Batteries & Fuel Cells


Maximise the run time in automotive battery stacks even as cells age


Samuel Nork, director, Boston Design Centre and Tony Armstrong, product marketing director, Power by Linear Group, Analog Devices Inc. talks about the battery requirements in electric vehicles


L


arge battery stacks consisting of series- connected, high energy density, high peak power Lithium polymer or


Lithium-Iron Phosphate (LiFePO4) cells are commonplace in applications ranging from all-electric (EV or BEV) and hybrid gas/electric vehicles (HEVs and plug-in hybrid electric vehicles or PHEVs) to energy storage systems (ESS). The electric vehicle market in particular is forecast to create tremendous demand for large arrays of series/parallel connected battery cells. The 2016 global PHEV sales were 775,000 units, with a forecast of 1,130,000 units for 2017. Despite the growing demand for high capacity cells, battery prices have remained quite high and represent the highest priced component in an EV or PHEV with prices typically in the $10,000 range for batteries capable of a few 100s of kilometres of driving range. The high


Figure 1: Typical cell balancing topologies


cost may be mitigated by the use of low cost/refurbished cells, but such cells will also have a greater capacity mismatch, which in turn reduces the usable run time, or drivable distance on a single charge. Even the higher cost, higher quality cells will age and mismatch with repeated use. Increasing stack capacity with mismatched cells can be done in two ways: either by starting with bigger batteries, which is not very cost effective, or by using active balancing, a new technique to recover battery capacity in the pack which is quickly gaining momentum.


All series-connected cells need to be balanced


The cells in a battery stack are balanced when every cell in the stack possesses the same state of charge (SoC). SoC refers to the current remaining capacity of an individual cell relative to its maximum capacity as the cell charges and discharges. All battery cells must be kept within a SoC range to avoid damage or lifetime


16 April 2018


degradation. The allowable SoC min and max levels vary from application to application. In applications where battery run time is of primary importance, all cells may operate between a min SoC of 20 per cent and a max of 100 per cent (or a fully charged state). Applications that demand the longest battery lifetime may constrain the SoC range from 30 per cent min to 70 per cent max. These are typical SoC limits found in electric vehicles and grid storage systems, which utilise very large and expensive batteries with an extremely high replacement cost. The primary role of the battery management system (BMS) is to carefully monitor all cells in the stack and ensure that none of the cells are charged or discharged beyond the min and max SoC limits of the application. With a series/parallel array of cells, it is generally safe to assume the cells connected in parallel will auto- balance with respect to each other. That is, over time, the SoC will automatically equalise between parallel connected cells as long as a conducting path exists between the cell terminals. It is also safe to assume that the SoC for cells connected in series will tend to diverge over time due to a number of factors. Gradual SoC changes may occur due to temperature gradients throughout the pack or


differences in impedance, self-discharge rates or loading cell to cell. Although the battery pack charging and discharging currents tend to dwarf these cell-to-cell variations, the accumulated mismatch will grow unabated unless the cells are periodically balanced. Compensating for gradual changes in SoC from cell to cell is the most basic reason for balancing series connected batteries. Typically, a passive or dissipative balancing scheme is adequate to re-balance SoC in a stack of cells with closely matched capacities. Cell-to-cell mismatch in either capacity or SoC may severely reduce the usable battery stack capacity unless the cells are balanced. Maximising stack capacity requires that the cells are balanced both during stack charging as well as stack discharging.


High efficiency bi-directional balancing provides highest capacity recovery The LTC3300-2 is a new product


Components in Electronics


Figure 2: Stack capacity loss example due to cell-to-cell mismatch


Figure 3: LTC3300-2 high efficiency bi-directional multicellular active balancer


designed specifically to address the need for high performance active balancing. The LTC3300-2 is a high efficiency, bi- directional active balance control IC that is a key piece of a high performance BMS system. Each IC can simultaneously balance up to 6 Li-Ion or LiFePO4 cells connected in series. Each balancer in the LTC3300-2 uses a non-isolated boundary mode synchronous flyback power stage to achieve high efficiency charging and discharging of each individual cell. Each of the six balancers requires its own transformer. The primary side of each transformer is connected across the cell to be balanced, and the secondary side is connected across 12 or more adjacent cells, including the cell to be balanced. The number of cells on the secondary side is limited only by the breakdown voltage of the external components. Cell charge and discharge currents are programmed by external sense resistors to values as high as 10+ amps with corresponding scaling of the


external switches and transformers. High efficiency is achieved through synchronous operation and the proper choice of components. Individual balancers are enabled via the BMS system processor and they will remain enabled until the BMS commands balancing to stop or a fault condition is detected.


Balancer efficiency matters! One of the biggest enemies faced by a battery pack is heat. High ambient temperatures rapidly degrade battery lifetime and performance. Unfortunately, in high current battery systems, the balancing currents must also be high in order to extend run times or to achieve fast charging of the pack. Poor balancer efficiency results in unwanted heat inside the battery system, and must be addressed by reducing the number of balancers that can run at a given time or through expensive thermal mitigation methods.


www.analog.com www.cieonline.co.uk


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