LTC3300IUK-2

TECHNICAL ARTICLE | Share on Twitter | Share on LinkedIn | Email Active Battery Cell Balancing Kevin Scott and Sam Nork Analog Devices, Inc. With passive and active cell balancing, each cell in the battery stack is monitored to maintain a healthy battery state of charge (SoC). This extends battery cycle life and provides an added layer of protection by preventing damage to a battery cell due to deep discharging because of overcharging. Passive balancing results in all battery cells having a similar SoC by simply dissipating excess charge in a bleed resistor; it does not, however, extend system run time.1 Active cell balancing is a more complex balancing technique that redistributes charge between battery cells during the charge and discharge cycles, thereby increasing system run time by increasing the total useable charge in the battery stack, decreasing charge time compared with passive balancing, and decreasing heat generated while balancing. Active Cell Balancing During Discharge The diagram in Figure 1 represents a typical battery stack with all cells starting at full capacity. In this example, full capacity is shown as 90% of charge because keeping a battery at or near its 100% capacity point for long periods of time degrades its lifetime faster. The 30% discharge represents being fully discharged to prevent deep discharge of the cells. Charged 90% Lower Capacity Cells Discharge Faster Charged 90% Discharged 30% Unused Capacity Figure 2. Mismatched discharge. It can be seen that even though there may be quite a bit of capacity left in several batteries, the weak batteries limit the run time of the system. A battery mismatch of 5% results in 5% of the capacity being unused. With large batteries, this can be an excessive amount of energy left unused. This becomes critical in remote systems and systems that are difficult to access. As a result, there is a portion of energy that cannot be used, which results in an increase in the number of battery charge and discharge cycles. Furthermore, this unused energy reduces the lifetime of the battery and leads to higher costs associated with more frequent battery replacement. With active balancing, charge is redistributed from the stronger cells to the weaker cells, resulting in a fully depleted battery stack profile. Discharged 30% Charged 90% Figure 1. Full capacity. Over time, some cells will become weaker than others, resulting in a discharge profile, as represented by Figure 2. Discharged 30% Figure 3. Full depletion with active balancing. Visit analog.com  2 Active Battery Cell Balancing Active Cell Balancing While Charging When charging the battery stack without balancing, the weak cells reach full capacity prior to the stronger batteries. Again, it is the weak cells that are the limiting factor; in this case they limit how much total charge our system can hold. The diagram in Figure 4 illustrates charging with this limitation. With active balancing charge redistribution during the charging cycle, the stack can reach its full capacity. Note that factors such as the percentage of time allotted for balancing and the effect of the selected balancing current on the balancing time are not discussed here, but are important considerations. Analog Devices Active Cell Balancers Analog Devices has a family of active cell balancers, with each device targeting different system requirements. The LT8584 is a 2.5 A discharge current, monolithic flyback converter used in conjunction with the LTC680x family of multichemistry battery cell monitors; charge can be redistributed from one cell to the top of the battery stack or to another battery cell or combination of cells within the stack. One LT8584 is used per battery cell. Lower Capacity Cells Charge Faster Charged 90% The LTC3300 is a standalone bidirectional flyback controller for lithium and LiFePO4 batteries that provides up to 10 A of balancing current. Since it is bidirectional, charge from any selected cell can be transferred at high efficiency to or from 12 or more adjacent cells. A single LTC3300 can balance up to six cells. Discharged 30% Figure 4. Charging without balancing. Module+ 2.5 A Average Discharge + Bat 12 • • Module+ Module– V+ Read Cell Parameters C12 S12 LT8584 Measurable Cell Parameters Enable Balancing 2.5 A Average Discharge + Bat 2 • • Module+ Module– LTC6804 Battery Stack Monitor Read Cell Parameters C2 S2 LT8584 2.5 A Average Discharge + Bat 1 • Module– Read Cell Parameters LT8584 Enable Balancing Module– Figure 5. 12 cell battery stack module with active balancing. ► Temperature plus RCONNECTOR Faults ► Switching ► Undervoltage ► Serial Faults Counting ► Coulomb Module+ • ► VREF ► RCABLE Extractable Cell Parameters Enable Balancing ► VCELL ► IDISCHARGE C1 S1 V+/C0  Visit analog.com Next Cell Above Charge Supply (ICHARGE 1 to 6) + Charge Return (IDISCHARGE 1 to 6) + • IDISCHARGE + Charge Return Cell 12 Serial Data Out 3 to LTC3300-1 3 Above LTC3300-1 Cell 7 Cell 6 • Charge Return 3 LTC3300-1 • + ICHARGE Conclusion Both active and passive cell balancing are effective ways to improve system health by monitoring and matching the SoC of each cell. Active cell balancing redistributes charge during the charging and discharging cycle, unlike passive cell balancing, which simply dissipates charge during the charge cycle. Thus, active cell balancing increases system run time and can increase charging efficiency. Active balancing requires a more complex, larger footprint solution; passive balancing is more cost effective. Whichever method works best in your application, Analog Devices offers solutions for both, which are integrated into our battery management ICs (such as the LTC6803 and LTC6804) and complementary devices that work in conjunction with these ICs to provide a precise, robust battery management system. References Cell 1 • 3 Serial Data In from LTC3300-1 Below 1 K evin Scott and Sam Nork. “Passive Battery Cell Balancing.” Analog Devices, Inc. Next Cell Below Figure 6. High efficiency bidirectional balancing. The LTC3305 is a standalone, lead acid battery balancer for up to four cells. It uses a fifth reservoir battery cell (Aux) and continuously places it in parallel with each of the other batteries (one at a time) to balance all battery cells (lead acid batteries are rugged and can handle this). 10 µF 25 V VREG CM En1 En2 Mode Term 1 Term 2 1 µF 6V 100 kΩ Each 10 nF 100 nF 10 nF CP Boost 10 µF 25 V Ngate 1 to 9 3.01 kΩ 10 µF 25 V 6.04 kΩ 10 µF 25 V 9 12.1 kΩ Aux P VH Aux N Ngate 1 Ngate 6 6.04 kΩ Ngate 7 6.04 kΩ PTC 27.4 kΩ Ngate 4 Ngate 2 V1 Ct Bat Ngate 5 Ngate 3 10 µF 25 V V2 LTC3305 Ct Off ISET 6.04 kΩ V3 VL 42.2 kΩ 6.04 kΩ 10 µF 25 V V4 UVFLT OVFLT DONE BAL PTCFLT BATX BATY Ct on Charger Supply 1.33 kΩ 249 Ω 10 µF 25 V + Aux 6.04 kΩ Ngate 8 Gnd 6.04 kΩ Ngate 9 Figure 7. Four battery balancer with programmed high and low battery voltage fronts. 6.04 kΩ ICHARGE + + + + Bat 4 Bat 3 Bat 2 Bat 1 Battery Stack Charger 3 About the Authors Kevin Scott works as a product marketing manager for the Power Products Group at Analog Devices, where he manages boost, buckAbout the Author boost and isolated converters, LED drivers, and linear regulators. He Thomas began career at Analog Devices, Inc., in creating Munich previouslyBrand worked as ahis senior strategic marketing engineer, in October 2015 as part of his master’s thesis. From May 2016 to technical training content, training sales engineers, and writing January 2017, he was part of a trainee programadvantages for field applicanumerous website articles about the technical of the tion engineers at Analog in February 2017, he company’s broad productDevices. offering.Afterward, He has been in the semiconducmoved into the role as field applications engineer. Within this role, tor industry for 26 years in applications, business management, and he is mainly responsible for large industrial customers. Furthermore, marketing roles. he specializes in the subject area of industrial Ethernet and supportsKevin graduated from Stanford University in 1987 with a B.S. in electrirelated matters in Central Europe. cal engineering and started his engineering career after a brief stint in He engineering at the University of Cooperative the studied NFL. Heelectrical can be reached at kevin.scott@analog.com. Education in Mosbach before completing his postgraduate studies (now Sam Nork joinedSales Linearwith Technology part of Devices) as a in International a master’s degree at Analog the University senior product engineer the company’s Milpitas, CA headquarters in of Applied Sciences in at Constance. He can be reached at 1988. In 1994, he relocated to the Boston area to start up and manage thomas.brand@analog.com. an analog IC design center where he continues to work today. Sam has personally designed and released numerous integrated circuits in the area of portable power management, and is inventor/co-inventor on seven issued patents. As director of ADI’s Boston Design Center, Sam leads a team of nearly 100 people and oversees day-to-day development activity for a wide variety of analog integrated circuits in areas including portable power management, high speed op amps, industrial ADCs, system monitors, and energy harvesting. Previously, Sam also worked for Analog Devices in Wilmington, MA as a product/ test development engineer. He received A.B. and B.E. degrees from Dartmouth College. He can be reached at samuel.nork@analog.com. Online Support Community Engage with the Analog Devices technology experts in our online support community. Ask your tough design questions, browse FAQs, or join a conversation. 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