Low Level Battery Indicator

Some home appliances powered by Li-Ion Batteries, does not contain a low battery indicator. In my case it is a rechargeable floor sweeper with one 3.7 V battery. It is not easy to determine an exact time to recharge it and attach it on main socket. Usually, I recharge the sweeper in time, when battery is totally dead and electric motor is not running. Such situation is not very comfortable, especially, if you need to use sweeper immediately.

I was looking for simple solution, how to detect voltage level at which charging should occurs. In this article a simple low level Li-Ion Battery indicator is described. Designed circuit could be used in any electronic device powered by Li-Ion Battery and can help user to charge battery in proper time. The Battery indicator is dedicated for one cell, but could be easy modified to more cells. The indicator could be used for any battery with small modification of circuit.

Main advantage of indicator is very law current consumption, in average less than 10 microAmps. Current consumption depends on indicator state

There are three Functions States of Level Indicator:

• Indicator LED lights up continuously: the battery is fully charged.
• Indicator LED is flashing: the battery need to be charged.
• Indicator LED is not lit: the battery is charged and device is ready to use

Step 1: Introduction Li-Ion Battery Level Indicator

Parts:

All parts could be purchased for less than 5 Euro.

Here is the list:

• IC1 MC33164-3P, Micropower Undervoltage SensingCircuit TO-92, LCSC PN C145176
• IC2 ICM7555, CMOS Timer, LCSC PN C34608
• R1, R2 resistor 10K, all resistors, capacitors and small components LCSC
• R3 resistor 680K
• R4 resistor 680
• C1 capacitor M1
• C2 capacitor 1M
• C3 capacitor 10M
• D1,D2,D3 diode 1N5819, LCSC PN C2474
• LED1 diode led 3mm, red
• T1 screw terminal

Resistors are for 0.25 W or less, capacitors for 12V or more.

Tools:

• Soldering Iron
• Cordless Drill
• Hot glue gun

Step 2: Circuit Description

Integrated circuit MC33164-3P is heart of the level indicator. Detailed information for this component is here.

Simple description of circuit: It is micropower under voltage sensing IC, in three pins plastic package, similar to low power transistor. MC33164 is designed as reset circuit for microprocessor, in case of power drop.

It detects voltage on pin 2. Compares detected voltage to reference voltage, in our case 2.7V. Result could be evaluated as voltage value on pin 1. If detected voltage is less than 2,7V, output is low and close to 0V. If input voltage is over 2,7V, presented value on pin 1 is about 3V, or more.

Typical reference value for MC33164-3P (3 after dash means 3V), is 2,71V. On this value exactly, output value is changed. (Do not consider hysteresis.) Voltages for one cell Li-Ion battery are: maximum voltage is 4.2V, typical voltage 3.7V and minimum voltage is from 2.8 to 3V, assume 2.9V. Minimum voltage is present at the end of discharge cycle and this voltage level should activate our low level indicator.

Reference voltage for MC33164 is too low comparing our requirements. There are 2 solutions to reduce voltage. The first and the simplest is voltage divider. But, divider consume extra current. Less current consuming is the second solution, using some components in series to reduce 2.9V to 2.7V. Diodes are components with some voltage drop in forward direction and they could be used successfully. Because of very low current value, the best diode type I have choose by tests.

Function of R1, D1, D2, D3 is to reduce input voltage.Jumper J1 could eliminate the last diode voltage drop and input voltage could be slightly decreased. Output IC1 is fed to timer IC2. Its active value is low and function is to enable timer. Unfortunately, there is not any input pin on IC2, which allow to enable this IC without any invert circuit.

I decided to enable timer ICM7555 by applying output IC1 as minus voltage to pin 1 of IC2. Components C2,R3 determine period of timer, it is adjusted for about 2 seconds. Resistor R4 limits current for indicating LED1 diode. Tested voltage from battery is connected to terminal with pins 1 (plus) and 2 (minus). Values for R2, C1 are recommended from data sheet.

Timer ICM7555 is CMOS equivalent of 555. Its advantage is working voltage from 2.5V and very low current consumption. On the second picture there is very simple circuit as voltage monitor recommended by datasheet. This schema could be used as well, but using ICM7555 is advantage, because of low level voltage indicated by flashing LED, which is more noticeable.

Step 3: Construction

Parts are soldered on one piece of prototyping board with size 20x35mm. Outside of the board is LED diode, could be mounted on visible place. Monitored Li-Ion battery is connected via screw terminal block. Board is small enough to be inserted to any device.

Connection inside device is simple: just connect wires from terminal block to battery and drill hole for LED and fix it. Wires could be connected directly to battery poles on battery holder.. In this case current is drain independently, in relation to switch position and indicator is working whole time.

In my case, I have connected low level indicator after main (low voltage) switch. Because of charger board inside device, which is connected separately to switch and separately to battery, the connecting place "after switch" is not clear. I use simple solution, connect indicator directly to load, DC motor.

Prototyping board require more time to connect all components by wires. To save this time, I have designed PCB, size 20x40mm, with through hole components. PCB contains just one layer. Using SMD components can decrease board size. I did not make this design because of more complex soldering and manipulating with very small parts. Gerber files for PCB fabrication is attached.

Step 4: Conclusion

Described, low battery level indicator can be used for any battery with voltage more than 2.5V. In such a case skip diodes D1, D2 and D3 and add one resistor R5 as a part of voltage divider to R1. Value of R1 depend on detected voltage level U and could be calculated by:

R5 = 2.7*R1/(U-2.7)

Construction is done on a small PCB with through hole components. If you have got in your stock some SMD parts, I recommend to use SMD components.

Size of board could be smaller and construction allows you practise using SMD parts.

Thank you for reading and have a nice time with construction.