Lithium-Ion Batteries. M.F.M. Elias, K.M. Nor, Senior Member IEEE. A.K.Arof. Department of ElectricalEngineering. Department of Physics. Faculty of Engineering ...
IEEE PEDS 2005
Design of Smart Charger for Series Lithium-Ion Batteries M.F.M. Elias, K.M. Nor, Senior Member IEEE Department of Electrical Engineering Faculty of Engineering, University of Malaya 50603 Kuala Lumpur, Malaysia Abstract-This paper presents the development of battery charger for charging series Li-ion batteries. It discusses the basic concepts of Li-ion battery charging such as the method of charging, cell balancing, charging control, monitoring, safety protection as well as the design constraints. The experimental results obtained are also presented. Keywords-Battery charger, Cell balancing, Charging control algorithm, Lithium-ion, Smart charger. I. INTRODUCTION In recent years, most of electronics products equipped with rechargeable Li-ion or Li-polymer batteries as the energy sources due to its high energy density and light weight. For certain applications, which consume less energy, they can be powered by using a single Li-ion battery for example mobile phones and PDAs. But, in some applications which require high energy and power, Li-ion batteries need to be stacked in both parallel and series as used in digital cameras, camcorders and notebook computers for instance. By doing so, in terms of charging the batteries it has becoming more difficult especially when the batteries are configured in a series string in order to increase its energy potential. Series-connected batteries need to be balanced while charging in order to preserve its capacity since intemal impedance of each battery is not purely identical [1-6]. Unlike those batteries which configured in parallel, they have no balancing issue because of their terminals are tightened together, giving the potential remains the same at all time. Besides cell balancing, Li-ion batteries must not be overcharged, hence require monitoring to be carried out during charging and thus need a controller. This paper concentrates on the implementation of Li-ion battery charger for charging series connected Li-ion batteries. The batteries used are 10 Li-ion batteries that has the capacity of 6Ah and maximum voltage of 4.2V each.
II.
DESIGN AND IMPLEMENTATION
A. Design Concepts The design of Li-ion battery charger as shown in Figure 1 consists of an 8-bit microcontroller, which connected to the constant-current/constant-voltage source, sensors, analog to digital converters, cell balancing circuit and liquid crystal display/personal computers. The input voltage for the charger
0-7803-9296-5/05/$20.00 © 2005 IEEE
A.K. Arof Department of Physics Faculty of Science, University of Malaya 50603 Kuala Lumpur, Malaysia
is fixed to 50V DC, which is taken from a buck converter. In this design, the whole operation of charging will be
controlled by using the microcontroller. This includes to start and stop charging, acquire charging current, individual cell voltage, control cell balancing, display charging status as well as to provide indication of errors.
B. Charging Method
The method of charging applied is based on constantcurrent/constant-voltage. In this method, initially constantcurrent is applied until the batteries have reached the maximum value of 42V. At this stage, the batteries have about 70% of its full capacity. In order to fully charge the batteries, constant-voltage need to be applied which equal to its maximum voltage while monitoring the charging current to drop below 0.1C or less [1]. Main DC
SPy
Sulp_
l LCD/PC
Fusel Relay
Currnt
CC/CV
ADC
.
Voltag U4on
Sensor
Micrcontroller
Switching Control
Balancing
Circuit
Figure 1. Block diagram of Li-ion battery charger
It should be noted that while charging the batteries to its maximum capacity, these batteries tend to be unbalanced and need to be balanced by using an appropriate cell balancing method, which will be discussed next. Constant-current charging is not applied continuously since actual voltage measurement of the individual battery is
needed from time to time in order to obtain the charging status. This makes the method merely identical to pulse charging method and takes the advantages of both methods. This helps in charging the batteries faster by applying higher charging current as well as reducing stress on the battery that can develop heat and release gaseous.
1485
In this design, the Li-ion batteries are charged at 0.5C by using the method mentioned earlier. The typical charging and discharging profile showing the relation between battery's voltage and current is shown in Figure 2 [2]. Depending on the types of Li-ion batteries, some of them would have a very large voltage range such as from 2.5V to 4.2V, but some of them just have 3.5V to 4.2V. The difference is due to the fabrication and assembly technologies as well as the quality of materials used. Vmax
OA
V min mm
Fok eI Cc
Charging Mode
As can be seen from Figure 4, each battery iS associated
with 2 power switches that have built-in parasitic diodes and 2 extemal diodes. The extemal diodes are used to prevent free-
wheeling current flowing from higher to lower voltage potential which can cause short-circuit between the batteries' terminal.
Current, Voltage vs. rime
Ima
charging will be stopped. Cell balancing will be done by using dissipative resistor. Another advantage of this circuit configuration is that cell balancing is not limited to one battery at one time but it can be more with suitable resistor.
Having both methods that share the same
circuit creates difficulties in determining the proper value for the shunting or dissipating resistor even if the intemal impedance of Li-ion batteries is known. Moreover, the actual Li-ion battery model is not available and series configuration of resistor and capacitor only cannot represent its actual characteristics. Therefore, experimental approach is used to determine the appropriate resistor value. This is shown in Figure 5.
Cv
Charging Mode Time
CC
Res
Discharging Mode
Figure 2. Typical Li-ion battery charge and discharge profile
Constan
Cntro
Buck~ ~~~~~~~ _1_ Figure 3. Block diagram of constant-current/constant-voltage souarce
Figure 3 above shows the design of constant-
current/constant-voltage source. The input to the constantcurrent/constant-voltage source is taken from buck converter
I
,
~'
thing
Switchmg
Figulre4. Charge shunting/dissipative reststor balancing method output is executed by using microcontroller. In the case of constant-current charging mode, microcontroller periodically From Figure 5, Ic is the constant charging current, 'b is the tumns on an off the constant-current circuit in order to enable current flowing into the battery through an intemnal impedance, voltage measurement to be made.Control~~~~~~~~~~~~~~~~~~~~~~~~oto This is also same for the Rb and Is is the current flowing through the case of constant-voltage charging mode. shunting/dissipating resistor, Rs. Vb and Vd are battery's voltage and diode forward drop voltage respectively whereas C. Cell Balancing R¢(0c) is the on-state resistance of the power switch. There are several cell balancing methods that can be used which are charge shuttling, charge shunting, dissipative b Rds(on)resistor and energy converter [3]. All of them have their own_, advantages and disadvantages, which suit different ', CVd applications. In this design, the combination of charge Ic 4 l I Switching shunting and dissipative resistor is used. Figure 4 shows Re VrlCtir cI J charge shunting and dissipative resistor cell balancing Vd _! methods sharing the same configuration circuit. With charge iBatte cell balancing canbe done while constant-current votg1n4doefrar86pvlag Rds(on)- epcivl hra shunting, charging. However, in certain case where the batteries are still 5. Ehpri e shunting/dissipati resistor ng unbalanced at the end of charging, therefore constant-current Figureto value 1486 batte ofcostn-vltg mode. shunting/dissipating resistor
having 50V output. The switching control for selecting the
Relation between Ic, Ib and Is is given by the following equation; I
=I+Is(1)balancing
= 'b + I~ (1) Since the voltage across the branches is same, therefore; Vb + IbRb = 2Vd + 2IsRds(on) + IS RS (2)
This equation can be used to determine the value of Rs if Rb is known while other parameters are available and measurable. In this design, it is intended that at maximum voltage of about 4.2V, all charging current, Ic is diverted to the shunting/dissipating resistor in order to provide faster cell balancing. Consequently, the term Ib becomes zero, by setting Vb = 4.2V, Is = Ic and substituting the value of Vd and Rds(on) therefore the value of Rs can be obtained and validated experimentally. Another reason of choosing Vb = 4.2V is to avoid the battery from being discharged if cell balancing is activated at this voltage level. If lower value of Vb is chosen the value of Rs will be smaller and cause the voltage drop is lower as compared to the battery's voltage. Hence, when cell balancing is activated, current will be drawn out of the unbalanced battery. The effectiveness of cell balancing during charging increases towards its maximum voltage. This means that more current will be diverted to the resistor instead of charging the unbalanced battery. In the case where charge shunting is no more effective, dissipative resistor method is used instead. The instantaneous discharge current can be determine by setting I, =0, and solve for I, by using the equation (2).
kc Charging/
Charging/
I Balancing time _l
t cl
t-tr Rest time 1-
Balancing time
t c2
and the lowest voltage of the batteries. If the difference is large, the balancing time will be short in order to enable faster since the batteries' voltages are monitored more frequently. If the difference is small, the balancing period will be long. However, if the voltage difference is small and within the allowable range, cell balancing will not be executed. In this case, all the batteries will be charged at the same rate.
tcl AV2> AV3
AL
p
P1 AVI
0
P3
c, AV2
AV3
-
Figure 7. Relation between voltage difference and charging/balancing time
For better assignment of the charging/balancing time, wellknown controllers such as PID or fuzzy logic can be used. But, implementation of this controller requires extra processing time and more powerful DSP microcontroller since it has to perform some mathematical calculations. D. Charging Control Algorithm In order to ensure that the batteries are fully charged and balanced at the end of charging, therefore charging control algorithm is required. The algorithm is implemented by using an 8-bit microcontroller. In this design, PIC16F877 from Microchip Technology Inc. is used and the program is developed by using MPLAB software with PIC Basic Pro compiler from microEngineering Labs. Figure 8(a) and 8(b) show the flowchart of the charging control algorithm implemented in this design. Initially, once the microcontroller has been powered up it will connect to the batteries through relays and then followed by measuring the total voltage. If the batteries are fully charged, it will be disconnected and the charging is terminated. However, if the batteries is not fully charged, voltage and temperature of each batteries will be measured and compared with the normal where the values are out of range, operating range. In the case ihaloeain tpe.Tems error will be indicated with all operations stopped The most unbalanced battery will be determined and the balancing will be decided. After that, constant-current will be actlon applied for one charging time. During the charging time, the amount of current delivered to the battery is measured and compared If the value is abnormal, all operation will be terminated immediately, otherwise it will be continued. After charging delay completed and before the charging current is removed, the data is sent to LCD or PC for the purpose of display and real-time monitoring. Once charging current is removed, the delay or rest time for allowing the batteries to become stable is started. Next, after completed the individual batteries is measured again and the same procedure repeated. Iere I h aeweeoeo oebteishv ece t maximum voltage, but the voltage difference exceeded preset ero
1487
wilb.niae
value, charging will be stopped and the unbalanced batteries will be discharged until the difference between the highest and the lowest voltage lies within allowable range. START
Select battery
oduleNo
Masurtot l STOP
Yea
Fully x
|Stat CC charging
M charging 1 FIIeasur e
indicate onnalNo ErrorYes Ye_Fully
No 0
