battery cell balancing for improved performance in evs.docx

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Battery Cell Balancing for Improved

Performance in EVs - Part II: Active

Balancing TechnologiesPor Lee H. Goldberg

Colaboracin de Hearst Electronic Products

12/07/2011

Techniques that equalize the charge/discharge characteristics of a batterys individual cells areessential for extending the range and service life of electric vehicles and many portable

electronic products. In fact, the passive cell balancing technologies discussed inPart I of thisseriesare already part of the protection and management systems in most of the 5 kWh-20 kWhbatteries used by todays hybrid-electric and plug-in-hybrids (PHEVs). These smart passivebalancing systems use impedance tracking, coulomb counting, and other state-of-chargemonitoring techniques (Figure 1).

Figure 1: Battery management plays a critical role in modern EV propulsion systems. (Courtesyof Maxim Integrated Circuits).

Since even these advanced passive balancing systems allow cells with higher capacity to fullycharge by repeatedly bleeding off the energy in weaker cells, they can only unlock a portion of abatterys stranded capacity. As a result, there is a lively debate about whether the next

generation of pure EVs can tolerate the lost range and lost charge/discharge cycles if their 25-100 kWh batteries are passively balanced.

At least in theory, the better alternative is an active cell balancing system which re-distributes the
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charge from stronger cells to prop up the weaker cells within a battery stack. But, whileelectronics manufacturers are promoting the virtues of their early active cell balancingtechnologies, many battery and electric vehicle manufacturers have concerns about whetheractive systems added cost and complexity are worth the extended range and operational lifetimethey provide.

In this article, we will take a closer look at active cell balancing techniques and where they mayplay a role in the next generation of electric vehicles and other high-capacity storage applications.

Why active cell balancing?

As discussed inPart I,passive (top) balancing prevents cells with lower capacities fromlimiting the charge that cells with larger capacities can accept. This is accomplished (Figure 2)using a bleed resistor to unload over-charged cells so their output voltage falls below thechargers voltage regulation point, so the remainder of the stack can continue charging. To guard

against damage, individual cell conditions must be sampled frequently, with laptop batteries

typically monitored at 4-10 samples per second (sps) and EV/HEV batteries from 20-100 sps.

Figure 2: Passive cell balancing can be implemented using general-purpose components (Figure

2a) or an application-specific IC (2b). (Courtesy of Infineon and Analog Devices).

While passive top balancing eliminates the risk of catastrophic cell failure and provides someimprovement in run time and service life, it effectively reduces a battery stacks overall current

capacity to that of the weakest cell. Since passive balancing is normally done only when thebattery is charging, it cannot fix the imbalances that develop during operation (due to internalimpedance and self-discharge) which further diminish capacity.

The alternative is active balancing techniques which transfer a stronger cells excess charge toone or more cells which need it. Most active systems use switching MOSFETs similar to thoseused in passive mechanisms except that they substitute inductors for the bleed resistors whichserve as the secondary side of a transformer whose primary sits across the entire battery stack(Figure 3).
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Figure 3: Inductively-coupled active cell balancing using an MCU to monitor cell voltage andcontrol the inductively-coupled charge pumps. A filter circuit on the MCUs A/D input allows

measurement of cell voltages from the primary side of the charge pumps transformer. (Courtesy

of Infineon).

Active top balancing (Figure 4) is accomplished by momentarily connecting the cell with thehigher voltage to its secondary winding of the balancing circuit, creating an induced voltage onthe primary winding. The donor cells switch is then opened and the switch on an acceptorcell is closed, allowing the primarys energy to be drivenback into its secondary winding. Thistechnique allows energy to be transferred between cells during charge, standby, or discharge

with efficiencies in the neighborhood of 85 percent.


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Figure 4: Top balancing using an active magnetic switching circuit (4a) is performed by

energizing one of the transformers secondary windings to induce currents in its primary

winding (4b). (Courtesy of Infineon).

Active inductive balancing can also used to perform bottom balancing which allows stronger

cells to share their charge with weaker cells. Bottom balancing can be done at any time but isusually done during the discharge cycle as cells with less capacity approach their maximumdischarge limits. In this case, the primary winding is energized with a pulse of the battery stackvoltage with the secondary switches on all of the cells open. Once the primary is energized, theconnection to the secondary winding for the cell to be charged is closed, allowing the storedenergy to be transferred (Figure 5). Bottom balancing unlocks energy that would have otherwisebeen stranded inside a battery.

Figure 5: Bottom balancing using an active magnetic switching circuit (5a) is performed by

energizing the transformers primary side to induce currents in one of the its secondary windings.

(5b). (Courtesy of Infineon).

Most active balancing designs employ variations of the technique shown above, but TexasInstruments has developed an alternative architecture they refer to asPowerPump.Instead of theclassical one-to-many architecture described earlier, PowerPump shuttles charges betweenadjacent cells using a simple charge-coupled switch (Figure 6). Operating at roughly 200 kHz,

the circuit can switch Q1 to push current from the top cell to the bottom cell through the bodydiode of Q2. Likewise, charge can be shuttled from the lower to the upper cell by applying theswitching waveform to Q2. Since the losses involved are relatively low, its practical for a

PowerPump circuit to boost a non-adjacent cell by passing the excess charge across several othercells to where its needed in a bucket brigade fashion.
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Figure 6: Texas Instruments PowerPump cell balancing technology uses a simple charge

shuttling scheme to transfer energy between adjacent cells.

The TIbq78PL114master gateway battery controller, for example, is part of a complete Li-Ioncontrol, monitoring, and safety solution designed for large series cell strings. Texas instruments

also has recently introduced an automotive-grade active balancing solution based on PowerPumptechnology which will be discussed in the following section.

Implementation strategies

All battery balancing techniques must work within the framework of the battery packs otherbattery management and protection functions. In most automotive designs, the software for cellbalancing algorithms and control functions will be run on an automotive-qualified host MCU,typically located within the battery management system (BMS) itself (Figure 7). The BMS MCUcan often use the same electronics used to determine cell voltage, charge/discharge current, andstate of charge (SOC) for the batterys fuel gauge and charge management systems to perform

similar measurements required for its cell balancing operations. As discussed in Part I of thisseries, a battery cells SOC can be determined using precise voltage measurements or, if greater

accuracy is desired, combined with coulomb counting, or some other technique which measuresthe total current flowing in and out of the cell. In either scenario, the cell voltage measurementrequires an A/D converter with 12 to 14-bits of resolution.
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Figure 7: A block diagram of a typical battery management system. (Courtesy of Texas

Instruments).

At present, most manufacturers do not offer complete integrated solutions for active cellbalancing, but they do offer off-the shelf products that handle necessary A/D, multiplexing, levelshifting, and communication functions that can reduce their parts count, module size, and BOM

cost.

For example, Maxims offers a 12-Channel, high-voltage battery monitor developed specificallyfor Lithium-based automotive storage systems (Fig.8a) combining a 12-bit SAR-type A/D, ahigh-voltage switch bank input which is controlled by a simple state machine. It is equipped witha high-speed I2C bus for SMBus-laddered serial communication. Rather than rely on the hostprocessor for fault protection, a companion chip, theMAX11080,is used to provide aninstantaneous over-voltage or under-voltage fault indication when any of the cells cross the user-selectable threshold for longer than the set program-delay interval (Figure 8b). At present,Maxims product line only offers passive balancing solutions but these devices are equally usefulas the measurement and protection elements of an active solution.
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Figure 8: An integrated battery cell monitoring and protection solution, capable of supporting

up to 12 Li-Ion cells.

An active balancing circuit also can be implemented using an addressable driver that allows thehost MCU to control a series of power MOSFETS that serve as the switches on the balancing

transformers primary and secondary legs. MOSFET power devices with fast switchingcharacteristics and low Rdsonlike InfineonsOptiMOS series,MicrosemisCoolMOS devicesandFairchildsPowerTrench integrated FET+Schottkyproducts have the speed, current capacity,and low switching losses necessary for efficient charge transfer between cells.

If active cell balancing technology finds acceptance in mainstream applications, it is likely thatmost IC manufacturers involved with battery management will offer more highly-integratedproducts that support an active balancing scheme. But, for the moment, the only company tooffer an automotive-grade battery management IC with active cell balancing capabilities is TexasInstruments. Thebq76PL536-Q1(Figure 9), based on TIs PowerPump charge-shuttlingtechnology, provides battery monitoring, electrical/thermal protection and active balancing for

up to six cells. Its high-speed (SPI) bus allows the devices to be stacked vertically, providingreliable communications across a high-voltage battery cell stack of up to 192 cells withoutadditional isolation.
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Figure 9: A block diagram of Texas Instruments bq76PL536-Q1, an automotive-grade battery

monitor/protection device with integrated active cell balancing capabilities. (Courtesy of TexasInstruments).

The bq76PL536-Q1 has a second I2C bus that is used to communicate with a host MCU whichprograms and controls its on-chip monitoring and balancing functions. Fault (secondary)

protection for over-voltage, under-voltage, and over-temperature conditions are detected usingcomparators, so they are independent of the ADC system and host controller to insure rapid,deterministic response. All protection thresholds and detection delay times can be programmedvia the I2C host interface and stored in an internal EPROM.

Conclusions

Highly-integrated solutions such as TIs bq76PL536-Q1 will help narrow the cost differential


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between active and passive cell balancing systems. But with the low cost of some passivesolutions, it is unclear whether the added run time and battery life afforded by active technologywill ever outweigh the added cost and complexity it brings to an EVs BMS. At least part of theanswer to this question lies how much improvement manufacturers can deliver in the quality anduniformity of their automotive batteries. If the capacity and impedance variations of todays cells

(typically 2 percent-3 percent) can be further narrowed through advances in manufacturingprocesses, battery chemistry, and nano-materials, balancing all but the largest EV batteries couldbe performed with minimal losses using simple, low-cost passive systems. A recent studyconducted by Maxim Semiconductor calculated the value of the 10 percent-12 percent extracharge/discharge cycles that active balancing can add to an automotive battery pack. Assumingthe battery pack cost $5000, active balancing unlocks around $500 worth of added value,although it is spread out over the vehicles 8.5 year service life. If this study is correct,

manufacturers may have a difficult time justifying the $70/yr in savings unless the added cost ofactive balancing is relatively low and it also gives the vehicle a tangible range boost (5 percent-10 percent).

On the other hand, proponents of active balancing technologies say that once highly-integratedsolutions arrive, they will cost less to implement than todays passive systems. These costsavings would mostly be due to eliminating the costly high-power resistors, thermal managementcomponents, and the high-current wiring required in the 100+ kWh battery packs that will powerthe next generation of EVs.

While its still too early to determine whether active cell balancing will become the de facto

standard for EV/HEV battery management systems, its a sure bet that it will be the technologyof choice in high-performance vehicles and other applications where run time, energy efficiencyand service life are essential.

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