A BETTER BATTERY CHARGER

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A BETTER BATTERY CHARGER

Lessons from the Trenches Part 2: Hardware and Software Implementation
by Thomas Richter

Start ı The Buck Converter ı Voltage Reference and Battery Temperature ı AT90S2333 Battery Charger ı ATtiny15 Battery Charger ı Charge Current ı Software Implementation ı User Settings ı Source Code ı INT Battery Function ı The Stable_ADC Function ı BC.H ı B_DEF.H ı Charge MethodıSLA.C ı Charge MethodıNiCd.C and NiMH.C ı Charge MethodıLilon.C ı Suggested Improvement ı Sources and PDF

Last month, I introduced you to the AVR Battery Charger Reference Design and went through the basics of battery technologies, safe charging methods, and termination methods. This month, Iıd like to get down deep into the design by discussing the hardware and software.

The reference design includes two complete battery charger designs and is divided into five main blocks (see Photo 1). The power supply includes analog reference, pushbutton, and LEDs (for power supply and reference voltage schematics). The input voltage is rectified through D9ıD12 and then filtered by C13. The rectified input voltage can be measured at the test point marked V_IN. V_IN is supplied to both the buck converter and LM7805 voltage regulator. The LM7805 delivers 5 V for the microcontrollers. This voltage can be measured at the test point marked V_CC. When the power is on, the LED marked "5V OK" is lit.

Photo 1ıThe main blocks of the Battery Charger Reference Design can be seen here. Also, see the diagram of the main blocks.

The PC interface is connected to the UART interface on the 2333 and can be used to log battery data while charging. The data can be imported in a spreadsheet to display the charging characteristic for a certain battery. For more information about logging data, see application note "AVR451: Data Logger for Battery Charger Reference Design." In the app note, youıll also find source code examples and Microsoft Excel spreadsheets for importing data. The 2333 can also be used as a data logger when using the ATtiny15 battery charger.

The board has several LEDs and switches for debug/monitoring purposes. Only a few are used in the current applications, but the rest can be added easily when needed. The table below shows a quick rundown of the connections and functions for the LEDs and switches.

Label	Description
LED0	Connected to Port B, pin 0 on the 2333. Used in the current application for visualizing the charge mode fast or trickle.
LED1	Connected to Port B, pin 2 on the 2333.
LED2	Connected to Port B, pin 3 on the 2333.
LED3	Connected to Port B, pin 4 on the 2333. Used to display "Error" in the 2333 application.
LED4	Not connected, can be connected to test points on the board for extended debug/monitoring.
LED5	Not connected, can be connected to test points on the board for extended debug/monitoring.
LED6	Connected to Port B, pin 1 on the ATtiny15. Used in the current application for visualizing the PWM frequency.
LED7	Indicates power status.
S1	Connected to Port D, pin 4 on the 2333. Used to start the charger in the current 2333 application.
S2	Connected to Port D, pin 5 on the 2333.
S3	Connected to Port D, pin 6 on the 2333.
S4	Connected to Port D, pin 7 on the 2333.
S5	Restarts the prog Restarts the program and is used to recover from charge errors.

Both designs have a 10-pin ISP header on the test board. The flash program memory and EEPROM data memory can be downloaded from AVRISP PC programming software.

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