All energy-harvesting-based systems need energy storage for times when the energy cannot be harvested (e.g., at night for solar-powered systems). Rechargeable batteries ‒ known as “secondary” cells to differentiate them from “primary” or single-use cells ‒ are usually specified for this task.
For rechargeable batteries, however, battery management depends on the best possible measurement of what is known as the state-of-charge (SOC) of battery cells. For lithium-ion batteries, the characteristics of Li-ion cells complicate SOC measurement and can challenge engineers looking to maximize Li-ion battery lifetime. To simplify design of Li-ion battery management systems, engineers can leverage a variety of SOC measurement techniques supported in ICs from Atmel, Linear Technology, Maxim Integrated Products, STMicroelectronics, and Texas Instruments.
Battery SOC is defined as the percentage of charge remaining in a battery, and thus ranges from 0% to 100%. Because SOC measurement fills the same purpose as a gas gauge in an automobile, ICs that provide SOC measurement are typically called “gas gauge” or “fuel gauge” ICs.
SOC measurement underlines intelligent battery management systems. As SOC changes, battery management systems calculate optimum charging voltage and current values. Consequently, SOC measurement ICs are typically paired with battery charger ICs in designs or included as functionality within more comprehensive charge management and battery protection designs.
SOC measurements are used by the host system to manage power usage and by the application to notify users as battery charge becomes low. In electric vehicles, for example, SOC measurements are central to estimations of remaining range available to the vehicle and appear on the driver’s display panel as a familiar fuel gauge and range estimate. Indeed, automotive applications require reliable SOC measurements to reduce “range anxiety” as drivers begin to accept these new types of vehicles.
In fact, reliable SOC measurement is essential for ensuring safety and maximizing battery life in rechargeable batteries in general and for Li-ion cells in particular. Poor estimates of SOC can result in over-charging and over-discharging, resulting in diminished battery performance and lifetime. Worse, uncontrolled charging can even give rise to battery breakdown, thermal runaway and even uncontrolled venting and explosion.
For Li-ion cells, however, accurate measurement of SOC is difficult at best. Li-ion cells maintain a near-constant voltage output across much of their discharge range (Figure 1). As a result, the common method of simply relating voltage measurements to charge remaining in the battery cannot be used for these cells.
Figure 1: For a typical lithium-ion battery, voltage remains relatively flat across a wide range of the battery’s capacity, complicating conventional methods of reporting state-of-charge simply based on battery terminal voltage. (Courtesy of Infinite Power Solutions.)
As a result, determining SOC for Li-ion cells is largely a process of estimation and remains the subject of active research to find better methods for refining SOC estimates. For large, complex battery packs such as those used in electric vehicles, the need to maximize cost-efficiency of the battery requires very sophisticated SOC estimation methods based on neural networks, fuzzy logic, and adaptive filters. For many other energy harvesting applications, however, less complex methods based on current measurements, voltage measurements, or model-based methods provide adequate information about battery SOC needed for the application.
Current-based method
The current-based method tracks the change in charge remaining in the battery by measuring discharge and charge currents. In this method, called coulomb counting, the battery management system estimates SOC by calculating net increase and decrease in charge based on current measurement. Although this method is highly accurate in theory, the practical characteristics of circuits leave it prone to error, particularly over time. Uncertainties in current sensor accuracy, parasitic, and cell aging can introduce errors that accumulate over time, requiring periodic recalibration.
Because of this method’s simplicity and relative accuracy, engineers will find current-based SOC measurement supported in a wide range of ICs. For example, the Linear Technology LTC2941 and LTC2942 feature dedicated coulomb counting circuitry. The LTC2942 infers charge flow by integrating voltage measurements of battery current taken across a sense resistor.
Read More: Advanced ICs Simplify Accurate State-of-Charge Measurement for Lithium-Ion Batteries