A Novel Microprocessor-Based Battery Charger Circuit With Power Factor Correction Aziz E. Demian Jr., Carlos A. Gallo, Fernando L. Tofoli, João Batista Vieira Jr., Luiz Carlos de Freitas, Valdeir J. Farias, Ernane A. A. Coelho Federal University of Uberlândia Department Of Electrical Engineering Av. João Naves de Ávila, 2160, Campus Santa Mônica, Bloco "3N" CEP 38400-902, Uberlândia, MG, Brazil, +55-34-32394166 E-mail: batista@ufu.br Abstract— An effective microcontrolled battery charger circuit that monitors the charging process avoiding the battery damage by overcharge is described, where a PWM Forward topology with power factor correction is employed in order to provide dc/dc conversion. Depending on the battery charge state, which is determined by microcontroller PIC 16F873, the charging process is modified. In this way, fast charging does not have negative effects on the effective capacity of the battery and on battery cycle-life. Keywords: battery charger, microcontrollers, battery state. I. INTRODUCTION Determining battery state is the first stage to minimize charging time without negative effects on battery life. Different methods (“memorizing of battery charging/discharging history” or “measurement of internal impedance” [1]) have so far been proposed to give an accurate indication of battery charge state (remaining energy) but the implementation of these methods becomes difficult, as they are not practical. Constant voltage and constant current charging are usual ways to charge batteries. The constant voltage charging approach employs an equivalent series resistance to control power flow to charge the battery and keep the battery voltage constant. The constant current charging approach keeps the charging current constant until the battery voltage reaches a designated value. When constant voltage charging is employed, the converter delivers highest output power when the battery voltage is low. This usually happens in the initial charging period and only lasts a few minutes, which corresponds to a short period if compared to the total charging period. When constant current charging method is employed, the converter delivers highest power in the final charging period when the battery voltage is high. Again this period only lasts a short period of time, usually a few minutes. In such time periods, the converter delivers the highest power and has the highest loss [1]. Therefore, the battery remains the most fragile element. Its useful life depends much on the processes of charging and discharge. Most batteries must be protected from overcharge and excessive discharge, which can cause electrolyte loss and can even damage or ruin the battery plates. Protection is usually achieved using a charge controller which also maintains system voltage. Most charge controllers also have a mechanism that prevents current from flowing from the battery back into the array at night during charging period [2] [3]. This paper proposes a microcontroller-based battery charger circuit that employs PIC 16F873 to control the charging process. If the battery is lowly charged, the charging is performed with constant current until the battery voltage reaches an optimum value. Otherwise if the battery is nearly fully charged, the charging process occurs with constant voltage. II. CHARGING PROCESS AND DESCRIPTION OF THE MICROCONTROLLER The use of microcontrollers in battery chargers implies several advantages, as the battery intrinsic characteristics, the number of series-connected units and the load curve of the battery can be defined in the internal memory of the processor via software. Thus the charging process provides more controllable results, becoming reliable and preserving the battery life. An additional control loop can even be implemented in order to monitor the battery temperature, since the excessive heating in the fast-charging process may damage the battery. The battery charger proposed in this paper employs PIC 16F873, which has four A/D converters, to determine the battery state. The block diagram of the circuit is shown in Fig. 1, where one can see that a Boost converter is responsible for the power factor correction (Fig. 2), and a two-switch forward converter provides the dc-dc conversion (Fig. 3). One must mention that, in this case, a forward converter is preferred instead of a flyback one due to the improved efficiency and reduced control complexity. The charging process is performed according to the manufacturer specifications, which define the maximum current and voltage that can be applied to the battery. The process comprehends two current levels and one voltage level. The first stage employs constant current, equal to 10% of the charging nominal current, as this stage finishes when the voltage reaches the constant voltage level. In the second stage, the voltage is maintained constant, and it finishes when the 0-7803-8269-2/$17.00 (C) 2004 IEEE. 1407