.7 V. For which I will have a discharge indicator. Probably, you should not tell , how harmful to the battery crane is their low discharge. And for batteries of all varieties. Proper operation of batteries will prolong their life several times and save you money.

Charging Indicator

The circuit is quite universal and can work in the range of 3-15 volts. The threshold can be adjusted with a variable resistor. So the device can be used for almost any battery, whether it is acid, nickel-cadmium (nicd) or lithium-ion (Li-ion). The circuit monitors the voltage and as soon as it falls below a predetermined level - the LED lights up, signaling low battery discharge. The circuit uses an adjustableZener diode TL431(link where it took). In general, this zener diode is a very interesting radio element, which can greatly facilitate the life of radio amateurs, in the construction of circuits tied to stabilization or threshold operation. So take it into service, especially when building power supplies, current stabilization circuits, etc. The transistor can be replaced with any other NPN structure, the domestic analogue of KT315, KT3102. R2- adjusts the brightness of the LED. R1 is a variable resistor of 50 to 150 kΩ. The value of R3 can be added up to 20-30 kΩ to save energy if you use a transistor with a high transmission coefficient. If you do not have an adjustable stabilizer TL431,you can use the proven Soviet circuit on two transistors.
Simple indicator of the discharge of Li-ion batteries
The threshold is set by resistors R2, R3. Instead, you can solder a single variable to allow adjustment and reduce the number of elements. Soviet transistors can be replaced by BC237, BC238, BC317 (КТ3102) and BC556, BC557 (КТ3107).
Simple discharge indicator of Li-ion batteries
The circuit can be assembled on a board or mounted mounting. Put on a heat shrink tube and blow a heat gun. Stick on double-sided tape to the back of the case. I personally installed this board in a screwdriver and now I don’t drive its batteries to a critical level. You can also connect a buzzer (squeaker) parallel to a resistor with a LED, and then you will know for sure about critical thresholds.