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A BETTER BATTERY CHARGER

Part 1: Charging and Termination
by Thomas Richter

Start ı Battery Technologies ı Safe Charging of Batteries ı Termination Methods ı Sources and PDF

As more electronic equipment becomes portable, the rush for better batteries with higher capacity, smaller size, and lower weight will increase. The continuing improvements in battery technology calls for more sophisticated charging algorithms to ensure fast and secure charging. Higher accuracy monitoring of the charge process is required to minimize charge time and use the maximum capacity of the battery while avoiding battery damage.

In this two-part series, Iım going to introduce a battery charger that fully implements the latest technology in battery charger designs. In the first part of this series, Iıll concentrate more on the battery chemistries and charging algorithms, and then next month, Iıll get into the actual hardware and firmware of the charger.

The charger can fast-charge all popular battery types without any hardware modifications. It allows a full product range of chargers to be built around a single hardware design. And, a new charger model is designed simply by reprogramming the desired charge algorithm into the microcontroller using in-system programmable flash memory. This allows minimum time to market for new products and eliminates the need to stock more than one version of the hardware. The charger design contains complete libraries for sealed lead acid (SLA), nickel cadmium (NiCd), nickel metal hydride (NiMH), and lithium-ion (Li-Ion) batteries.

The Battery Charger Reference Design includes two battery chargers built with the AT90S2333 and ATtiny15 microcontrollers (see Photo 1). However, it can be implemented using any microcontroller with an A/D converter, PWM output, and enough program memory to store the desired charging algorithm.

Photo 1ıHere you can see the Battery Charger Reference Design board.

Atmelıs AVR 8-bit RISC microcontroller offers flash memory, EEPROM, and a 10-bit A/D converter in one chip. Flash memory eliminates the need to stock microcontrollers with multiple software versions and can be efficiently programmed in production just before shipping the finished product. Programming after mounting is made possible through fast in-system programming (ISP), allowing up-to-date software and last minute modifications.

The EEPROM data memory can be used for storing calibration data and battery characteristics, it also allows charging history to be permanently recorded, allowing the charger to optimize for improved battery capacity. The integrated 10-bit A/D converter gives superior resolution for the battery measurements compared to other microcontroller-based solutions. Improved resolution allows charging to continue closer to the maximum capacity of the battery and eliminates the need for external op-amps to "window" the voltage. The result is reduced board space and lower system cost.

The AVR microcontroller is designed for high-level languages like C. The design for the 2333 is written entirely in C, which makes this design easy to adopt and modify for todayıs and tomorrowıs batteries. The 2333 can be used for voltage and temperature monitoring with a UART interface to a PC for data logging. The design for ATtiny15 is written in assembly to achieve maximum code density. Table 1 shows the differences in the designs.

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