IEEE Power & Energy Magazine - September/October 2017 - 28

Charge/Discharge
Control

Driver Interface

State Monitor
e-

SOC, SOH, SOP
Prediction

+
Li-
Charge

Database
Current

Li+
Graphite

Voltage

Discharge

Temperature

Li+ Conducting
Electrolyte
Li+
Migration
Path

Cell
Balancing

Data Acquisition

Thermal
Management

Safety
Protection

figure 6. The main functions of BMSs. [Modified based on E. Aleksandrova, "Lithium-ion batteries for electric cars:
Opportunities and challenges," presentation from Honda R&D Europe (Deutschland) GmbH, 2010.]

SOC/SOP/SOH Estimation

Cell Balancing

SOC is an indicator of the available charge stored in a battery. Accurate measurement of SOC is necessary not only for
charge/discharge safety and battery health but also for systemlevel energy management in a smart grid. SOH is related to
battery aging status and remaining lifetime. This information
is crucial for a BMS to perform battery lifetime prolongation,
fault diagnosis, and replacement of certain cells in a pack. SOP
denotes the peak charge and discharge power a battery can
sustain instantaneously or within a time horizon of interest.
None of these states is directly measurable using current sensors, leading to the emergence of various estimation algorithms.
The sophistication of a state estimation algorithm depends to
a large extent on the battery model employed. Open-loop and
model-free methods are often vulnerable to pitfalls such as unknown disturbances, measurement noise, initial deviation, slow
convergence, and parameter uncertainties. Model-based approaches are more promising, particularly those reflecting battery electrochemical dynamics. Model-based approaches ensure
a closed-loop feedback mechanism, high estimation accuracy,
and resiliency under changing operational conditions.

For a battery pack composed of a number of battery cells,
even though they are all of the same type and specification,
there may exist cell variances that increase with battery usage. To circumvent this bucket effect, it is desirable to safeguard cell inconsistency by deliberate balancing of SOC,
voltage, and temperature. As for SOC/voltage equalization,
passive and active solutions are being developed with energy
dissipation and nondissipation, respectively. The passive scheme
consumes the extra capacity of individual cells through external circuits, whereas the active scheme transfers energy from
one "rich" cell to another "poor" cell. For temperature equalization, advanced battery thermal management is required,
which features high-fidelity characterization of heat generation and transfer, pack-structure optimization, and efficient
thermal controls, e.g., a hybrid air/liquid/heat pipe cooling.

ieee power & energy magazine

Charging Control
Battery charging must be carefully manipulated to guarantee
charge acceptance within a prespecified charge duration while
also meeting safety requirements. Typically, different batteries
september/october 2017

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2017