November 2011

Doc ID 2074 Rev 2

1/21

AN417

Application note

From nickel-cadmium to nickel-hydride fast battery charger

1

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 the enormous contribution made by rechargeable batteries to our comfortable

lifestyle. NiCd battery chargers charging in one hour and even less are already widespread.

Ultra fast charging of NiCd batteries in less than 15 minutes is a very attractive feature in

applications where 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 may

lead to a significant reduction of the battery service life. This could cause a prejudice against

the appliance manufacturer's image, as when the battery service life is reduced, the user is

practically led to a costly replacement of the complete battery pack.

The trend is now to replace NiCd batteries by the more environmentally friendly NiMH

batteries. Several charger applications such as notebook computers and portable phones

already require NiCd /NiMH compatible battery chargers. In this case, the most common

charge monitoring method used for a NiCd battery (negative delta voltage: [-

longer suited to the NiMH battery.

In this application, the charge termination method is based on the detection of the inflexion

point in the battery voltage curve. This inflexion point detection method is not only "NiCd-

NiMH compatible", it also significantly increases the NiCd battery life-time when fast

charging.

Such a high performance charger can be totally managed by a low cost 8-bit microcontroller

(MCU), the ST6210. Safe charging is achieved by the combination of three back-up charge

termination methods: [-

additional benefit of using such a 20 pin standard microcontroller lies in its high adaptability

of application 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 a

15W/100kHz DC/DC chargers are described in this note.

www.st.com