2001-01-0959.pdf

2001-01-0959
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.
ABSTRACT
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 [8].
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) INTRODUCTION
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 REFERENCES
continuously, with the resistors turning on and off asrequired. The effectiveness of the dissipative technique [1] 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.
[2] 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 [3] 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 CONCLUSIONS
[4] 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 [5] 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 [6] 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.
[7] 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 [8] 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 [9] 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.

Source: http://baoxian.dianyuan.com/bbs/u/31/1120192376.pdf