1. Introduction
The project was born from the need to have a universal charger that is able to charge both 1 cell and a battery pack for child car or scooter .
The main concept to make Li-ion charger was using the block for fast prototyping.
I chose as hardware for this role 5 modules:
1. MT3620
2. LM2596
3. INA226
4. MCP4725
5. OLED Display
2. Description of concept
On the diagram bellow are present the connection between all additional boards.
The main processor, in this role MT3620, is responsible for collecting information about actual voltage and current provided to charged battery and correct them if they are out of scope.
The main module responsible for controlling the voltage is based on LM2596 and is controlled via 12-bit DAC - MCP4725. To collecting all necessary information I use wide range input ADC - INA226.
Additionally few information about charging process like:
- charging voltage
- charging current
- capacity
- passed time form the begin of charging
- actual charging mode
are presented on OLED display.
All this three modules are connected to ISU2 and controlled via I2C bus.
Furthermore Azure module is also responsible for communicating with Azure IoT Central where all information of charging process are presented.
In Azure IoT Central is also possible to change parameters for charging process.
3. The Software
After power on the charger, the first what is done, is initialization the I2C by calling InitPeripherals() function.
In this function are set the speed all parameters of I2C bus and then, the external peripherals like ADC, DAC, display and GPIO are initialized.
In the next step is checked state of Buttons A and B from main board. Depending of it’s combination, charger is running in preferred mode:
Button A | Button B | Mode |
Release | Release | Standalone mode |
Pressed | Release | Configured via Azure |
Release | Pressed | Test mode |
Pressed | Pressed | Configure via Azure and send data |
In “Standalone mode” all parameters for charging process are predefined in file liion_charger_algorithm.h
In “Configured via Azure” mode the parameters are taken from Azure. In this case program is waiting for connecting to Azure and after receiving all parameters, sets them by calling function SetDefaultChargingParameters(&azureChargingParameters).
In “Test mode” user has possibility to check all peripheries. Program read state of button A and B and increase / decrease value of DAC register. The result of this is changing voltage on output. By holding the button for time longer than one second, DAC value is changing in step +/- 20. When button is pressed and released in time shorter than one second, DAC value is changing in step of +/- 1.
Information about Voltage on output and actual current are show in display.
In “Configure via Azure and send data” mode, program in first step do this same like in “Configured via Azure” mode, but during charging process, is sending information about actual voltage and current.
4. Charging algorithm
Whole charging algorithm is contained in liion_charger_algorithm.c file.
The function named LiionChargerProces() is called periodically from main() function.
In first step, (called S0) program is checking if battery is connected. This is realized by measuring voltage on output during the voltage on regulator is setting to lowest value.
If battery was connected program compare voltage of battery with set values and makes a choice which charging mode should be start.
Charging process is divided on five steps:
Step 1 (S1) – precharging. This step is ran when battery voltage is < 3 V
Step 2 (S2) – Constant Currenc (CC)
Step 3 (S3) – Constant Voltage (CV)
Step 4 (S4) – End of charging. When battery voltage has 4,2 V and charging current drop under 0,1 C
The LiionChargerProces() is also responsible for calculating capacity and charging time.
Appendix A.
To calculate the R_DAC please visit: https://www.fischl.de/dcdccontrol/
Appendix B.
Graph of charging battery from Azure
Setting form on Azure portal
