DIY Arduino Battery Capacity Tester - V2.0




Nowadays fake Lithium and NiMH batteries are everywhere which is sold by advertising with higher capacities than their true capacity. So it is really difficult to distinguish between a real and a fake battery. Similarly, it is difficult to know the capacity retained in the salvaged 18650 laptop batteries. So, a device is required to measure the true capacity of the batteries.

In the year 2016, I have written an Instructable on " Arduino Capacity Tester - V1.0 " which was a very straight forward and simple device. The earlier version was based on Ohms Law. The battery to be tested is discharged through a fixed resistor, current and time duration is measured by Arduino and capacity is calculated by multiplying both the readings ( Discharge current and time ).

The drawback of the earlier version was that during the testing, as as the battery voltage decrease, the current also decreases which make the calculations complex and inaccurate. To overcome this, I have made the V2.0 which is designed in such a way that the current will remain constant throughout the discharging process. I made this device by inspiring the original design by MyVanitar

The main features of Capacity Tester V2.0 are :

1. Capable of measuring the capacity of AA / AAA NiMh / NiCd, 18650 Li-ion, Li-Polymer, and Li FePO4 battery. It is suitable for almost any kind of battery rated below 5V.

2. Users can set the discharge current by using the pushbuttons.

3. OLED user Interface

4. The device can be used as an Electronic Load

Update on 02.12.2019

Now you can order the PCB and components together in a kit from PCBWay

Disclaimer:Please note that you are working with Li-Ion battery which is highly explosive and dangerous. I cannot be held responsible for any loss of property, damage, or loss of life if it comes to that. This tutorial was written for those who have knowledge of rechargeable lithium-ion technology. Please do not attempt this if you are a novice. Stay Safe.


Components Used

Now Order PCB and all the components to build this project in a kit from PCBWay


2. Arduino Nano: Amazon / Banggood

3. Opamp LM358: Amazon / Banggood

4. 0.96" OLED display : Amazon / Banggood

5. Ceramic Resistor: Amazon / Banggood

6. Capacitor 100nF : Amazon / Banggood

7. Capacitor 220uF: Amazon / Banggood

8. Resistors 4.7K & 1M : Amazon / Banggood

9. Push Button: Amazon / Banggood

10. Push-Buttons Cap: Aliexpress

11. Screw Terminal: Amazon / Banggood

12. Prototype Board: Amazon / Banggood

13. PCB Stand-off: Amazon / Banggood

14. Heatshrink Tubing: Amazon/ Banggood

15. Heatsink: Aliexpress

Tools Used

1. Soldering Iron: Amazon / Banggood

2. Clamp Meter: Amazon / Banggood

3. Multimeter: Amazon / Banggood

4. Hot Air Blower: Amazon / Banggood

5. Wire Cutter: Amazon / Banggood

6. Wire Stripper: Amazon / Banggood

Teacher Notes

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Step 1: Schematic Diagram

The whole schematic is divided into to followings sections:

1. Power Supply Circuit

2. Constant Current Load Circuit

3. Battery Voltage Measurement Circuit

4. User Interface Circuit

5. Buzzer Circuit

1. Power Supply Circuit

The power supply circuit is consists of a DC Jack ( 7-9V) and two filter capacitors C1 and C2. The power output (Vin) is connected to the Arduino pin Vin. Here I am using the Arduino on-board voltage regulator to step down the voltage to 5V.

2. Constant Current Load Circuit

The core component of the circuit is Op-amp LM358 which contains two operational amplifiers. The PWM signal from the Arduino pin D10 is filtered by a low-pass filter ( R2 and C6 ) and fed to the second operational amplifier. The output of the second op-amp is connected to the first op-amp in voltage follower configuration. The power supply to LM358 is filtered by a decoupling capacitor C5.

The first op-amp, R1, and Q1 build a constant current load circuit. So now we can control the current through the load resistor (R1) by changing the PWM signal pulse width.

3. Battery Voltage Measurement Circuit

The battery voltage is measured by the Arduino analog input pin A0. Two capacitors C3 and C4 are used to filter out the noises coming from the constant current load circuit which can degrade the ADC conversion performance.

4. User Interface Circuit

The user interface circuit is consists of two push-buttons and a 0.96" I2C OLED display. The Up and Down push-button is to increase or decrease the PWM pulse width. R3 and R4 are pull-up resistors for the Up and Down push-buttons. C7 and C8 are used to debounce the push-buttons. The third push-button (RST) is used for resetting the Arduino.

5. Buzzer Circuit

The buzzer circuit is used to alert the starting and end of the test. A 5V buzzer is hooked to Arduino digital pin D9.

Step 2: How Does It Work?

The theory is based on the voltage comparison of the inverting (pin-2) and the non-inverting (pin-3) inputs of the OpAmp, configured as a unity amplifier. When you set the voltage applied to the non-inverting input by adjusting the PWM signal, the output of the opamp opens the gate of MOSFET. As the MOSFET turn-on, the current runs through R1, it creates a voltage drop, which provides negative feedback to OpAmp. It controls the MOSFET in such a way that the voltages at its inverting and non-inverting inputs are equal. So, the current through the load resistor is proportional to the voltage at the non-inverting input of the OpAmp.

The PWM signal from the Arduino is filtered by using a low pass filter circuit ( R2 and C1). To test the PWM signal and filter circuit performance, I hooked up my DSO ch-1 at the input and ch-2 at the output of the filter circuit. The output waveform is shown above.

Step 3: Capacity Measurement

Here Battery is discharged to its low-level threshold voltage ( 3.2V).

Battery Capacity (mAh) = Current ( I ) in mA x Time (T ) in Hours

From the above equation it is clear that to calculate battery capacity (mAh), we have to know the current in mA and time in Hour. The designed circuit is a constant current load circuit, so the discharge current remains constant throughout the testing period.

The discharge current can be adjusted by pressing the Up and Down Button. The time duration is measured by using a timer in the Arduino code.

Step 4: Making the Circuit

In the previous steps, I have explained the function of each of the components in the circuit. Before jump to make the final board, test the circuit on a breadboard first. If the circuit works perfectly on the breadboard, then move to solder the components on the prototype board.

I used the 7cm X 5cm prototype board.

Mounting the Nano: First cut two rows of female header pin with 15 pins in each. I used a diagonal nipper to cut the headers. Then solder the header pins. Be sure the distance between the two rails fits the Arduino nano.

Mounting OLED Display: Cut a female header with 4pins. Then solder it as shown in the picture.

Mounting the terminals and components: Solder the remaining components as shown in pictures.

Wiring: Make the wiring as per the schematic. I used colored wires to make the wiring so that I can identify them easily.

Step 5: OLED Display

To display the Battery Voltage, discharge current and capacity, I used a 0.96" OLED display. It has 128x64 resolution and uses an I2C bus to communicate with the Arduino. Two pins SCL (A5), SDA (A4) in Arduino Uno are used for communication.

I am using the Adafruit_SSD1306 library to display the parameters.

First, you have to download the Adafruit_SSD1306. Then installed it.

The connections should be as follows

Arduino --> OLED

5V --->VCC


A4----> SDA

A5----> SCL

Step 6: Buzzer for Warning

To provide alerts during the start and competition of the test, a piezo buzzer is used. The buzzer has two terminals, the longer one is positive and the shorter leg is negative. The sticker on the new buzzer has also " + " marked to indicate the positive terminal.

As the prototype board doesn't have enough room to place the buzzer, I have connected the buzzer to the main circuit board by using two wires. To insulate the bare connection, I have used heat shrink tubing.

The connections should be as follows

Arduino --> Buzzer

D9--> Positive terminal

GND--> Negative terminal

Step 7: Mounting the Standoffs

After soldering and wiring, mount the standoffs at 4 corners. It will provide sufficient clearance to the soldering joints and wires from the ground.

Step 8: PCB Design

I have drawn the schematic by using EasyEDA online software after that switched to the PCB layout.

All of the components you added in the schematic should be there, stacked on top of each other, ready to be placed and routed. Drag the components by grabbing on its pads. Then place it inside the rectangular borderline.

Arrange all the components in such a way that the board occupies minimum space. Smaller the board size, the cheaper will be the PCB manufacturing cost. It will be useful if this board has some mounting holes on it so that it can be mounted in an enclosure.

Now you have to route. Routing is the most fun part of this entire process. It’s like solving a puzzle! Using the tracking tool we need to connect all the components. You can use both the top and the bottom layer for avoiding overlap between two different tracks and making the tracks shorter.

You can use the Silk layer to add text to the board. Also, we are able to insert an image file, so I add an image of my website logo to be printed on the board. In the end, using the copper area tool, we need to create the ground area of the PCB.

You can order it from PCBWay.

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When you place an order, I will get 10% donation from PCBWay for a contribution to my work. Your little help may encourage me to do more awesome work in the future. Thank you for your cooperation.

Step 9: Assemble the PCB

For Soldering, you will need a decent Soldering Iron, Solder, Nipper and a multimeter.It is good practice to solder the components according to their height. Solder the lesser height components first.

You can follow the following steps to solder the components :

1. Push the component legs through their holes, and turn the PCB on its back.

2. Hold the tip of the soldering iron to the junction of the pad and the leg of the component.

3. Feed solder into the joint so that it flows all around the lead and covers the pad. Once it has flowed all around, move the tip away.

Step 10: Software & Libraries

First, download the attached Arduino Code. Then download the following libraries and install them.


Download and install the following libraries:

1. JC_Button:

2. Adafruit_SSD1306:

In the code, you have to change the following two things.

1. Current Arrays values: This can be done by connecting a multimeter in series with the battery. Press the up button and measure the current, the current values are the elements of the array.

2. Vcc: You use a multimeter to measure the voltage at the Arduino 5V pin. In my case it is 4.96V.

Updated on 20.11.2019

You can change the Low_BAT_Level value in the code as per the battery chemistry. It is better to take a little margin over the cut off voltage stated below.

Here is the discharge rates and cutoff voltages for various Lithium-Ion Battery chemistries:

1. Lithium Cobalt Oxide: Cut-off Voltage = 2.5V at 1C discharge rate

2. Lithium Manganese Oxide: Cut-off Voltage = 2.5V at 1C discharge rate

3. Lithium Iron Phosphate: Cut-off Voltage = 2.5V at 1C discharge rate

4. Lithium Titanate: Cut-off Voltage = 1.8V at 1C discharge rate

5. Lithium Nickel Manganese Cobalt Oxide: Cut-off Voltage = 2.5V at 1C discharge rate

6. Lithium Nickel Cobalt Aluminum Oxide: Cut-off Voltage = 3.0V at 1C discharge rate

Step 11: Conclusion

To test the circuit, first I charged a good Samsung 18650 battery using my ISDT C4 Charger. Then connect the battery to the battery terminal. Now set the current to according to your requirement and long-pressed the “UP” button. Then you should hear a beep and the test procedure starts. During the test, you will monitor all the parameters on the OLED display. The battery will discharge until its voltage reaches its low-level threshold (3.2V). The test process will be finished by two long beeps.

Note: The project is still under development stage. You can join me for any improvements. Raise comments if any mistakes or errors. I am designing a PCB for this project. Stay connected for more updates to the project.

Hope my tutorial is helpful. If you like it, don't forget to share :) Subscribe for more DIY projects. Thank You.

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    7 Discussions


    14 days ago

    wonderful instructable. where to find IRLZ44 in india?


    18 days ago on Introduction

    Very nice project.

    One suggestion:
    There are at least six formulations for Lithium-ion cathode chemistries in the market today, each having an array of unique properties. Two properties which are critical to measuring residual and rechargeable capacity is the rate of discharge, expresses as "C", and cut-off voltage. Here is a brief review of discharge rates and cutoff voltages for each of the six chemistries:

    Lithium Cobalt Oxide: Cut-off Voltage = 2.5V at 1C discharge rate
    Lithium Manganese Oxide: Cut-off Voltage = 2.5V at 1C discharge rate
    Lithium Iron Phosphate: Cut-off Voltage = 2.5V at 1C discharge rate
    Lithium Titanate: Cut-off Voltage = 1.8V
    Lithium Nickel Manganese Cobalt Oxide: Cut-off Voltage = 2.5V at 1C discharge rate
    Lithium Nickel Cobalt Aluminum Oxide: Cut-off Voltage = 3.0V at 1C discharge rate

    Below is an excellent source of information on battery technology.
    To expand on the warning posted in your project review. Lithium battery products are very popular today, however they are far more hazardous to use that industries previous batteries. Lithium is sensitive to under/over voltage conditions and charge/discharge rates more so than traditional chemistries, thus the reason for their respective mandatory battery management systems. Lithium-ion cells when in thermal run-away burn hot enough to vaporize the entire metal canister, it's contents, and anything in remote proximity. Lithium fires can not be extinguished with the traditional fire extinguisher. Lithium-ion chemistry burns similar to Phosphor, white hot and un-extinguishable.

    Good luck with your design, and thank you for your design and your publication.

    2 replies

    Question 5 weeks ago

    Nice project. But I would question the precision. The current value takes from predefined table based on pwm duty cycle. This table must be calibrated for each device. Otherwise it might be way off based on type of transistor and actual values of resistor capacitor that are placed before operational amlifier. The other issue comes from the way how Arduino reads voltage. You need to provide stable reference voltage. Otherwise Arduino will use power supply as a voltage reference. USB voltage could varry from 4.5 to 5.3 volts. This will create an error in voltage measurement for up to 15%. From my experience I didn't manage to get stable analog reads without zenner diode.

    2 answers
    Open Green Energykasedy

    Answer 5 weeks ago

    Thank You for your valuable feedback. I agree with you.
    I will rectify the issues in my next revision.