A Review of Cell Equalization Methods for Lithium Ion and
Lithium Polymer Battery Systems
Stephen W. Moore
Peter J. Schneider
Copyright 2001 Society of Automotive Engineers, Inc.
Traditionally, cell-to-cell imbalances in lead-acid batterieshave been solved by controlled overcharging [6,7]. Lead-
Lithium-based battery technology offers performance
acid batteries can be brought into overcharge conditions
advantages over traditional battery technologies at the
without permanent cell damage, as the excess energy is
cost of increased monitoring and controls overhead.
released by gassing. This gassing mechanism is the
Multiple-cell Lead-Acid battery packs can be equalized by
natural method for balancing a series string of lead acid
a controlled overcharge, eliminating the need to
battery cells. Other chemistries, such as NiMH, exhibit
periodically adjust individual cells to match the rest of the
similar natural cell-to-cell balancing mechanisms .
pack. Lithium-based based batteries cannot be equalizedby an overcharge, so alternative methods are required.
Because a Lithium battery cannot be overcharged, there
This paper discusses several cell-balancing
is no natural mechanism for cell equalization. Therefore,
methodologies. Active cell balancing methods remove
an alternative method must be employed. This paper
charge from one or more high cells and deliver the charge
discusses three categories of cell balancing
to one or more low cells. Dissipative techniques find the
methodologies: charging methods, active methods, and
high cells in the pack, and remove excess energy through
a resistive element until their charges match the low cells.
This paper presents the theory of charge balancing
Cell balancing is necessary for highly transient lithium
techniques and the advantages and disadvantages of the
battery applications, especially those applications where
charging occurs frequently, such as regenerative brakingin electric vehicle (EV) or hybrid electric vehicle (HEV)
applications. Regenerative braking can cause problemsfor Lithium Ion batteries because the instantaneous
Lithium Ion and Lithium Polymer battery chemistries
regenerative braking current inrush can cause battery
cannot be overcharged without damaging active materials
voltage to increase suddenly, possibly over the electrolyte
[1-5]. The electrolyte breakdown voltage is precariously
close to the fully charged terminal voltage, typically in therange of 4.1 to 4.3 volts/cell. Therefore, careful monitoring
Deviations in cell behaviors generally occur because of
and controls must be implemented to avoid any single cell
two phenomenon: changes in internal impedance or cell
from experiencing an overvoltage due to excessive
capacity reduction due to aging. In either case, if one cell
in a battery pack experiences deviant cell behavior, thatcell becomes a likely candidate to overvoltage during high
Single lithium-based cells require monitoring so that cell
power charging events. Cells with reduced capacity or
voltage does not exceed predefined limits of the
high internal impedance tend to have large voltage swings
chemistry. Series connected lithium cells pose a more
when charging and discharging. For HEV applications, it
complex problem: each cell in the string must be
is necessary to cell balance lithium chemistry because of
monitored and controlled. Even though the pack voltage
may appear to be within acceptable limits, one cell of theseries string may be experiencing damaging voltage due
For EV applications, cell balancing is desirable to obtain
maximum usable capacity from the battery pack. Duringcharging, an out-of-balance cell may prematurelyapproach the end-of-charge voltage (typically 4.1 to 4.3
volts/cell) and trigger the charger to turn off. Cell
ACTIVE CELL BALANCING METHODS
balancing is useful to control the higher voltage cells untilthe rest of the cells can catch up. In this way, the
Active cell balancing methods employ an active charge-
charger is not turned off until the cells simultaneously
shuttling element or voltage or current converters to move
energy from one cell to another. These devices can beeither analog or digitally controlled. The two major
END-OF-CHARGE CELL BALANCING METHODS
classifications of active cell balancing methods are chargeshuttling and energy converting.
Typically, cell-balancing methods employed during and atend-of-charging are useful only for electric vehicle
purposes. This is because electric vehicle batteries aregenerally fully charged between each use cycle. Hybrid
Charge shuttling cell balancing mechanisms consist of a
electric vehicle batteries may or may not be maintained
device that removes charge from a selected cell, stores
fully charged, resulting in unpredictable end-of-charge
that charge, and then delivers it to another cell. There are
conditions to enact the balancing mechanism.
several embodiments of charge shuttling schemes, themost notable being a 'flying capacitor' (Figure 2).
Hybrid vehicle batteries also require both high powercharge (regenerative braking) and discharge (launch assist
or boost) capabilities. For this reason, their batteries are
usually maintained at a SOC that can discharge therequired power but still have enough headroom to acceptthe necessary regenerative power. To fully charge theHEV battery for cell balancing would diminish chargeacceptance capability (regenerative braking).
The charge-shunting cell balancing method selectively
shunts the charging current around each cell as theybecome fully charged (Figure 1). This method is most
Figure 2. Flying Capacitor Charge Shuttling Method
efficiently employed on systems with known charge rates.
The shunt resistor R is sized to shunt exactly the
The control electronics close the proper switches to
charging current I when the fully charged cell voltage V is
charge capacitor C across cell B1. Once the capacitor is
reached. If the charging current decreases, resistor R will
charged, the switches are opened. The switches are then
discharge the shunted cell. To avoid extremely large
closed to connect capacitor C across cell B2. The
power dissipations due to R, this method is best used
capacitor then delivers charge to B2 based on the
with stepped-current chargers with a small end-of-charge
differential of voltage between B1 and B2 (Eq.1).
The capacitor is then connected in the same manner
across B3, B4,…,Bn, B1,… The highest charged cells willcharge C and the lowest charged cells will take charge
from C. In this way, the charge of the most charged cellsare distributed to the least charged cells. The onlyelectronic controls needed for this method is a fixed
switching sequence to open and close the properswitches.
A variation on the 'flying capacitor' method is intelligently
Disadvantages of the charge shunting method are the
select which cells to balance. In this way, the capacitor
requirement for large power dissipating resistors, high
can be charged from the highest cell and selectively
current switches, and thermal management requirements.
discharged to the lowest cell. This method can
This method is best suited for systems that are charged
dramatically reduce the time to charge balance the cells,
especially if the highest and lowest charged cells are onthe opposite ends of the pack. Additional controls arenecessary to detect and select the target cells.
This method requires a large number of switches (n+5)
rated at the peak charging current for C. For a ideal
system (no ESR in the capacitor or switching losses) with
a very large cell imbalance (Bn = 3.0V, Bm = 4.0V), a flyingcapacitor could balance these cells at an initial rate of1Ahr per hour per 1000uF of capacitance switching at1kHz with an average switch current of 1A. Figuring in thecapacitor ESR and switching losses dramatically
increases the system's time constant for charging and
discharging, effectively reducing actual balancing currentby at least an order of magnitude and increasing the peak
switch current. The larger the capacitor used, the longerit will take to transfer a usable charge and the clock rate
will have to be decreased and the peak switch current will
increase. A large (100Ahr) battery pack would require acharge shuttling device with a very large capacitor with
Figure 4. Charge Shuttling with Several Cells
extremely large switch currents. A significant amount ofenergy is dissipated as resistive heating in the switches
Charge shuttling techniques are of limited usefulness for
and capacitor. A large portion of balancing is simply
HEV applications. Lithium chemistries offer a relatively
achieved by dissipating the charge from the higher
flat open cell terminal voltage across a broad range of
SOC from 40%-80% (Figure 5). A cell at a high SOCdoes not have a significantly large ? V from a low SOC
Another charge shuttling method shares a 'flying
cell, unless one of those cells are on a voltage 'knee' over
capacitor' for every two battery cells (Figure 3). The
90% SOC or below 20% SOC. HEV batteries operate in
capacitor constantly switches between the two cells,
the mid-SOC range, and this is where the cell-to-cell
thereby swapping charge from the higher charged cell to
voltage differentials are the smallest, thus limiting the
the lower charged cell. Each capacitor only needs simple
usefulness of charge shuttling techniques.
Figure 3. Charge Shuttling Between Two Cells
Figure 5. Open Cell Voltage of Lithium Polymer Battery
Several charge shuttling blocks can be cascaded forhigher voltage packs (Figure 4). Because cells B2….Bn-1
Charge shuttling techniques are useful for EV
share flying capacitors with their two neighboring cells,
applications. Because an EV can be routinely fully
charge can travel from one end of the cell string to the
charged, the voltage differential between a fully charged
other. This method would take a large amount of time to
cell and a lesser-charged cell is greater near the ends of
transport charge from high cells to low cells if they are on
the voltage curve (Figure 5). This increases the
the opposite ends of the pack because the charge would
have to travel through every cell with time and efficiencypenalties. This method has a packaging advantage: for
every two cells, the control circuitry, power supply andcapacitor can be packaged in a single unit powered from
Cell balancing utilizing energy conversion devices employ
the cells they are balancing. Units can be added as cell
inductors or transformers to move energy from a cell or
group of cells to another cell or group of cells. Two activeenergy converter methods are the switched transformerand the shared transformer.
The switched transformer method shares the sameswitching topology as the flying capacitor method (Figure
6). Current I is taken from the entire pack and is switched
secondary's rectifier. The balancing circuit would have to
into transformer T. The transformer output is rectified
be designed for the maximum expected number of cells;
through diode D and delivered into cell Bn, which is
additional secondary taps could not be easily added.
determined by the setting of switches S. Electroniccontrol is required to select the target cell and set
Several transformers can be used with the same result by
coupling the primary windings instead of coupling via asingle magnetic core (Figure 8). The benefit of thismethod is each cell can have its own magnetic core, thus
allowing additional cells to be added to the string withoutaltering the host controller.
This method can rapidly balance low cells at the cost of
The shared transformer method is suitable for both EV
removing energy from the entire pack. Disadvantages
and HEV applications. If current I1 is designed to be
include high complexity, high parts count in terms of
small (< 100mA/Ahr capacity), the device could operate
control, magnetics, and switches, and low efficiency due
continuously at a higher efficiency than any of the other
to switching losses and magnetics losses.
A shared transformer has a single magnetic core with
PASSIVE CELL BALANCING METHODS
secondary taps for each cell (Figure 7). Current I from thecell stack is switched into the transformer primary and
induces currents in each of the secondaries. Thesecondary with the least reactance (due to a low terminal
The dissipative method shunts selected cells with high
voltage on Bn) will have the most induced current. In this
value resistors to remove charge from the highest cells
way, each cell receives charging current inversely
until they match the charge of the lowest cells (Figure 9).
This circuit is the simplest and cheapest cell balancingimplementation. If the resistor value is chosen so that I is
small (<10mA/Ahr capacity), the physical resistor size
and switch rating can be small. A 10mA/Ahr resistorcould balance severely high cells at a rate of 1% per hour.
If operated continuously, such a technique could drain theentire battery pack in a few days.
The only active component in the shared transformer isthe switching transistor for the transformer primary. No
closed-loop controls are required. The shared transformercan rapidly balance a multicell pack with minimal losses.
Disadvantages of this cell balancing method includes
complex magnetics and high parts count due to each
The dissipative cell balancing method can be operated
continuously, with the resistors turning on and off asrequired. The effectiveness of the dissipative technique
 N. Furukawa et all, “EV 24h Travel Distance Record
can be improved by the application of adaptive and
Challenge,” the 17th International Electric Vehicle
Symposium (EVS-17), Montreal, Canada, 2000
The dissipative technique is suitable for HEV applications.
 Ph. Blanchard, D. Cesbron, G. Rigobert and G. Sarre,
Advantages are low cost and low complexity.
“PERFORMANCE OF SAFT LI-ION BATTERIES FOR
Disadvantages are high energy losses. For EV
ELECTRIC VEHICLES,” the 17th International Electric
applications, a 10mA/Ahr resistor could specify a 1A
Vehicle Symposium (EVS-17), Montreal, Canada, 2000
resistor current for a 100Ahr battery pack, meaning a 4Wresistor per lithium cell (4V/cell). Such large values could
 M. Okada, H. Yasuda, M. Yamachi, E. Yagasaki, and
result in a costly design with thermal management
S. Hashizume, “Porous Polymer Electrolyte Li Ion Battery
with Superior Performance,” Electric Vehicle Symposium16 (EVS16), 1999
 Segawa, M., S. Hitomi, H. Yasuda, M. Yamachi,
Electric vehicle applications can benefit from cell-
“Effects of Porous Polymer Electrolyte on Electrochemical
balancing devices, especially for lithium-based battery
Characteristics for LiNi1-xCoxO2/C System Lithium Ion
chemistries. Since battery pack charging is limited by
Cell for Electric Vehicles,” the 17th International Electric
any one single cell reaching its end-of-charge voltage (4.1
Vehicle Symposium (EVS-17), Montreal, Canada, 2000
V to 4.3 V), it is useful to control high voltage cells untilthe lower voltage cells catch up. This way, each cell can
 H. Horiba, K. Hironaka, T. Matsumura, T. Kai, M.
be charged to its end-of-charge voltage.
Koseki and Y. Muranaka, “Manganese Type Lithium IonBattery for PEV and HEV Use,” the 17th International
Several cell-balancing methods are suitable for EV
Electric Vehicle Symposium (EVS-17), Montreal, Canada,
applications. Charge shunting methods work well but are
limited by the amount of current that must be dissipated.
The shared transformer method is applicable, but it is
 Keyser, M., A. Pesaran, M. Mihalic, “Charging
costly in terms of magnetics and parts count. The
Algorithms for Increasing Lead Acid Battery Cycle Life for
dissipative method is applicable, and is the most cost
Electric Vehicles, “ the 17th International Electric Vehicle
effective. Charge shuttling methods would be prohibitively
Symposium (EVS-17), Montreal, Canada, 2000
expensive due to the switches required to handle the largepeak capacitor charging currents.
 E. Sexton, “Improved Charge Algorithms for ValveRegulated Lead Acid Batteries,” IEEE 00TH8490, in
Hybrid electric vehicle applications typically feature
Proceedings of the 15 th Annual Battery Conference on
regenerative braking, battery charging and electric
Applications and Advances,
Long Beach, California,
motoring. These features put high demands on the
battery pack for both charging and discharging. Thebattery pack is usually not kept in a fully charged
 Dennis Corrigan, et al., "Ovonic Nickel-Metal Hydride
condition; rather it is marginally charged, leaving room at
Electric Vehicle Batteries", the 12th International Electric
the top for charge acceptance. Thus, charge shunting is
Vehicle Symposium (EVS-12), Anaheim, CA, Dec., 1994
 W. Josefowitz, D. Macerata, H. Mettlach, D.
Since some HEV designs features battery packs
Porcellato, N. Farin, J. Hansson, “EV Energy Bench
significantly smaller than their EV counterparts, charge
Testing - European Testing Report,” the 17th International
shuttling methods become more attractive with smaller
Electric Vehicle Symposium (EVS-17), Montreal, Canada,
peak switch currents. However, the amount of energy
dissipated in capacitor ESR and switching losses maynot justify the increased complexity and expense. Thedissipative method is effective without the complexity andexpense. However, the algorithm development issignificantly more involved.
Curriculum Vitae Dr. Loay Ahmed Al-Momani Personal Details Date of birth: 1970 Place of birth: Al-Ramtha, Jordan Nationality: Jordanian Marital status: Married, have two daughters _________________________________________________________ Contact Information Working address Tafila Technical University Phone:+96232250326 Ex.1285 Department of Chemistry Fax:+96232250431 Jordan.
RSV – Recommended Literature Down Syndrome/Trisomy 21 • Aboussouan LS, O’Donova PB, Moodie DS, Gragg LA, Stoller JK Hypoplastic trachea in Down’s Syndrome. Am Rev Resp Dis, 1993; 147: 72-5. • Bloemers BL, Bont L, de Weger RA, Otto SA, Borghans JA, and Tesselaar K. Decreased Thymic Output Accounts for Decreased Naïve T Cell numbers in Children with Down Syndrome. J Im