APPLICATION NOTE
AN INTELLIGENT ONE HOUR MULTICHARGER
FOR Li-Ion, NiMH and NiCd BATTERIES
by J-M. Ravon and L. Wuidart
INTRODUCTION
An increasing number of cordless and portable appliances are powered by rechargeable batteries. Products such as portable audio equipment, mobile phones, and cordless power tools
illustrate the enormous contribution which rechargeable batteries make to our comfortable lifestyle. As equipment becomes lighter and more compact, battery performance must follow suit,
hence NiMH batteries have overtaken NiCd batteries, due to their superior capacity for a given
weight and size.
More recently, the new Li-Ion battery technology has been used on camcorders, and will soon
spread to other applications. Other new battery technologies, such as Zinc-Air, are already appearing on the market.
Rapid charging of NiCd and NiMH batteries in one hour or less is already commonplace. Many
battery chargers already cope with both these battery types and, in future, chargers will have
to be compatible with emerging battery technologies.
This paper presents a new multicharger concept which is fully compatible with Li-Ion, NiCd
and NiMH. Moving from NiCd/NiMH to Li-Ion technology is easy when battery charging is
monitored by a flexible industry standard 8-bit microcontroller.
AN859/0496
1/12
1
A ONE HOUR MULTICHARGER CONCEPT...
In this example of an intelligent one hour multicharger, the battery is charged by a constant
current source while an industry standard 8-bit microcontroller monitors both battery voltage
and temperature, using its on-chip Analog to Digital converter as illustrated in Figure 1.
Figure 1. Intelligent multicharger concept: Li-Ion/NiCd/NiMH compatibility is ensured by
an STMicroelectonics low-cost 8 bit microcontroller.
Vin
cell voltage
I charge
standard 8 bit
microcontroller
ST6220
temperature
current
Li-Ion
or
NiMH
or
NiCd
The key to the charging methods used to ensure compatibility with different battery technologies, is the ability to monitor the charging parameters using a low-cost industry standard
STMicroelectonics 8-bit microcontroller: the ST6220. The inherent programmability of a microcontroller based charger concept allows the existing NiCd/NiMH design [1] to be upgraded for
use with the new Li-Ion rechargeable batteries, simply by modifying the software, while retaining the same hardware design.
Safe charging is ensured by combining several termination methods, guaranteeing results on
a par with high-end dedicated monitoring circuits. These back-up termination criteria can be
optimised by software for any specific application requirement. For example, the termination
criteria can be easily adapted by software to specific cell technologies or to dedicated battery
pack shapes.
The intrinsic adaptability to new battery technologies and charging methods represents a
major benefit with respect to dedicated solutions.
2/12
A ONE HOUR MULTICHARGER CONCEPT...
1 NiCd AND NiMH FAST CHARGING
Safe fast charging relies on the charging cycle being monitored intelligently. This has already
been successfully achieved in a design based on a standard ST6220 low-cost 8-bit microcontroller. This charger design is capable of charging both NiCd and NiMH batteries, using a
charge termination method based on detecting the inflexion point in the battery voltage
curve.[1]
However, sophisticated termination methods, such as inflexion point detection, require battery
voltage measurement with a resolution of at least 10 bits. This can be achieved by using an
external operational amplifier to improve the resolution of the on-chip 8-bit Analog to Digital
converter [1].
A cheaper means of obtaining high resolution voltage measurement lies in measuring the
charge time of a capacitor, using the microcontroller’s on-chip timer.
A simple RC circuit (using a polyester capacitor for its temperature stability), connected to a
triggered input port, allows battery voltage variations to be converted to time measurements,
with an equivalent resolution of 11 bits.
,,,,,,
,,,,,,
,,,,,,
,,,,,,
Figure 2. RC charge time voltage measurement technique
ST6
Vbat
Vcap
TIMER
T
VTH
V
Vcap = Vbat (1 - exp(-t/RC))
VTH
T = - RC Ln (1 - VTH/Vbat)
Vcap
T
t
3/12
A ONE HOUR MULTICHARGER CONCEPT...
The effective resolution achieved using this technique has been measured as better than 11
bits, allowing more reliable detection of the inflexion point on the battery voltage curve.
Charge termination based on inflexion point detection improves NiCd/NiMH battery life, by
avoiding excessive overheating. This avoids the user having to prematurely replace costly
battery packs.
Safe charging, in one hour or less, is possible using this low-cost microcontroller design which
combines three other back-up termination methods, worthy of high-end dedicated control circuits: -∆V detection, battery temperature monitoring and timed shutdown.
Figure 3. Safe charging of NiCd and NiMH batteries is ensured by combining three backup termination methods: -∆V detection, battery temperature monitoring and timed
shutdown.
battery
cell voltage
V
battery
temperature
°C
1.6
STOP
BACK-UP
1
60
-DV
inflexion point
1.5
50
1.4
40
voltage
1.3
30
BACK-UP
2
1.2
temperature
20
charging time
4/12
BACK-UP
3
!
A ONE HOUR MULTICHARGER CONCEPT...
2 EXTENDING THE CONCEPT TO Li-Ion
The inherent programmability of such a microcontroller based charger concept allows it to be
upgraded from the NiCd/NiMH version to cope with the new Li-Ion batteries.
Such a multicharger concept is fully Li-Ion/NiCd/NiMH compatible: charging any of these battery types becomes transparent to the user .
Li-Ion batteries can be charged using two different methods, both of which are easily monitored by means of an industry standard low-cost 8-bit microcontroller.
In the conventional method, charging begins under constant current conditions.
Once a specified battery voltage (4.2V) has been reached, the charger applies a regulated
constant voltage to the Li-Ion cell (C1 phase in Figure 4.).
Current thus decreases as charging proceeds, until current flow ceases when full capacity is
reached, preventing overcharging (C2 phase in Figure 4.).
Figure 4. Conventional Li-Ion charging method:
- C1 phase: constant current charge
- C2 phase: decreasing current charge
Battery
voltage
4.2V
2.7V
0
0
30mn
2.5h
Current
1A
stop charge
C1
C2
0.1A
5/12
A ONE HOUR MULTICHARGER CONCEPT...
Although a 1 Ah NiCd/NiMH battery pack is fully charged in one hour with a 1 Amp output
charger, it will take 2.5 hours to complete the charge of the corresponding Li-Ion 1Ah pack as
can be seen in Figure 5.
An alternative technique consists in using a constant current source, just as in NiCd/NiMH battery chargers.
This new Li-Ion method may be applied because the capacity level is known during the entire
charging process. Once this “capacity level” indicates full charge, the monitoring microcontroller stops the constant current flow, thus avoiding any overcharging.
Such a constant current source based charging method has an obvious benefit, when one
hour charging is required. To reach full capacity in a 1 Ah Li-Ion pack in one hour, a charger
must deliver twice the output power using a conventional method, compared to a constant current source method.
Referring again to our 1 Ah battery example, a “Conventional one hour multicharger” must deliver
2 Amps for 20 minutes in order to reach full capacity of the Li-Ion battery, whilst NiMH/NiCd batteries
require only 1 Amp to be fully charged under the same conditions, as can be seen in Figure 6.
Figure 5. A 1 Amp output multicharger will reach full capability of 1Ah pack in:
- in 1 hour for NiCd and NiMH batteries
- in 2.5 hours for Li-Ion batteries
Ich (A)
2
1 Ah NiMH or NiCd
1
1 Ah Li-Ion
1
6/12
2
2,5
time (hour)
A ONE HOUR MULTICHARGER CONCEPT...
Figure 6. To reach full capacity of a 1 Ah Li-Ion pack in one hour, a multicharger must
deliver twice the output power with a conventional method, compared to a constant
current source method.
Ich (A)
2
1 Ah NiMH or NiCd
1
1 Ah Li-Ion
20
60 time (min)
40
Figure 7. New Li-Ion charging method using a constant current source charger: battery
voltage and current envelopes
Battery
voltage
envelop 4.2V
,,,,
,,,,
,,,,
2.7V
Current
envelop
0
30mn
1h
2.5h
1A
7/12
A ONE HOUR MULTICHARGER CONCEPT...
This new charging method is based on measurements of the Li-Ion cell voltage under “no
charge” conditions, i.e. when charging current is stopped. The value of this “no charge” Li-Ion
cell voltage gives a good indication of the charge capacity level.
Since a 4.2V “no charge” voltage corresponds to a fully charged battery, charging is stopped
as soon as the “no charge” voltage reaches 4.2V, as can be seen in Figure 7.
To measure this “no charge” battery voltage, the charging current delivered by the switchmode power supply is interrupted by the microcontroller every 20 seconds, as can be seen in
Figure 8.
The “true” value of the “no charge Li-Ion cell voltage” is measured by the on-chip 8-bit Analog
to Digital converter, some seconds after interrupting the charging current, to allow the battery
to recover.
Under charging current conditions, the Li-Ion cell voltage can be higher than 4.2.V (reaching
for example 4.4.V), mainly because of the additional voltage drop due to its internal resistance.
This additional voltage drop does not lead to overchargeof the Li-Ion battery.
Figure 8. To measure “no charge” battery voltage, the charging current delivered by the
switching power supply is interrupted by the microcontroller every 20 seconds.
Battery
voltage
4.2V
2.7V
0
30mn 1h
2.5h
1A
Current
Battery
voltage
0
1A
Current
8/12
Stop charge
4.2V
1.5s
20s
1.5s
60mn
A ONE HOUR MULTICHARGER CONCEPT...
LI-ION VOLTAGE CALIBRATION
Whatever the charging method, for a Li-Ion cell, a 100 mV cell voltage difference represents in
a change of more than 7% in cell capacity.
Since the absolute value of the Li-Ion cell voltage is used to stop the charging process, battery
manufacturers recommend charging methods with an overall voltage accuracy of +/- 1%.
To acheive such an accuracy level, conventional discrete solutions require expensive components (0.2% resistors, 0.5% voltage references, etc...).
This microcontroller based multicharger concept offers a cheaper solution by using a voltage
reference software calibration technique, as can be seen in Figure 9.
The microcontroller memory allows compensation of the voltage reference.
The voltage reference only needs to be temperature stable. For example, a standard UC3843
voltage reference circuit provides a temperature stability of 0.2 mV/°C, which is more than sufficient.
Calibration and memorisation of the Li-Ion reference battery voltage is carried out during factory testing.
Figure 9. The microcontroller memory allows Vref calibration. Vref only needs to be
temperature stable (UC3843 Vref has a sufficient temperature stability: 0.2 mV/°C).
Calibration and memorisation of the Li-Ion reference battery voltage is carried out
during factory testing.
UC3843
Vref
Li-Ion
battery
ST6220
n x 4V2
A/D
conv.
EPROM
calibration
9/12
A ONE HOUR MULTICHARGER CONCEPT...
Figure 10. Electrical diagram of the Li-Ion/NiCd/NiMH multicharger:
DC/DC 100 kHz SMPS/10 Watts
R1
10K
C2
1nF
+ P1
+
2A
11 TO25V
DC
GND
U1
UC3843N
7 VIN
OUT
8 VREF
VFB
4 RT/CT COMP
G
C/SEN
N
D
5
F1
C1
220uF
25V
C8
4.7uF
10V
- P2
L4962
L1
100uH
2A
STPS340U
D7
out 7
U4
5 Osc
3 Comp
CNF 2
gnd Sof/St
6
4
R2
15K
1%
6
2
1
1
Vin
P3 T
+
P5
D2
C4
100uF
STPS340U 25V
R3
C3
22nF
P4 P6 contact
R4 NiCfor
d/NiMH
0.5R
1.5W
I=1.2A
12K
1%
C10
2.2uF
10V
3
4.7uF C9
16V
X1
2MHz
C6
33pF
C7
33pF
R21
10K
1
2
3
4
5
6
8
9
10
VDD
TIMER
OSCIN
OSCOUT
NMI
VPP/TES
PB7
PB6
PB5
U2
ST62E20
VSS
PA0
PA1
PA2
PA3
PB0
PB1
PB2
PB3
PB4
20
19
18
17
16
15
14
13
12
11
R14
6.8K
use software
10/12
+Vbatt
LED
D3
D4
D5
D6
R13
R6
R7
R8
R17
15K
R11
6.8K
C11
2.2uF
10V
green
680 On
red
1K Temp
680 Stop yellow
680 Type green
"On":NiCd/NiMH
R12
475K
1%
C5
0.22uF
63V
Polyester
R15
16.2K
1%
R16
16.2K
1%
BATTERY
PACK
6V1A
NiCd/NiMH
or
7.2V1.2A
LI-ION
A ONE HOUR MULTICHARGER CONCEPT...
CONCLUSION
Li-Ion is an emerging battery technology because of its higher capacity for a given weight and
size, compared to the latest NiMH rechargeable batteries. For this reason, the next generation
of battery chargers must be fully Li-Ion/NiCd/NiMH compatible.
This paper describes an example of such an intelligent multicharger concept, using a DC/DC
switch mode power supply providing a constant current to the battery pack as illustrated in
Figure 10.
Battery type may be determined by means of sensing contacts on the battery pack which are
read by I/O ports on the microcontroller.
The internal memory of this 8-bit standard microcontroller provides a cost effective calibration
solution to acheive the high level of voltage accuracy required for Li-Ion charging.
The intrinsic programmability of such a microcontroller based charger has allowed an easy upgrade path from the previous NiCd/NiMH version [1] to the new Li-Ion rechargeable batteries,
without major hardware changes.
Other charging and back-up termination methods can be easily implemented simply by modifying the control software, while retaining the same hardware design. This simplifies the design task compared to a fully dedicated control circuit. A standard microcontroller based approach can thus significantly reduce the overall design cycle time of battery chargers.
Software adaptability to new battery technologies and charging methods represents a major
benefit compared to dedicated solutions.
The inherent flexibility of the microcontroller approach will allow rapidly evolving battery technologies, such as Zinc-Air or Zinc-Silver, to be catered for.
References:
[1] “From NiCd To NiMH Fast Battery Charger”
AN417/L.WUIDART, J. NICOLAI (STMicroelectonics)
11/12
A ONE HOUR MULTICHARGER CONCEPT...
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