Harnessing Power Technologies: Striking the Perfect Balance

Tony Armstrong, the marketing director of power products at Analog Devices, and Sam Nork, director of the company’s Boston design center, discuss how to extend the runtime of automotive battery stacks even as the cells get older.

Big battery stacks made of high-energy density, high-peak power lithium polymer or lithium-iron phosphate (LiFePO4) cells are common in electric vehicles (EVs and BEVs), hybrid petrol-electric vehicles (HEVs and PHEVs), and energy storage systems (ESSs).

The EV market is expected to drive significant demand for large arrays of series and parallel connected battery cells. In 2016, global PHEV sales reached 775,000 units, with a forecast of 1.13 million units for 2017. Despite the rising demand for high-capacity cells, battery prices remain stiff, often around $10,000 for those enabling a few hundred kilometers of driving range.

One way to lower costs is by using cheaper and refurbished cells, but these tend to have greater capacity discrepancies, which reduce the usable runtime or driving distance on a single charge. Even the more expensive, higher-quality cells will age and experience mismatches over time.

To boost the capacity of stacks with mismatched cells, you have two options: either start with larger batteries, which isn’t very cost-effective, or use active balancing, a rapidly growing technique to recover battery capacity in the pack.

Balancing Act

Cells in a battery stack are balanced when each has the same state of charge (SoC). SoC indicates the current remaining capacity of a cell relative to its maximum capacity during charge and discharge cycles. For example, a 10A/hr cell with 5A/hr remaining capacity has a 50% SoC.

Keeping all battery cells within a specific SoC range is crucial to avoid damage or reducing their lifespan. The minimum and maximum allowable SoC levels vary by application. For applications where run time is critical, cells might operate between 20% and 100% SoC. Applications that prioritize the longest battery life may restrict the SoC range from 30% to 70%. These typical SoC limits apply to electric vehicles and grid storage systems, both of which use very large and costly batteries that are expensive to replace.

A battery management system (BMS) is essential to monitor all cells in the stack to ensure none charge or discharge outside the SoC limits of the application.

In a series-parallel array of cells, it’s generally safe to assume that parallel-connected cells will naturally balance with each other, provided a conducting path exists between their terminals. Conversely, the SoC for series-connected cells tends to diverge over time due to factors such as temperature gradients, differences in impedance, self-discharge rates, or loading cell-to-cell.

Although charging and discharging currents may overshadow these variations, the mismatches will progressively worsen unless the cells are periodically balanced. Addressing gradual SoC changes from cell to cell is the most basic rationale for balancing series-connected batteries. Typically, a passive or dissipative balancing scheme is sufficient to rebalance SoC in a stack of cells with closely matched capacities.

Passive balancing is straightforward and low-cost, but it’s slow, generates unwanted heat inside the battery pack, and balances by reducing the…