Part # Application Note General Purpose datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: ST6 - From Nickel-Cadmium to Nickel-Hydride fast battery charger

Part Details:

APPLICATION NOTE From Nickel-Cadmium To Nickel-Hydride Fast Battery Charger J. NICOLAI, L. WUIDART INTRODUCTION Today, many cordless and portable equipment are supplied by a rechargeable battery (Nickel-Cadmium, NiCd or Nickel-Hydride, NiMH). Individual applications such as portable phones,camcorders, cordless power tools, portable appliances and audio equipment highlight theenormous contribution made by rechargeable batteries to our comfortable lifestyle.NiCd battery chargers charging in one hour and even less are already widespread. Ultra fastcharging of NiCd batteries in less than 15 minutes is a very attractive feature in applicationswhere the battery is rapidly discharged, as in power tools such as cordless drills [1]. Nevertheless, when fast charging, the use of a non-adapted charge termination method maylead to a significant reduction of the battery service life. This could cause a prejudice againstthe appliance manufacturer s image, as when the battery service life is reduced, the user ispractically led to a costly replacement of the complete battery pack. The trend is now to replace NiCd batteries by the more environmentally friendly NiMHbatteries. Several charger applications such as notebook computers and portable phonesalready require NiCd /NiMH compatible battery chargers. In this case, the most commoncharge monitoring method used for a NiCd battery (negative delta voltage :[- V]) is no longersuited to the NiMH battery. In this application, the charge termination method is based on the detection of the inflexionpoint in the battery voltage curve. This inflexion point detection method is not only "NiCd-NiMHcompatible", it also significantly increases the NiCd battery life-time when fast charging. Such a performant charger can be totally managed by a low cost 8-bit microcontroller (MCU),the SGS-THOMSON ST6210. Safe charging is achieved by the combination of three back-upcharge termination methods :[- V] detection, temperature monitoring and timer cut-off. Anadditional benefit of using such a 20 pin standard microcontroller lies in its high adaptability ofapplication features. The proposed charging power converters use the Switched Mode Power Supply technology(SMPS), operating from AC mains or DC voltage sources. A 35W/100kHz offline and a15W/100kHz DC/DC chargers are described in this note. AN417 / 01,94 FROM NICKEL-CADMIUM TO NICKEL-HYDRIDE FAST BATTERY CHARGER 1 CHARGE TERMINATION METHODS Basically, NiCd and NiMH batteries are charged by a constant current source (see Figure 1). Figure 1. A battery charger is made of a constant current source controlled by amicrocontroller which monitors the battery voltage variation with its internal analog-to-digitalconverter Vin cell voltage I charge battery micro controller NiCd ST62xx or NiMH As soon as the full capacity of the battery has been detected by the microcontroller, thecharging is stopped by turning the current off. Schematic examples of power convertersoperating as current sources are given later. The same converter hardware can be used intwo different charging methods depending upon the appliance requirements. 2/16 FROM NICKEL-CADMIUM TO NICKEL-HYDRIDE FAST BATTERY CHARGER 1.1 The [- V] method When a NiCd battery reaches full charge, its voltage decreases slightly (see Figure 2). Thenegative delta voltage method [- V] consists of stopping the charge as soon as the voltageslope versus time becomes negative. This first charge termination technique is optimized tofast charge a NiCd battery to its full capacity. Figure 2. The negative delta voltage method fast charge is terminated at point A. V deg C cell voltage temperature 1.6 60 A -dV 1.5 50 1.4 battery voltage 40 1.3 30 battery temperature 1.2 20 charging time In fact, a NiCd battery charged with the [- V] method is slightly overcharged: Figure 2 showsthat the battery temperature has substantially increased at point A when charge is terminated,which may decrease the life-time of the battery. More precisely in Figure 3, most of the currentfed to the battery between point B and the negative voltage drop A is not directly convertedinto active battery charge, but into heat. This can be seen in the temperature curve shown inFigure 3. The point B corresponds to the inflexion point of the battery voltage curve versustime. The [- V] method is definitely no longer suited when it comes to charging NiMH batteries: theNiMH charging reaction is permanently exothermic (releases heat), so the battery temperaturewould become excessive in its [- V] area of the voltage curve (see Figure 3).Another characteristic of the NiMH batteries makes the [- V] method unsuitable: some typesof NiMH batteries do not exhibit a significant voltage drop as NiCd batteries do when reachingtheir full capacity. 3/16

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