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: [-
Δ
V]) is no
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: [-
Δ
V] detection, temperature monitoring and timer cut-off. An
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.
November 2011
Doc ID 2074 Rev 2
1/21
www.st.com
Contents
AN417
Contents
1
2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Charge termination methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1
2.2
The [-Δ V] method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The inflexion point method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
4
5
6
Principle of the inflexion method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Charge control program description . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Charger schematic examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1
6.2
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Battery charger examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2.1
6.2.2
Offline charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
DC/DC charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7
8
9
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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List of tables
List of tables
Table 1.
Table 2.
Charge of different battery types with an 2.2 A current source. . . . . . . . . . . . . . . . . . . . . . 13
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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List of figures
AN417
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Battery charger circuit diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The negative delta voltage method fast charge is terminated at point A . . . . . . . . . . . . . . . 6
Fast charge terminates at point B in the inflexion point method . . . . . . . . . . . . . . . . . . . . . . 7
NiMH versus NiCd charging characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Inflexion point method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Simplified program flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Charge of a 1.4 Ah NiCd battery with the -[Δ V] method: charging current 2.2 A, total
time 48 mn, temperature increase 9.6°C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Charge of a 1.4 Ah NiCd battery with the inflexion method: charging current 2.2 A,
total time 41 mn, temperature increase 5°C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Charge of a 2.2 Ah NiMH battery with the -[Δ V] method: charging current 2.2 A, total
time 63 mn, temperature increase 18.2°C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Charge of a 2.2 Ah NiMH battery with the inflexion method: charging current 2.2 A,
total time 57 mn, temperature increase 7.5°C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Block diagram of an off-line SMPS charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Block diagram of a DC/DC charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
This 35W/100kHz off-line charger is an asymmetrical half-bridge regulated in current
mode from its primary side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
This 15W/100kHz DC-to-DC charger is also driven by a low-cost PWM control
integrated circuit, the UC3843 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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Doc ID 2074 Rev 2
AN417
Charge termination methods
2
Charge termination methods
Basically, NiCd and NiMH batteries are charged by a constant current source (see
Figure 1).
A battery charger is made of a constant current source controlled by a microcontroller which
monitors the battery voltage variation with its internal analog-to-digital converter
Figure 1.
Vin
Battery charger circuit diagram
cell voltage
I charge
battery
micro controller
NiCd
or
NiMH
ST62xx
As soon as the full capacity of the battery has been detected by the microcontroller, the
charging is stopped by turning the current off. Schematic examples of power converters
operating as current sources are given later. The same converter hardware can be used in
two different charging methods depending upon the appliance requirements.
2.1
The [-Δ V] method
When a NiCd battery reaches full charge, its voltage decreases slightly (see
Figure 2).
The
negative delta voltage method [-
Δ
V] consists of stopping the charge as soon as the voltage
slope versus time becomes negative. This first charge termination technique is optimized to
fast charge a NiCd battery to its full capacity.
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