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Journal of Mechatronics, Electrical Power, and Vehicular Technology 9 (2018) 81–88
Journal of Mechatronics, Electrical Power,
and Vehicular Technology
e-ISSN: 2088-6985
p-ISSN: 2087-3379
www.mevjournal.com
doi: https://dx.doi.org/10.14203/j.mev.2018.v9.81-88
2088-6985 / 2087-3379 ©2018 Research Centre for Electrical Power and Mechatronics - Indonesian Institute of Sciences (RCEPM LIPI).
This is an open access article under the CC BY-NC-SA license (https://creativecommons.org/licenses/by-nc-sa/4.0/).
Accreditation Number: (LIPI) 633/AU/P2MI-LIPI/03/2015 and (RISTEKDIKTI) 1/E/KPT/2015.
Implementation of a LiFePO4 battery charger
for cell balancing application
Amin*, Kristian Ismail, Abdul Hapid
Research Centre for Electrical Power and Mechatronics, Indonesian Institute of Sciences
Jl. Sangkuriang 21, Building 20, 2nd Floor, Bandung, West Java 40135, Indonesia
Received 14 May 2018; received in revised form 26 November 2018; accepted 28 November 2018
Published online 30 December 2018
Abstract
Cell imbalance always happens in the series-connected battery. Series-connected battery needs to be balanced to maintain
capacity and maximize the batteries lifespan. Cell balancing helps to distribute energy equally among battery cells. For active
cell balancing, the use of a DC-DC converter module for cell balancing is quite common to achieve high efficiency, reliability,
and high power density converter. This paper describes the implementation of a LiFePO4 battery charger based on the DC-DC
converter module used for cell balancing application. A constant current-constant voltage (CC-CV) controller for the charger,
which is a general charging method applied to the LiFePO4 battery, is presented for preventing overcharging when considering
the nonlinear property of a LiFePO4 battery. The prototype is made up with an input voltage of 43 V to 110 V and the maximum
output voltage of 3.75 V, allowing to charge a LiFePO4 battery cell and balancing the battery pack with many cells from 15 to
30 cells. The goal is to have a LiFePO4 battery charger with an approximate power of 40 W and the maximum output current of
10 A. Experimental results on a 160 AH LiFePO4 battery for some state of charge (SoC) shows that the maximum battery voltage
has been limited at 3.77 V, and maximum charging current could reach up to 10.64 A. The results show that the charger can
maintain battery voltage at the maximum reference voltage and avoid the LiFePO4 battery from overcharging.
©2018 Research Centre for Electrical Power and Mechatronics - Indonesian Institute of Sciences. This is an open access
article under the CC BY-NC-SA license (https://creativecommons.org/licenses/by-nc-sa/4.0/).
Keywords: cell balancing; constant current-constant voltage (CC-CV); DC-DC converter module; LiFePO4 battery.
I. Introduction
Recently, Li-ion batteries have been widely used in
different applications, such as portable electronic
devices and electric vehicles, due to their several
advantages of high energy density, low self-discharge,
long life cycles, and no memory effect [1][2]. The life
cycles of Li-ion batteries are affected by undercharging
or overcharge condition. It is because overcharge
would damage the physical component of the batteries,
and undercharge could reduce the energy capacity of
the batteries.
In electric vehicle application which requires high
power and energy, the LiFePO4 traction battery needs
to be connected in series or series-parallel in order to
increase its energy potential. This traction battery is the
most critical part of an electric vehicle because it affects
the driving range, and also the battery types mainly
influence the cost of the vehicle. Since the internal
impedance of each battery is not identical, a series-
connected battery needs to be balanced to maintain
their capacity. It become more difficult to charge when
the batteries are configured in a series. The battery pack
tends to imbalance after consecutive charge/discharge
process [3].
Cell imbalance always occurs in the series-
connected battery which leads in the degradation of an
individual cell. Furthermore, the capacity of the battery
pack will be reduced quickly and shorten the batteries
lifespan. A battery management system (BMS) to
observe LiFePO4 batteries is crucial for safety and
operational reasons. It avoids cell breakdowns caused
by undercharging or overcharging, keeps the balance of
the voltage among battery cells, each cell in safe
operating condition, and monitors the battery
temperature [4][5]. One of the most crucial features of
a BMS is cell balancing. Cell balancing helps to dispart
energy equally among battery cells.
* Corresponding Author. Tel: +62 823 1721 1215
E-mail address: amin_hwi@yahoo.co.id; amin@lipi.go.id
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82
Passive and active cell balancing are two types of
cell balancing method. The major distinction between
both methods is that passive cell balancing removing
the extra energy of the most charged cell through the
passive element (resistor) and the active cell balancing
is transferring the energy of the strong cell to the weak
cell. Active cell balancing methods work to reduce the
high energy losses observed in passive cell balancing
methods. According to the active element used for
storing the energy, active cell balancing has various
topologies namely capacitor based, inductive based,
and converters based [6]. For converter based active
cell balancing, the DC-DC converter module is
commonly used to achieve high efficiency, reliability,
and high power density converter. This DC-DC
converter module is used to charge every LiFePO4
battery in the battery pack to balance the battery pack.
In other words, the DC-DC converter module acts like
a LiFePO4 battery charger. In [6], DC-DC converter
modules were used for balancing among battery
modules through the auxiliary battery with maximum
current rating of 6 A. A method using a single equalizer
circuit that can be switched to a target cell via a set of
sealed relays was proposed in [7] and [8] for balancing
battery cells. In this method, a DC-DC converter
module was used for cell balancing with a maximum
current rating from 4 to 5 A. In [9] and [10], a DC-DC
converter module was used for balancing the selected
weak cell via a matrix of electronic relays with a
balance current close to 6 A.
The previous studies explained that the DC-DC
converter module could charge or balance the battery
with a maximum current rating of 6 A. In this work, the
DC-DC converter module was designed to charge or
balance LiFePO4 battery with maximum current rating
of 10 A. This bigger current rating is used to achieve
faster balancing time in the electric vehicle battery.
This study discusses the implementation of a
LiFePO4 battery charger for cell balancing application
with a maximum current rating of 10 A. To obtain a
clear and comprehensive analysis of the effect of 10 A
of current rating, the components value of the LiFePO4
battery charger circuit was calculated and followed by
efficiency calculation. In order to validate the charger
circuit, the charger prototype was build and followed
by an experimental test. The detailed analysis of the
voltage and current characteristics of the LiFePO4
battery with the different initial state of charge are
discussed as the focus of this study.
II. Materials and methods
In LiFePO4 batteries applications, a battery system
contains the battery and battery management system
(BMS). One important requirement for LiFePO4 BMS
is to observe the voltage across each cell when more
than one cell is connected in series to assure charge
equalization and voltage balancing of the cells. A
protection circuit is usually added to the charger circuit
to manage cells voltage. LiFePO4 batteries have very
crucial charging requirements that must be fulfilled
during charging to assure safe operating condition.
Battery charging plays an important role in the BMS,
where the charging method has a strong effect on
battery performance and life cycles.
A charger has three main functions: supplying
charge to the battery; optimizing the charge rate; and
stopping the charge [1]. The charge can be supplied to
the battery through a different charging method,
depending on the battery chemistry. For LiFePO4
batteries, a constant current-constant voltage (CC-CV)
charging method is very popular and commonly used in
charging LiFePO4 batteries because of implementation
easiness and simplicity [1][11][12]. In this paper, a
constant current-constant voltage (CC-CV) method
was used for preventing overcharging of the LiFePO4
battery.
This CC-CV method also the most widely adopted
charging method to develop a charger for a Li-ion
battery with some improvement methods. In [2] and
[13], Li-ion battery internal resistance compensation is
used in CC-CV based charger. Not only for low power
Li-ion charger, but the CC-CV method was also used
for high power Li-ion battery charger [14]. Other
improvement methods for CC-CV based charger are
using on-off duty cycle control zero computational
algorithms [15], and inductive power transmission with
temperature protection [16]. The CC-CV based charger
was also used in different charger topology, they are
LLC resonant converter [17] and PFC Sheppard Taylor
converter [18].
LiFePO4 batteries require a constant current (CC) to
charge the battery until the battery voltage achieves a
predefined safety limit (maximum charging voltage) at
which a constant voltage (CV) begins. Then, the
charging voltage is kept at a maximum charging
voltage, while simultaneously, the charging current is
exponentially reduced as shown in Figure 1. A constant
voltage (CV) charging is used to limit the current and
thus prevent the battery from overcharge. The charging
process ends when the charging current achieves a
small preset current. The charging curve of the CC-CV
charging method is shown in Figure 1 [11].
A. LiFePO4 battery charger circuit
The DC-DC converter module is voltage regulating
device and makes it feasible to utilize them as efficient
high-power current sources because of their wide trim
range. Current regulation of the DC-DC converter can
be performed through a current-sense resistor and the
addition of an external control loop. The isolated DC-
DC converter module circuit that is used as a LiFePO4
battery charger is shown in Figure 2 [19].
Figure 1. Charging curve of the CC-CV method
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83
From Figure 2, the output voltage of the DC-DC
converter can be adjusted by controlling the SC pin. In
order to meet the low-cost requirement, an analog
circuit was used to control the DC-DC converter
module. Figure 3 shows the detail of the LiFePO4
battery charger circuit based on an analog circuit [20].
The LiFePO4 battery charger was created based on
isolated DC-DC converter module with an input
voltage of 43 V to 110 V, allowing to charge a LiFePO4
battery cell and balancing the battery pack with many
cells from 15 to 30 cells. Thus, a LiFePO4 battery
charger with an approximate power of 40 W, and with
a maximum output current (Imax) of 10 A was obtained.
Table 1 summarizes the charger specifications.
Considering a LiFePO4 battery with a nominal
voltage of 3.2 V, the selected float voltage (Vfloat) is
3.75 V. The required maximum output voltage of the
DC-DC converter module (Vmax) is given as:
VVVV Ffloat 05.43.075.3
max
(1)
where VF is a voltage drop on the Schottky protection
rectifier D2.
According to [20], the components value of the
LiFePO4 battery charger were calculated and
summarized in Table 2.
B. LiFePO4 battery charger efficiency
Based on LiFePO4 battery charger circuit as shown
in Figure 3, the components with significant power
dissipation in this charger system are the DC-DC
converter module, the shunt resistor (PRshunt), and the
Schottky diode (PD2). In this derivation of an estimate
of LiFePO4 battery charger efficiency, other sources of
power dissipation will be neglected.
At the end of the constant current (CC) phase, the
output power to the battery (POUT) will be:
max max 4.05 10 40.5
OUT
P V I W
(2)
Figure 2. DC-DC converter module circuit
Figure 3. LiFePO4 battery charger circuit
Table 2.
Components value of the LiFePO4 battery charger
Component
Value
R1
10 kΩ
R2
357 Ω
R3
47 kΩ
R4
4.7 kΩ
R5
14.7 kΩ
R6
20 kΩ
R7
100 kΩ
R8
20 kΩ
R9
20 kΩ
R10
20 kΩ
R11
22 Ω
Rshunt
12.5 mΩ
C1
470 nF
C2
680 nF
C3
470 pF
D1
BAT85
D2
MBR1545
U1
LM10
U2
TL431
Table 1.
Charger specifications
Parameters
Values
Input voltage
43 to 110 V
Output voltage
4.05 V
No. li-ion cells
15-30 cells
Max output current
10 A
Max power
40 W
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84
The power dissipated on the Schottky diode D2
(PD2) and the shunt resistor (PRshunt) can be calculated
using Equation (3) and (4).
2 max 0.3 10 3
DF
P V I W
(3)
22
max 10 0.0125 1.25
Rshunt shunt
P I R W
(4)
Therefore, the output power from the DC-DC
converter module (PTOT) is:
W
PPPP RshuntDOUTTOT
75.4425.135.40
2
(5)
Considering a worst-case efficiency of 81.3% for
the DC-DC converter module [21], the input power of
the DC-DC converter module (PIN) will be:
W
Efficiency
P
PTOT
IN 043.55
813.0
75.44
(6)
The overall efficiency of the LiFePO4 battery
charger (EffTOT) is:
%58.737358.0
043.55
5.40
IN
OUT
TOT P
P
Eff
(7)
III. Results and discussions
A. Charger prototype
According to the LiFePO4 battery charger
specification described in section II, the circuit will be
implemented using Vicor Power V72C5E100BL DC-
DC converter module. A LiFePO4 battery charger
prototype has been implemented to validate the
effectiveness of the charger design. The prototype of
the LiFePO4 battery charger is shown in Figure 4 and
experimental test for charge a LiFePO4 battery has been
carried out as shown in Figure 5.
From Figure 4, the core of the charger circuit is an
LM10 (U1) operational amplifier and voltage reference.
A shunt regulator (U2) is adjusted to supply a voltage
of 2.5 V from the output of the DC-DC converter
module to the LM10. The op-amp (U1A) acts as an
error amplifier and is designed as an integrator using
capacitor C1 and resistor R4. Resistor R9 and R10 are
used to set the internal reference voltage at the non-
inverting op-amp input. Subsequently, the reference
voltage is compared with the current-sense resistor
(Rshunt) voltage to control the load current. The op-amp
(U1A) activates the Schottky diode (D1) cathode to
adjust the DC-DC converter module output. An initial
condition when a voltage is off can be done by fully
discharging the voltage across capacitor C1 using
resistor R3. The diode D1 with a low forward voltage
increased the output voltage of the DC-DC converter
module and used to avoid the op-amp (U1A) from
overdriving the SC pin. The resistor R1 adjust the float
voltage by decreasing the converter maximum output
voltage. The battery voltage (B1) rise to a constant float
voltage as the increases of battery state of charge. The
diode D2 connected series to the positive terminal of
the battery to isolate the output of the DC-DC converter
module in case of malfunction and avoid the battery
activates the circuit when the charger is off.
B. Experimental test
Figure 5 shows the experimental apparatus to test
the charger prototype. It consists of a power supply unit
as an input of the charger. This power supply unit is
used to replace the voltage of the battery pack in the
battery management system with a number of the cell
from 15 to 30 cells. A current sensor based on ACS712
was used to measure the current to the 160AH LiFePO4
battery. The charge current and voltage of the LiFePO4
battery was stored using LGR-5327 Datalogger to
know the behavior and effectiveness of the charger.
C. Testing result
The LiFePO4 battery charger was used to charge a
160AH LiFePO4 battery with a different initial voltage
that represents a different state of charge (SoC) to test
the CC-CV charging method and simulate the battery
balancing system. Figure 6, Figure 7, and Figure 8
present cell voltage and charging current for some
charging process with different initial voltage.
Experimental results show that at the beginning of the
Figure 4. LiFePO4 battery charger prototype
Amin et al. / Journal of Mechatronics, Electrical Power, and Vehicular Technology 9 (2018) 81–88
85
charging process, the LiFePO4 battery was charged
with certain current and decrease slowly. At the end of
the charging process (CV stage), the LiFePO4 battery
voltage was kept at the maximum reference voltage and
avoid the battery from overcharging.
In Figure 6, the initial voltage of the LiFePO4
battery was 2.8 V. At the beginning of the charging
process, the charge current was 10.64 A. The charge
current was decreased slowly, and at the time of 27
hours, the charge current was 0.78 A, and the LiFePO4
battery voltage was 3.64 V. At the end of the charging
process, the LiFePO4 battery voltage was maintained at
3.77 V.
In Figure 7, the starting voltage of the LiFePO4
battery was 3.27 V. In the start of the charging process,
the charge current was 7.54 A. The charge current was
also decreased slowly, and at the time of 21 hours, the
charge current was only 0.42 A and the LiFePO4 battery
Figure 6. Charging process with initial voltage of 2.8 V
Figure 7. Charging process with initial voltage of 3.27 V
2.5
2.7
2.9
3.1
3.3
3.5
3.7
3.9
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35 40 45 50
voltage (V)
current (A)
time (h)
Icharge (A) Vbat (V)
2.5
2.7
2.9
3.1
3.3
3.5
3.7
3.9
0
1
2
3
4
5
6
7
8
0 5 10 15 20 25 30 35 40 45 50
voltage (V)
current (A)
time (h)
Icharge (A) Vbat (V)
Figure 5. Experimental test
Amin et al. / Journal of Mechatronics, Electrical Power, and Vehicular Technology 9 (2018) 81–88
86
voltage was 3.66 V. At the end of the charging process,
the LiFePO4 battery voltage was maintained at
approximately 3.78 V.
In Figure 8, the initial voltage of the LiFePO4
battery was higher than the previous two experiments,
which was 3.36 V. At the beginning of the charging, the
charge current was 5.87 A. The charge current was
dropped slowly and at the time of 8 hours, the charge
current was only 0.6 A and the LiFePO4 battery voltage
was 3.66 V. At the end of the charging process, the
LiFePO4 battery voltage was maintained at 3.78 V.
Figure 9 shows the LiFePO4 battery voltage for
various charging process with different initial voltage
to simulate cell balancing. The experimental result
shows that the LiFePO4 batteries voltage can be
balanced at 3.77 V at the end of the charging process
and avoid the LiFePO4 batteries from overcharging.
In this experimental results, the DC-DC converter
module was able to charge the LiFePO4 battery with a
maximum current rating of 10.64 A. The charge current
is bigger than previous papers which only can charge
the battery with a maximum current rating of 6 A. This
bigger current rating is used to achieve faster balancing
time in the electric vehicle battery.
D. Constraints and future works
In this paper, the LiFePO4 battery charger was used
especially for battery balancing in the battery
management system. Further works could be focused
on verifying the effectiveness of the charger equipment.
The charger will need to be tested on the battery
management system to balance the battery pack with a
larger number of installed cells. The cells could be
varied from 15 to 30 cells.
IV. Conclusion
A 40 W LiFePO4 battery charger was successfully
designed based on DC-DC converter modules. The
charger was used to charge LiFePO4 battery cell and
made up with an input voltage of 43 V to 110 V.
Experimental results on a 160 AH LiFePO4 battery for
some state of charge shows that the maximum battery
voltage has been limited at 3.77 to 3.78 V, and the
maximum charging current could reach up to 10.64 A.
This result shows that the maximum battery voltage is
well regulated and the LiFePO4 battery charger can
keep the LiFePO4 battery voltage according to the
maximum reference voltage. This result shows that the
charger can regulate the maximum battery voltage. It
can be concluded that the charger can be used to charge
a LiFePO4 battery cell and balance the battery pack
with a number of cells from 15 to 30 cells depending on
the input voltage.
Figure 8. Charging process with initial voltage of 3.36 V
Figure 9. Charging process with different initial voltage
2.5
2.7
2.9
3.1
3.3
3.5
3.7
3.9
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30 35 40 45 50
voltage (V)
current (A)
time (h)
Icharge (A) Vbat (V)
2.5
2.7
2.9
3.1
3.3
3.5
3.7
3.9
0 5 10 15 20 25 30 35 40 45 50
voltage (V)
time (h)
Vbat1 Vbat2 Vbat3
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87
Acknowledgement
The Authors would like to thank all members of the
Electric Vehicle research group, Research Centre for
Electrical Power and Mechatronics which have already
supported the battery management system research and
development.
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