IET Circuits, Devices & Systems
Research Article
Active balancing method for series battery
pack based on flyback converter
ISSN 1751-858X
Received on 8th January 2020
Revised 22nd February 2020
Accepted on 2nd March 2020
E-First on 25th November 2020
doi: 10.1049/iet-cds.2020.0008
www.ietdl.org
Guo Xiangwei1 , Geng Jiahao1, Liu Zhen1, Longyun Kang2, Xu Xiaozhuo1
1The
School of Electrical Engineering and Automation, Henan Polytechnic University, Jiaozuo, 454000, People's Republic of China
School of Electric Power, South China University of Technology, Guangzhou, 510000, People's Republic of China
E-mail: gxw@hpu.edu.cn
2The
Abstract: Lithium battery has become the main power source of new energy vehicles due to its high energy density and low
self-discharge rate. In the actual use of the series battery pack, due to the internal resistance and self-discharge rate of batteries
and other factors, inconsistencies between the individual cells are unavoidable. Such inconsistencies will reduce the energy
utilisation rate and service life of the battery pack, and even endanger the safety of the battery systems. To improve the
consistency of the series battery pack, a novel balancing method based on the flyback converter is proposed in this study. The
flyback converter with a simple and reliable structure is used to realise the energy transfer between the whole battery pack and
any single cell. Compared with the traditional balancing topology, the topology proposed in this study reduces the number of
components and the volume of the balancing system, and only needs one set of control signals on the converter primary side,
thus reducing the control difficulty. The experimental results show the effectiveness of the novel balancing method.
1 Introduction
Lithium-ion (Li-ion) battery has gradually become the main power
source of new energy vehicles due to its high energy density, high
output power, long cycle life, and other advantages [1, 2]. Since the
low voltage of lithium battery cells, it is generally necessary to
connect cells in series to form a battery pack in applications [3].
However, due to the influence of production technology and other
factors, the single-cell will have inconsistent phenomena after
cyclic charging and discharging for a period of time. This
phenomenon will reduce the energy utilisation and service life of
the battery pack, and easily lead to overcharge or over-discharge.
Therefore, balancing technology is of great significance for
improving the consistency of the battery pack.
At present, balancing technology is mainly divided into two
categories: passive balancing and active balancing [4]. Passive
balancing mainly uses a resistor as the shunt of each battery to
convert the extra energy of the high-voltage battery into thermal
energy for consumption. This method has the advantages of small
volume and low cost. However, the problems of energy dissipation
and heat dissipation are key shortcomings. Active balancing
achieves energy transfer through energy storage elements such as
capacitors, inductors, and transformers, which is also called nonenergy-consumption balancing or lossless balancing. The balancing
topology based on switched capacitors proposed in [5–7] has the
advantages of small size and easy control, but its balanced
efficiency is not high and the capacitance balancing time is long,
especially when the voltage difference between cells is small. The
inductor-based balancing topology proposed in [8–12] has high
balancing efficiency, but its circuit structure is complex, requiring a
large number of switching tubes and inductors, and usually, the
control is complicated and is not conducive to the reduction of the
volume of the balancing system. The transformer-based balancing
topology proposed in [13–16] has the advantages of high balancing
efficiency, simple control, and easy isolation. For example, Zhang
et al. [14] proposed a battery equaliser based on a multi-winding
transformer. The advantage is that energy can be transferred
between any single cells, but the multi-winding transformer has
problems such as large size, high complexity, and serious magnetic
leakage problems. Shang et al. [16] proposed a modular balancing
method based on a multi-winding transformer. Compared with the
former, it reduces the volume and cost, but the module structure is
IET Circuits Devices Syst., 2020, Vol. 14 Iss. 8, pp. 1129-1134
© The Institution of Engineering and Technology 2020
more complex. When the number of battery cells changes, it is not
conducive to the expansion of the balancing topology.
Based on the summary and analysis of the research contents of
the previous scholars, and the characteristics of a flyback
converter, such as simple structure, wide input voltage range, and
high conversion efficiency, an active balancing topology based on
flyback converter is proposed in this study. Only one set of the
flyback converters is used to achieve energy transfer. Compared
with the existing methods, the balancing method proposed in this
study has the advantages of simple structure, small size, simple
control, and easy expansion, and can be used in the new energy
vehicles power battery balancing system.
The remainder of this paper is organised as follows: in Section
2, the structure and principle of balancing topology are described.
In Section 3, the fundamental parameters of the balancing topology
are calculated. In Section 4, the experimental verifications are
presented. Section 5 is the comparison between the balancing
topology of this study and the current common balancing topology.
Finally, conclusions are given in Section 6.
2 Balancing topology and principle
This section first introduces the structure of the novel balancing
topology and then takes a battery pack composed of four cells as an
example to analyse the working principle of the novel balancing
topology in detail.
2.1 Balancing topology
The novel active balancing topology based on the flyback
converter is shown in Fig. 1. Each cell in the series battery pack is
sequentially labelled B1, B2, B3,…, Bn, and each metal oxide
semiconductor field-effect transistor (MOSFET) is sequentially
labelled S0, S1, S2,…, S2n. When all the MOSFETs are turned off,
the voltage stress on S2n is the largest; it is equal to the battery pack
voltage. The left side of the converter is the primary side and the
right side is the secondary side. Each cell Bn requires two
MOSFET S2n−2 and S2n−1 to be connected to the secondary side of
the converter, which is used to control the on–off of the secondary
side's current. To ensure the safe operation of the circuit, each
MOSFET has a reverse diode in series. The MOSFET S2n is an
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advantages of the novel active balancing topology are: (i) the
energy storage unit has only one single-winding converter. The
topology is simple and easy to control, which helps to reduce the
volume and cost of the balancing system. (ii) Balancing energy can
be transferred between the entire battery pack and any single cell,
which reduces the balancing path and improves the balancing
speed. (iii) When the number of cells included in the battery pack
changes, it is only necessary to increase or decrease the
corresponding number of MOSFETs, which is easy to expand.
2.2 Balancing principle
Fig. 1 Balancing topology
In this section, the principle of balancing is illustrated by taking a
battery pack with four cells connected in series as an example, as
shown in Fig. 2. The balancing circuit takes the terminal voltage of
the single cells as the battery pack inconsistency index [10]. When
the difference between the highest terminal voltage and the lowest
terminal voltage exceeds a given threshold, the balancing circuit
starts to work. The overall idea of the balancing circuit is to
transfer the energy of the entire battery pack to the cell with the
lowest terminal voltage through the flyback converter, so as to
achieve the energy balance of each cell. Assuming that the voltage
of cell B2 is too low to reach the balancing condition, the balancing
circuit starts working. In the first stage, as shown in Fig. 2a, the
control circuit drives the converter's primary side MOSFET S8 to
turn on. The battery pack charges the primary inductor, the primary
inductor current linearly rises, and the converter stores energy. At
this time, the polarity of the primary inductor voltage is ‘positive
up and negative down’. In the second stage, as shown in Fig. 2b,
the MOSFET S8 on the primary side of the flyback converter is
turned off. In the RCD buffer circuit, the capacitor absorbs the
leakage inductance energy stored in the flyback converter, and then
the resistor consumes the leakage inductance energy, thereby
reducing the voltage shock of the primary side MOSFET. In the
third stage, as shown in Fig. 2c, MOSFETs S2 and S3
corresponding to cell B2 are turned on. Since the inductor current
cannot be changed abruptly, the primary inductor will induce an
induced electromotive force with a polarity of ‘negative up and
positive down’. The polarity of the induced electromotive force
coupled to the secondary side inductor of the converter is ‘positive
up and negative down’, and the secondary side inductor will charge
cell B2. The whole process realises that the overall energy of the
battery pack is transferred to the cell with the lowest voltage
through the converter.
3 Calculation
parameter
Fig. 2 Balancing principle of the four-cell battery pack
(a) Charging process of converter primary, (b) RCD buffer circuit absorbs the spike
voltage, (c) Discharging process of converter secondary inductor
independent group connected to the primary side of the converter,
which is used to control the on–off of the primary side's current.
The residual-current device (RCD) buffer circuit is used to absorb
the leakage peak voltage of the converter. It is connected in parallel
to the primary side of the converter and consists of a resistor and a
capacitor connected in parallel and a diode connected in series. The
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of
the
balancing
topology
The parameters of the balancing topology mainly involve the turn's
ratio of the flyback converter, the inductance of the primary and
secondary sides, and the duty cycle of the MOSFET control signal.
The flyback converter has two modes of operation: continuous
current mode (CCM) and discontinuous current mode (DCM).
Before the primary side MOSFET is turned on, if the primary
inductor still has the energy and is not completely transferred to the
secondary side, the situation is called CCM. If the primary
inductor's energy is completely transferred to the secondary side,
the situation is called DCM. Owing to the incomplete transfer of
energy in CCM, there is a problem of reverse recovery of the
diode. Also, to prevent hysteresis saturation, the flyback converter
must work in DCM. The specific calculation process of the
parameters is as follows.
First, make the following rules: transformer primary winding
turns are recorded as NP, secondary winding turns are recorded as
NS. The primary inductor is recorded as LP and the secondary
inductor is recorded as LS. The primary side current is recorded as
IP and the secondary side current is recorded as IS. The primary
side voltage is recorded as VP and the secondary side voltage is
recorded as VS. The period of the control signal is recorded as T. In
one cycle, the rise time of the primary current is recorded as Ton,
the fall time of the secondary current is recorded as Toff, and the
IET Circuits Devices Syst., 2020, Vol. 14 Iss. 8, pp. 1129-1134
© The Institution of Engineering and Technology 2020
By selecting a suitable primary side balancing current IP, the
primary side's inductance LP can be calculated. According to (7),
the secondary inductance can be calculated
LS = LP /
Fig. 3 Converter operating current waveform
dead time is recorded as Td. The voltage reflected by the secondary
side voltage to the primary side is recorded as Vf.
Fig. 3 shows the designed primary and secondary side current
waveforms.
The current passing through the converter primary side
increases linearly when MOSFET S2n is turned on. The flyback
converter stores the energy transmitted from the entire battery pack
n
iLi =
∑j = 1V j ⋅ t
L
0<t < D⋅T
(1)
where T represents the switching cycle, D represents the duty
cycle, t represents the time, iLi represents the primary side current.
The average charging current of the converter in one period
Iic =
V i ⋅ D2 ⋅ T
1 Ip ⋅ D ⋅ T
=
⋅
2
2L
T
(2)
When the primary side MOSFET of the converter is turned off, the
polarity of the inductor voltage on the secondary side is ‘positive
up and negative down’. At the same time, the primary inductor will
induce a reflected voltage Vf with a polarity of ‘positive down and
negative up’
Vf =
NP
× VS
NS
(3)
Without considering the leakage inductance spike, the voltage drop
that the MOSFET withstand is VP + Vf, and VP is the overall
voltage of the series battery pack. So, the turn ratio determines the
shutdown voltage that the MOSFET can withstand. Therefore, the
choice of turn's ratio should make the maximum voltage pressure
of the MOSFET as small as possible.
After the turn's ratio is determined, it is also necessary to
determine the duty ratio of the control signal. To make the
converter work in DCM, the dead time Td must be set. Also, to
ensure the magnetic core is not saturated, the primary and
secondary sides of the converter should meet the volt-second
balance principle, i.e.
V P × T on =
NP
× V S × (T − T on − T d)
NS
(4)
Among them, VP and the turn ratio can be known, and the dead
time Td is usually set to 0.2 T [17]. The secondary voltage VS is
equal to the battery voltage plus the conduction voltage drop of the
two diodes. Ton can be obtained from (4) and duty ratio D can be
determined from (5) by selecting an appropriate switching
frequency f
D = T on × f
(5)
NP
NS
2
(7)
In summary, by selecting the appropriate balancing current and
switching frequency, other parameters required for the balancing
can be obtained.
Considering that the flyback converter will generate a very
large peak voltage at the moment when the primary side MOSFET
is turned off, its damage to the MOSFET is very large and
excessively high dv/dt will cause serious electromagnetic
interference. So it is necessary to add a buffer circuit to the
topology to suppress it. Common buffer circuits include RCD
buffer circuits, liquid crystal display buffer circuits, and transient
suppression voltage diode clamp circuits. Among them, the RCD
buffer circuit is widely used for its simple structure and low cost.
This study uses the RCD buffer circuit to reduce the impact of the
peak voltage on the MOSFET.
The RCD buffer circuit module is connected in parallel at both
ends of the primary coil of the flyback converter and is composed
of a resistor and a capacitor connected in parallel and a diode in
series. At the moment when the primary MOSFET is turned off,
the capacitor can absorb the leakage inductance energy stored in
the converter and consume it by the resistor, so the voltage stress of
the primary side MOSFET can be reduced. If R × C is too small,
the capacitor will charge faster and the leakage inductance energy
will be consumed faster. Before the primary side MOSFET is
turned on, the resistance will consume the primary magnetising
inductance energy. If R × C is too large, the capacitor charging will
be slower, causing the secondary side of the converter to delay
conduction, and part of the primary excitation inductance energy is
consumed by the resistor–capacitor circuit. Therefore, the
appropriate R × C value should be selected so that it only consumes
leakage inductance energy. The following is the design method of
the RCD absorption circuit.
The first step is to determine the voltage VRCD of the absorption
capacitor, which is usually 2–2.5 times the reflected voltage [17].
However, VRCD + Vin cannot exceed 0.85VDSS. VDSS is the drainsource breakdown voltage of the MOSFET.
The second step is to determine the absorption resistance. After
VRCD is determined, the loss of the RCD buffer circuit is
PRCD =
V RCD
1 2
f
L i
2 K peak V RCD − V f s
(8)
Among them, fs is the switching frequency of the MOSFET, ipeak is
the peak value of the primary side current, LK is the leakage
inductance of the converter, and the leakage inductance of the
sandwich winding method is usually 2–3% of the primary
inductance [17]. It can be seen from (6) that the smaller VRCD, the
2
greater the loss of the RCD loop. The loss on resistor R is V RCD
/R.
According to the principle of conservation of energy, we can obtain
R=
2V RCD(V RCD − V f )
LK i2peak f s
(9)
The third step is to determine the absorption capacitance. The
maximum ripple voltage on the absorption capacitor C is
ΔV RCD =
V RCD
CR f s
(10)
The value of the absorption capacitor C can be determined
according to the appropriate ripple voltage.
According to (6)
V P = LP ×
IP
T on
IET Circuits Devices Syst., 2020, Vol. 14 Iss. 8, pp. 1129-1134
© The Institution of Engineering and Technology 2020
(6)
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Fig. 4 Balancing experiment platform
Table 1 Balancing experimental parameters
Parameters
battery capacity
switching frequency
PWM1 duty cycle
PWM2 duty cycle
balancing current
balancing accuracy
flyback converter turns ratio
primary/secondary inductance
diode voltage drop
capacitance
resistance
charging current
discharging current
Value
3.2 Ah
10 kHz
20%
80%
1.2 A
ΔV < 0.05 V
1:1
50 μH
0.3 V
0.44 μF
100 Ω
0.7 A
0.7 A
Fig. 6 Voltage variation of four cells in one control signal period
Fig. 5 Primary side's control signal and the converter balancing current
4 Experimental verification
This section explains the parameters setting of the experimental,
lists and analyses the experimental results.
4.1 Parameter setting of the balancing experiment
To verify the effectiveness of the novel balancing topology, a
battery pack consisting of four cells is designed for experiments.
The experimental platform is shown in Fig. 4, the 18,650 type
ternary lithium battery with a rated capacity of 3.2 Ah and a rated
voltage of 3.7 V produced by the SANYO company of Japan was
selected. The balancing topology experimental parameters are
shown in Table 1. The drive signal of the primary MOSFET S8 in
the control circuit is pulse-width modulation 1 (PWM1), which is
used to control the charging process of the flyback converter, and
the control signal of the flyback converter in the discharging
process is PWM2.
1132
Fig. 7 Primary inductor voltage waveforms
(a) Primary inductance voltage waveform without RCD buffer circuit, (b) Primary
inductance voltage waveform with RCD buffer circuit
4.2 Balancing experimental results and analysis
Fig. 5 shows the control signal of the primary side MOSFET of the
flyback converter and the balancing current of the primary and
secondary sides. During a control signal period, the primary side
current linearly rises, indicating that the entire battery pack is
charging the converter, and the secondary side current linearly
falls, indicating that the flyback converter is charging the balanced
target. The peak value of the primary current is 1.5 A, and the
secondary current is 1.2 A. Each cycle of the converter works in
DCM, and the inductor can be reset.
Fig. 6 shows the changes in the voltage of four single cells
during a control signal period. In the first stage, the battery pack
IET Circuits Devices Syst., 2020, Vol. 14 Iss. 8, pp. 1129-1134
© The Institution of Engineering and Technology 2020
5
Discussion
As shown in Table 2, the novel balancing method proposed in this
study is compared with the existing balancing methods in terms of
component number, cost, and balancing efficiency. The evaluation
criteria are as follows: (i) the number of components is mainly
concerned with the number of switches, inductors, capacitors,
diodes, transformers, and resistors. (ii) Cost is calculated according
to the type, number, and market price of components required for
each system. (iii) Evaluate the balancing efficiency according to
the type and number of components through which the balancing
energy flows. (iv) The balancing speed mainly depends on the
balancing current and the switching steps of the balancing energy
transmission from the source to the target. The balancing topology
proposed in this study is to transfer the entire battery pack energy
to the lowest terminal voltage battery. The energy conversion only
requires two steps, and the balancing speed is fast. (v) System
complexity mainly depends on the difficulty of the control system
and the complexity of the circuit structure. The balancing topology
proposed in this study only needs two sets of complementary
control signals, and the controller only needs to judge the collected
terminal voltage signals. If the difference between the maximum
terminal voltage and the minimum terminal voltage is bigger than
the threshold, the controller sends two sets of control signals, one
to the MOSFET S2n on the primary side and the other to the
MOSFET S2n−2 and S2n−1 on the secondary side. The control
system is very simple and the proposed circuit structure is highly
symmetrical. When the number of battery cells increases, only the
corresponding MOSFET needs to be added. The balancing circuit
is easy to expand. (vi) Compared with the multi-winding
transformer-based balancing topology proposed in [18–21], the
topology proposed in this study only requires one set of converters,
and the size is greatly reduced, especially when the number of
series cells increases.
Fig. 9 shows the characteristics of the balancing topology
presented in this study more clearly. It should be noted that the
advantages of the number of components of the novel balancing
topology are not outstanding, which mainly depends on the number
of diodes required by the novel method. However, with the
development of semiconductor devices, the volume of diodes has
become very small and the cost is very low. In terms of balancing
speed, complexity, and volume, the novel balancing method based
on the flyback converter has obvious advantages. This mainly
depends on the use of only one set of flyback converters in the
energy storage unit, which greatly reduced the size and complexity
of the balancing system, and is suitable for the new energy vehicle
battery balancing system.
6 Conclusion
Fig. 8 Experimental results of four-cell battery balancing
(a) The shelved state with the initial voltage: ① 3.800 V, ② 3.740 V, ③ 3.300 V, ④
3.700 V, (b) The discharging process with the initial voltage: ① 3.960 V, ② 4.000, ③
3.700 V, ④ 3.950 V, (c) The charging process with the initial voltage: ① 3.550 V, ②
3.580 V, ③ 3.300 V, ④3.600 V
charges the primary inductor, and the voltages of the four cells first
fall overall. After the battery pack charges the converter, the
voltage of every single cell rises a period due to the polarisation
effect. In the second stage, the secondary inductor charges the
lowest terminal voltage cell, and the lowest terminal voltage rises
first. When the balancing current is zero, the lowest terminal
voltage falls a period due to the polarisation effect.
Fig. 7 shows the comparison of the effect of the RCD buffer
circuit on the primary inductor voltage. It can be seen that the RCD
buffer circuit absorbs the spike voltage generated by leakage
inductance, suppresses the excessively high dv/dt, and ensures the
safe and effective operation of the balancing topology (see Fig. 8).
IET Circuits Devices Syst., 2020, Vol. 14 Iss. 8, pp. 1129-1134
© The Institution of Engineering and Technology 2020
To improve the energy utilisation rate and service life of a series
battery pack for new energy vehicles, a novel active balancing
method based on the flyback converter was proposed. Only one set
of flyback converters with a simple structure and high conversion
efficiency is used to form an energy storage unit, and the balanced
energy is transferred between the entire group of the battery pack
and any single cell. The novel balancing topology has the
advantages of small size, low cost, fast balancing speed, and easy
expansion. By setting up an experimental platform, the
effectiveness of the novel balancing method in this study is
verified.
7 Acknowledgments
This work was supported by the National Natural Science
Foundation of China (61703145), the key scientific and
technological projects of Henan Province (202102210093), the key
scientific research projects of higher education institutions in
Henan Province (19A470001/18A470014). I would like to express
my deepest gratitude to my supervisor, Longyun Kang, who has
provided me with valuable guidance at every stage of writing this
paper. I would also like to thank the anonymous reviewers for
dedicating the time to review my paper despite their busy
schedules.
1133
Table 2 Comparison of various equalisation methods
Balancing topology
Components
Switch L C Diode T R Cost
resistance dissipation [22]
switching capacitor [23]
three-resonant-state [24]
switching inductor [25]
zero current switching [26]
multi-winding transformer [18]
flyback converter [19]
flyback/forward converter [20]
flyback–forward converter [21]
LC resonance type [27]
proposed topology
n
2n
4(n − 1)
n
2n
n
2n
2n
n
4n + 2
2n + 1
0
0
0
n
n
0
0
0
0
1
0
0
n
n
0
n
0
0
0
0
2
1
0
0
0
n
0
0
0
0
0
0
2n
0
0
0
0
0
n
n
n
n
0
1
n
0
0
0
0
0
0
0
0
0
1
E
E
G
G
G
G
G
G
G
VG
E
Performance parameter
Efficiency
Speed
Complexity
S
S
VG
VG
VG
E
E
E
E
E
VG
S
G
VG
VG
VG
E
E
E
E
VG
E
E
VG
G
G
S
G
G
G
G
S
E
Volume
E
VG
VG
G
G
G
G
G
G
VG
E
L, inductor; C, capacitor; T, transformer; R, resistor; n, number of single cells in the series battery pack; E, excellent; VG, very good; G, good; S, satisfactory.
[12]
[13]
[14]
[15]
[16]
Fig. 9 Comparison of several balancing topologies
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IET Circuits Devices Syst., 2020, Vol. 14 Iss. 8, pp. 1129-1134
© The Institution of Engineering and Technology 2020