TinyCharger - Single Cell Li-Ion Battery Charger with Monitoring

Microchip ATtiny10 Attiny13 ATtiny24 ATtiny25 ATtiny44 ATtiny45 ATtiny84 ATtiny85

TinyCharger is an ATtiny25/45/85-based, single-cell Li-ion battery charger with selectable charging current limit (100 mA - 1000 mA) and an OLED display for monitoring.

For Power Connecting device is equipped with a Micro-USB and a USB-C connector for power supply. Only one connector can be used at a time. The supply voltage must be 5 V. For Battery Connecting device is equipped with a JST-PH 2.0 mm socket and a 2.54 mm pin header for connection to the battery. Only one battery can be charged at a time (Figure 1).


Figure 1.	*TinyCharger - Single Cell Li-Ion Battery Charger with Monitoring.*

Characteristics:

Supply Voltage: 4.3 - 5.5 V
Charge Voltage Limit: 4.2 V
Charge Current Limits: 100, 350, 750, 1000 mA (selectable)
Voltage Measurement Resolution: 4 mV
Current Measurement Resolution: 1 mA

Hardware

For battery charging the TP4056 is used (Figure 2). The TP4056 is a complete constant-current/constant-voltage linear charger for single-cell lithium-ion batteries. The maximum charge voltage is fixed at 4.2 V and the charge current can be programmed externally with a resistor. The total resistance is determined by three resistors connected in parallel, two of which can be switched on and off by the ATtiny via MOSFETs. The TP4056 automatically terminates the charge cycle when the charge current drops to 1/10th the programmed value after the final float voltage is reached. Other features include current monitor, under voltage lockout and automatic recharge.


Figure 2.	*TinyCharger Schematic Diagram.*

An INA219 is used to measure voltage and current. The INA219 is a current shunt and power monitor with an I²C-compatible interface. The device monitors both shunt voltage drop and bus supply voltage, with programmable conversion times and filtering. A programmable calibration value, combined with an internal multiplier, enables direct readouts of current in amperes. The selected shunt resistance of 8 milliohms enables both a very small influence on the circuit and a measurement with a resolution of 1 milliampere. For an accurate measurement, a shunt resistor with a low tolerance (1% or better) should be selected.

All project files available in download section, in project page on Github.com [1] and in EasyEDA [2].

The user interface utilizes two buttons and a 128×64 pixels OLED display. An ATtiny25/45/85 microcontroller handles the user interface as well as the control and monitoring of the charging functions (Figure 3).


Figure 3.	*TinyCharger PCB view (bottom layer).*

Connectors and buttons description shown on Figure 4.


Figure 4.	*TinyCharger user interface description.*

The charging current is programmed via a resistor, the value of which can be set by the ATtiny via two MOSFETs and changed by pressing the SET button. An INA219 continuously measures the charging current and voltage and transmits the values to the ATtiny via I²C. From this, the ATtiny calculates the state of charge and capacity and displays the values on the OLED screen.

The I²C protocol implementation is based on a crude bitbanging method. It was specifically designed for the limited resources of ATtiny10 and Attiny13, but it works with some other AVRs (including the Attiny25/45/85) as well. The functions for the OLED are adapted to the SSD1306 OLED module, but they can easily be modified to be used for other modules. In order to save resources, only the basic functionalities which are needed for this application are implemented. For a detailed information on the working principle of the I²C OLED implementation visit TinyOLEDdemo.

Accuracy of Time and Capacity Determination

The internal oscillator of the ATtiny is used to determine the charging time and the electric charge. The accuracy of the internal oscillator is +/-10% with the factory calibration. This can be improved to ±2% or better by manual calibration or with TinyCalibrator device [3]. The calibration value determined in this way can be set in the source code. In addition to the time measurement, the internal resistance of the battery being charged also plays a role for the accuracy of the calculated capacity. Due to these factors, the displayed capacity is only a rough estimate.

Compiling and Uploading

Since there is no ICSP header on the board, you have to program the ATtiny either before soldering using an SOP adapter, or after soldering using an EEPROM clip. The AVR Programmer Adapter can help with this.

If using the Arduino IDE

Make sure you have installed ATtinyCore [4].
Go to Tools -> Board -> ATtinyCore and select ATtiny25/45/85 (No bootloader).
Go to Tools and choose the following board options:
- Chip: ATtiny25 or 45 or 85 (depending on your chip)
- Clock: 8 MHz (internal)
- Millis/Micros: disabled
- B.O.D.Level: B.O.D. enabled (2.7 V)
- Leave the rest at the default settings
Connect your programmer to your PC and to the ATtiny.
Go to Tools -> Programmer and select your ISP programmer (e.g. USBasp).
Go to Tools -> Burn Bootloader to burn the fuses.
Open TinyCharger sketch and click Upload.
If using the precompiled hex-file
Make sure you have installed avrdude [5].
Connect your programmer to your PC and to the ATtiny.
Open a terminal.
Navigate to the folder with the hex-file.
Execute the following command (if necessary replace "t85" with your chip and " usbasp" with the programmer you use):
avrdude -c usbasp -p t85 -U lfuse:w:0xe2:m -U hfuse:w:0xd5:m -U efuse:w:0xff:m -U flash:w:tinycharger.hex

Operating Instructions

Connect the device via the Micro-USB or USB-C port to a 5 V power supply that can provide sufficient current. Use the SET button to select the maximum charging current. Note that it can overshoot by a few milliamperes!

Connect the Li-Ion battery to one of the battery connectors. Pay attention to the correct polarity!

The battery is charged immediately. The SET button is locked during the charging process to prevent the charging current from being changed accidentally. The charging process stops automatically when the battery is fully charged. The total charging time and the charged capacity remain displayed as long as the device is supplied with power. These values can be reset using the RESET button.

References

Downloads

Schematic, PCB project, scketch, source files and hex file