DIY Arduino Battery Capacity Tester - V2.0

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.

Attachments

• Schematic_Arduino Battery Capacity Tester V2.0_Sheet_1_20191110161355.pdf
  Download

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.

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.

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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.

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

GND -->GND

A4----> SDA

A5----> SCL

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

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

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.

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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.

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

Libraries:

Download and install the following libraries:

1. JC_Button: https://github.com/JChristensen/JC_Button

2. Adafruit_SSD1306: https://github.com/JChristensen/JC_Button

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

Attachments

• Bat_Capacity_Tester_V2.0_PCB_09.11.2019.ino
  Download

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.

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