Power Technologies: Balancing facts

High efficiency, bidirectional, multi-cell active balancer

Tony Armstrong, marketing director of power products at Analog Devices, and Sam Nork, director of the company’s Boston design centre, explain how to increase the run time in automotive battery stacks even as cells age

Large 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 vehicles (EVs or BEVs) and hybrid petrol-electric vehicles (HEVs and plug-in hybrid electric vehicles or PHEVs) to energy storage systems (ESSs).

The EV market is projected to create tremendous demand for large arrays of series and parallel connected battery cells. The 2016 global PHEV sales were 775,000 units, with a forecast of 1.13 million 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 hundreds of kilometres of driving range.

The high cost may be mitigated by the use of low cost and 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 technique to recover battery capacity in the pack that is quickly gaining momentum.

 

Balance

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. For example, a 10A/hr cell with 5A/hr of remaining capacity has a 50% SoC.

All battery cells must be kept within a SoC range to avoid damage or lifetime degradation. The allowable SoC minimum and maximum levels vary from application to application. Where battery run time is of primary importance, all cells may operate between a minimum SoC of 20% and a maximum of 100% (or a fully charged state). Applications that demand the longest battery lifetime may constrain the SoC range from 30% to 70%. These are typical SoC limits found in electric vehicles and grid storage systems, which use very large and expensive batteries with an extremely high replacement cost.

The primary role of the battery management system (BMS) is to monitor all cells in the stack carefully and ensure that none of the cells are charged or discharged beyond the minimum and maximum SoC limits of the application.

With a series-parallel array of cells, it is generally safe to assume the cells connected in parallel will autobalance with respect to each other. That is, over time, the state of charge 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 state of charge 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 rebalance SoC in a stack of cells with closely matched capacities.

Passive balancing is simple and inexpensive. However, passive balancing is also very slow, generates unwanted heat inside the battery pack and balances by reducing the remaining capacity in all cells to match the lowest SoC cell in the stack. Passive balancing also lacks the ability to address SoC errors effectively due to another common occurrence – capacity mismatch.

All cells lose capacity as they age and they tend to do so at different rates for the same reasons state of charge cells in a series tend to diverge over time. Since the stack current flows in and out of all series cells equally, the usable capacity of the stack is determined by the lowest capacity cell in the stack. Only active balancing methods can redistribute charge throughout the stack and compensate for lost capacity, due to mismatch from cell to cell.

 

Mismatch

Cell-to-cell mismatch in either capacity or SoC may severely reduce the usable battery stack capacity unless the cells are balanced. Increasing stack capacity requires that the cells are balanced both during stack charging, as well as stack discharging.

In one example, a ten-cell series stack comprised of nominal 100A/hr cells with a ±10% capacity error from the minimum capacity cell to the maximum is charged and discharged until predetermined SoC limits are reached. If SoC levels are constrained to between 30 and 70% and no balancing is performed, the usable stack capacity is reduced by 25% after a complete charge-discharge cycle relative to the theoretical usable capacity of the cells.

Passive balancing could theoretically equalise each cell’s SoC during the stack-charging phase, but could do nothing to prevent cell ten from reaching its 30% SoC level before the others during discharge. Even with passive balancing during stack charging, significant capacity is lost (not usable) during stack discharge. Only active balancing can achieve capacity recovery by redistributing charge from high SoC cells to low SoC cells during stack discharging.

The use of ideal active balancing enables 100% recovery of the lost capacity due to cell-to-cell mismatch. During steady state use when the stack is discharging from its 70% SoC fully recharged state, stored charge must in effect be taken from cell one (the highest capacity cell) and transferred to cell ten (the lowest capacity cell), otherwise cell ten reaches its 30% minimum SoC point before the rest of the cells and the stack discharging must stop to prevent further lifetime degradation.

Similarly, charge must be removed from cell ten and redistributed to cell one during the charging phase, otherwise cell ten reaches its 70% upper SoC limit first and the charging cycle must stop.

At some point over the operating life of a battery stack, variations in cell aging will inevitably create cell-to-cell capacity mismatch. Only active balancing can achieve capacity recovery by redistributing charge from high SoC cells to low SoC cells as needed. Achieving maximum battery stack capacity over the life of the battery stack requires active balancing to charge and discharge individual cells efficiently to maintain SoC balance throughout the stack.

 

Recovery

The device in the diagram can address the need for high performance active balancing. It is a high efficiency, bidirectional, active balance control IC that is a key piece of a high performance BMS system. Each IC can simultaneously balance up to six Li-ion or LiFePO4 cells connected in series.

SoC balance is achieved by redistributing charge between a selected cell and a substack of up to 12 or more adjacent cells. The balancing decisions and balancing algorithms must be handled by a separate monitoring device and system processor that controls the IC.

Charge is redistributed from a selected cell to a group of 12 or more neighbouring cells to discharge the cell. Similarly, charge is transferred to a selected cell from a group of 12 or more neighbour cells to charge the cell. All balancers may operate simultaneously, in either direction, to reduce stack balancing time.

Each balancer in the device 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 10A+, 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 processor and they will remain enabled until the BMS commands balancing to stop or a fault condition is detected.

 

Efficiency

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 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.

The IC achieves better than 90% efficiency in both the charging and discharging directions, which allows the balance current to be more than doubled relative to an 80% efficient setup with equal balancer power dissipation. Furthermore, higher balancer efficiency produces more effective charge redistribution, which, in turn, produces more effective capacity recovery and faster charging.

 

Conclusion

New applications such as EVs, PHEVs and ESSs are growing rapidly. The consumer expectation of a long operating life for batteries and reliable operation without performance loss remains unchanged. Automobiles, whether they be battery or petrol powered, are expected to last for over five years without any perceptible degradation in performance.

In the case of an EV or PHEV, performance equates to drivable range under battery power. EV and PHEV suppliers must provide not only high battery performance, but also a multiyear warranty that covers a minimum range to remain competitive.

As the number and age of electric vehicles continues to grow, irregular cell aging within the battery pack is emerging as a chronic problem and primary source of run-time reduction. The operating time of a series-connected battery is always limited by the lowest capacity cell in the stack. It only takes one weak cell to compromise the whole battery.

For the vehicle suppliers, replacing or refurbishing a battery under warranty due to insufficient range is a very expensive proposition. Preventing such a costly event can be accomplished by using larger, more expensive batteries for each and every cell, or by adopting a high performance active balancer to compensate for cell-to-cell capacity mismatch due to non-uniform aging of the cells. With such a device, a severely mismatched stack of cells has nearly the same run time as a perfectly matched stack of cells with the same average cell capacity.

Tony Armstrong is marketing director of power products at Analog Devices and Sam Nork is director of the company’s Boston design centre

www.analog.com

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