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 "NiCdNiMH 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
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2
Charge termination methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1
The [-Δ V] method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2
The inflexion point method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
Principle of the inflexion method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4
Charge control program description . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5
Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6
Charger schematic examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2
Battery charger examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2.1
Offline charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2.2
DC/DC charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2/21
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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.
4/21
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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2
Charge termination methods
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.
Battery charger circuit diagram
Vin
cell voltage
I charge
battery
micro controller
ST62xx
NiCd
or
NiMH
-36
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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Charge termination methods
Figure 2.
AN417
The negative delta voltage method fast charge is terminated at point A
V
cell voltage
temperature
deg C
1.6
60
A
-dV
1.5
50
1.4
40
battery voltage
1.3
30
battery temperature
1.2
20
charging time
-36
In fact, a NiCd battery charged with the [-Δ V] method is slightly overcharged: Figure 2
shows that 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 current fed to the battery between point B and the negative voltage drop A is not
directly converted into active battery charge, but into heat. This can be seen in the
temperature curve shown in Figure 3. The point B corresponds to the inflexion point of the
battery voltage curve versus time.
The [-Δ V] method is definitely no longer suited when it comes to charging NiMH batteries:
the NiMH charging reaction is permanently exothermic (releases heat), so the battery
temperature would become excessive in its [-Δ V] area of the voltage curve (see Figure 3).
Another characteristic of the NiMH batteries makes the [-Δ V] method unsuitable: some
types of NiMH batteries do not exhibit a significant voltage drop as NiCd batteries do when
reaching their full capacity.
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2.2
Charge termination methods
The inflexion point method
A second charge termination method much more adapted to NiMH batteries consists of
detecting the inflexion point of the voltage curve, thus avoiding any excessive overheating of
the battery. This method therefore significantly increases the battery life-time.
Figure 3.
Fast charge terminates at point B in the inflexion point method
V
cell voltage
temperature
deg C
1.6
60
A
B
1.5
50
1.4
battery voltage
40
1.3
30
battery temperature
1.2
20
charging time
-36
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Charge termination methods
Figure 4.
AN417
NiMH versus NiCd charging characteristics
NiCd voltage
NiMH voltage
NiMH temperature
NiCd temperature
charging time
-36
Note:
The NiMH battery temperature rise is larger during fast charge, and its -[Δ V] is less
important, than its voltage slope variation.
Detecting the inflexion point of the voltage curve with the MCU requires the evaluation of the
first derivative of the battery voltage curve versus time, and to detect its summit.
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Principle of the inflexion method
3
Principle of the inflexion method
Every 4 seconds, the analog to digital converter (ADC) of the ST6210 microcontroller
measures the battery voltage and temperature. If the temperature is above a predetermined
threshold (40 degrees Celsius for example), fast charge is terminated.
The battery voltage is actually measured several times in series, and an average of the
measurements is made, to reduce measurement errors due to high frequency noise (for
example generated by a switched mode charging current).
Further, a rolling average of the present and previous measurements is made, to remove
low frequency noise due to electrochemical battery voltage variations (see previous
description of this technique in References [1.]).
This averaged battery voltage data is used to extract a time derivative, by calculating the
difference between present voltage and the voltage n samples earlier. Because of the
voltage digitization, which is measured by the ADC of the MCU, the first derivative curve
features a typical discontinuous aspect (see Figure 5).
For this reason, the MCU calculates a digital smoothing of this discontinuous derivative. The
inflexion point is then identified by detecting the first highest summit encountered on the
smoothed derivative curve (point B in Figure 5).
When this summit is detected, fast charge is terminated.
Figure 5.
Inflexion point method
battery voltage and derivative
B
smoothed derivative
voltage derivative
battery voltage
charging time ->
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Charge control program description
4
AN417
Charge control program description
The microcontroller detects the inflexion point in the battery voltage versus time curve while
charging. When full charge is detected, the MCU puts the charger in trickle charge mode.
As safety protection, the MCU also terminates fast charge if -[Δ V] is detected, or the battery
temperature exceeds a predetermined threshold, or a timer with programmable time
duration expires.
Figure 9 shows the simplified flowchart of the program for the complete charge control. The
overall system is reset after each new input source voltage connection. It is also reset when
a charged battery is removed, and replaced by a discharged battery.
Figure 6.
Simplified program flowchart
START
MEASURE BATTERY VOLTAGE 256 TIMES
NOISE FILTERING AND AVERAGING
CALCULATE DERIVATIVE
SMOOTH DERIVATIVE
ARE WE AT INFLEXION ?
Y
N
Y
ARE WE AT DELTA-V ?
N
IS TIMER EXPIRED ?
Y
N
WAIT 4 SECONDS
IS TEMPERATURE ABOVE THRESHOLD ?
Y
N
IS BATTERY PRESENT ?
N
Y
TRICKLE MODE
N
NEW BATTERY
INSERTED ?
Y
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Test results
5
Test results
Figure 7 and Figure 8 show curves of a NiCd battery charge respectively terminated by
-[Δ V] and inflexion point methods. A similar comparison is made with a NiMH battery in
Figure 9 and Figure 10. The charging current is 2.2 A, the NiCd battery was a 1.4 Ah type
and the NiMH battery a 2.2 Ah type. These measurement results clearly show that the
battery temperature increase is much smaller with the inflexion method than with the
conventional -[Δ V] method. Moreover, these curves demonstrate that a one hour charge of
NiMH batteries can be properly monitored by the ST6210 MCU.
Figure 7.
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.
cell voltage
temperature
32 deg
voltage
temperature
22.4 deg
charging time
-36
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Test results
Figure 8.
AN417
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.
temperature
cell voltage
smoothed derivative
26 deg
voltage
temperature
21 deg
charging time
-36
Figure 9.
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.
cell
voltage
temperature
voltage
41
deg
temperature
22.8
deg
charging
time
-36
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Test results
Figure 10. 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.
cell
voltage
temperature
smoothed voltage
derivative
voltage
26.5 deg
temperature
19 deg
charging time
Table 1.
-36
Charge of different battery types with an 2.2 A current source
Battery
Monitoring
Duration
Temperature increase
NiCd 1.4 Ah
-[ΔV]
inflexion point
48 min.
41 min.
9.6 °C
5 °C
NiMH 2.2 Ah
-[ΔV]
inflexion point
63 min.
57 min.
18.2 °C
7.5 °C
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Charger schematic examples
AN417
6
Charger schematic examples
6.1
Block diagram
The charger is a power supply operating as a constant current source. Such a current
source can be made with a SMPS working from the AC mains or from a DC voltage source.
Figure 11 and Figure 12 give block diagrams of an offline SMPS charger and a DC to DC
charger.
Figure 11. Block diagram of an off-line SMPS charger
NiMH
or
AC mains
SMPS
NiCd
battery
PWM
current
mode
MCU
temperature
ST6210
UC3845
voltage
-36
Figure 12. Block diagram of a DC/DC charger
NiMH
or
DC voltage
SMPS
NiCd
battery
PWM
current
mode
MCU
temperature
ST6210
UC3843
voltage
-36
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Charger schematic examples
6.2
Battery charger examples
6.2.1
Offline charger
Figure 13 gives an example of an offline 35 watt battery charger working at 100 kHz. This
SMPS can deliver up to 3.5 amps DC to a 6 cell battery. Typical charging time of a 1.4 Ah
7.2v NiCd battery pack is around 30 minutes. This offline charger is an asymmetrical half
bridge totally controlled from the primary side with a standard PWM control IC, the UC3845,
regulating in current mode (see References [3.]).
The asymmetrical half bridge structure allows the use of two standard 500V power
MOSFETS IRF820 without snubber network, as voltage across each power MOSFET is
systematically clamped to the input DC voltage by two demagnetization diodes
(BYT01/400). The MCU controls the SMPS section through a single optocoupler, either in
battery charge mode or in trickle mode.
Note that the high side power MOSFET is simply driven by an auxiliary winding of the power
transformer, avoiding the use of an additional pulse transformer.
The switching frequency is set at 100 kHz in order to keep the magnetic parts to a
reasonable manufacturing cost level (see References [2.]. The power transformer and the
output inductor can be integrated on a single ferrite core to allow a significant shrinking of
the power converter size. This integrated magnetic technique has been used in a 80 watts /
15 minutes charger (see References [2.]).
By a simple resizing of the discrete power devices ratings, and by applying the [-Δ V]
termination method, the same off-line converter hardware has been used to charge a typical
7V2/1.2 Ah NiCd pack of cordless drill in less than 15 minutes (see References [1.] [2.]).
6.2.2
DC/DC charger
Figure 14 shows a 15 watt 100 kHz battery charger supplied from a 12 Vdc input voltage.
The DC input voltage can be supplied from a car battery, or from a 50/60 Hz transformer
rectified voltage. This DC/DC charger is controlled by the PWM control circuit UC3843.
For example, such a 15 watt converter is able to deliver 1.5 amp DC, charging five 1500
mAh NiMH cells in approximately 1 hour.
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16/21
4.7uF
25V
BZX85C18V
39K
BC327
1N4148
Doc ID 2074 Rev 2
1nF
5
UC3845
VIN
OUT 6
VREF
VFB 2
8.2K
4
RT/CT COMP 1
G C/SEN 3
N
D
7
8
270K
1/2W
470K
680pF
220R
4.7R
IRF820
BYT01/400
1.8R
110
220
100uF
25V
1K
5
3
4
1
2
-
10K
PB4 11
PB0 15
PB1 14
PB2 13
PB3 12
20
VSS
PA0 19
PA1 18
PA2 17
PA3 16
ST62E10
PB6
PB5
RESET
PB7
NMI
VPP/TES
VDD
TIMER
OSCIN
OSCOUT
+
MAINS
90/260VAC
10
9
8
6
100K 7
2MHz
1uF
10V
1N4148
27pF 27pF
1A
1K
BZW04P15
10R
220uF
25V
78L05
IN G OUT
N
D
4.7uF
10V
70uH /4A
BYV10/40
STPS3045CT
EF25
BYT01/400
BZX83C18
4.7R
BZX83C18
IRF820
47UF
200V
47uF
200V
1A
400V
1.8K
1K
5.6K
1.5K
4N25
4.7uF
10V
22K
680
6.8V
3.3K
4.7uF
120K
6
TS271
5
41
7
27K
3
2
8
5.6K
5.6K
15K
5.6K
4.7uF
10V
Charger schematic examples
AN417
Figure 13. This 35W/100kHz off-line charger is an asymmetrical half-bridge regulated in current
mode from its primary side
-36
GND
11 TO 25V
+
-
+
DC
33pF
220uF
25V
2A
Doc ID 2074 Rev 2
33pF
2MHz
10
9
8
7
6
5
4
3
2
1
4
4.7uF
10V
8
7
OUT 6
VFB 2
ST62E10
PB5
PB6
PB7
RESET
VPP/TES
NMI
OSCOUT
OSCIN
TIMER
VDD
16
17
18
6.8K
PB4 11
PB2 13
PB3 12
6
22nF
15K
2.2uF
10V
6.8K
15K
680
2.2uF
10V
15K
Stop
On
1K Temp
680
2.2uF
10V
12K
2
10K
3
PB0 15 LED
PB1 14
PA3
PA2
PA1
VSS 20
PA0 19
4.7uF
16V
RT/CT COMP 1
G
C/SEN 3
N
D
5
VREF
VIN
UC3843
1nF
7
1
5
L4962
1K
uA741
2
3
1
9.1K
4
7
27K
100uF
25V
8
BZX55C3V3
6
BYV10-40
BYV10-40
4
80uH
1.5A
I=1.5A
0.33R
1.5W
-
+
6.8K
5 18K
R12
+Vbatt
6.2K
2.2uF
10V
PACK 6V
BATTERY
1500mAH
AN417
Charger schematic examples
Figure 14. This 15W/100kHz DC-to-DC charger is also driven by a low-cost PWM control
integrated circuit, the UC3843
-36
17/21
Conclusion
7
AN417
Conclusion
A relevant feature dominates today's electronic appliances - true portability. In these
cordless appliances, fast charging of the battery packs is often considered by the user as a
significant comfort improvement. Such an improvement can be achieved with a safe and
cost-effective charger concept using an off-the-shelf microcontroller, the ST6210.
Moreover, the present battery charger concept is NiCd/NiMH compatible, meeting the
current trend to progressively replace the NiCd battery by the more environmentally friendly
NiMH battery.
The charge termination method is based on the detection of the battery voltage inflexion
point in order to avoid any excessive overheating of the battery. Such a charge technique
significantly improves the battery service life, preventing the user from untimely replacing his
battery pack by a costly new pack. In addition, this low cost microcontroller provides a safe
charge by combining three other back-up termination methods typical of high end dedicated
control circuits: [-Δ V] detection, battery temperature monitoring and timer cut-off.
The natural programmability benefit of such a microcontroller-based charger design allows
the designer to easily implement additional user interface functions. For example, a "gas
gauge" function indicating the remaining battery capacity to the user could be easily added
to the present basic program, whilst retaining the same charger hardware structure. Finally,
the major benefit of using this off-the-shelf ST6210 approach lies in the high adaptability of
its application features.
18/21
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8
References
References
1.
Ultra-fast NiCd battery charger using ST6210 Microcontroller AN433
(STMicroelectronics)
2.
An ultra-fast NiCd battery charger concept 43rd International Appliance Technical
Conference, May 5-6, West Lafayette, Indiana U.S.A. / L. Wuidart
3.
A cost effective ultra-fast NiCd battery charger AN486 STMicroelectronics
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Revision history
9
AN417
Revision history
Table 2.
20/21
Document revision history
Date
Revision
Changes
01-Jan-1994
1
Initial release.
02-Nov-2011
2
Updated format and company logo.
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