Page 19 - PEN eBook July 2023

Smart Energy

high-voltage systems, state of charge (SOC), state of health (SOH) and extreme high-current flow.
All of those monitored values are necessary for the tasks of a BMS. In principle, a BMS is suited to
maximize SOC, optimize SOH and protect the battery against deep discharge and overvoltage by

keeping the values inside the given window, as shown in Figure 1.

Over- and undervoltage protection (cell balancing)

In a multi-cell battery, the cell with the lowest charge determines the capacity of the entire system.
As shown in Figure 1, the battery will suffer irreversible damage if the voltage drops below or rises
higher than the threshold voltage for which the battery is designed. In case of a lower voltage, the
anode copper dissolves. In case of a higher voltage, lithium plating will occur, and if the voltage
rises even more, the cell will start outgassing and ignite.

Cell balancing is normally performed by an integrated circuit (IC) with high-precision analog-
to-digital converters. The main types of cell balancing are active and passive balancing. In
active balancing, a higher charge of a single cell can be transmitted to another single cell, while
in passive balancing, the charge is dissipated with the help of a resistor. The individual cell
controllers can perform specific, particularly energy-saving housekeeping functions, such as
periodic cell measurements and condition analysis necessary for functional safety, independent
of the main BMS controller. Safety functions for signaling over- or undervoltage are triggered

autonomously.

Over-discharge protection/low-voltage cutoff

Over-discharge protection, also known as low-voltage cutoff, is an important safety feature that
many, and normally all Li-ion, battery packs have. It is meant as a protection against a voltage drop
below a certain level.

The consequences of a deeply discharged battery are diverse, but in nearly all cases, it leads to
irreversible damage. For example, reduced life-cycle performance or even thermal runaway can

then lead to fire.

Hence, different cell chemistries have different safety operating areas. In general, we use the IC to
determine the safe operating range and provide the necessary protection for the cell/pack in the
application.

Short-circuit protection

Overcurrent protection is needed when a short-circuit occurs on the battery. This leads to extreme
discharge behavior; hence, there is a high-current flow, the battery heats up rapidly and a thermal
runaway event occurs.

There are three ways to protect the battery: thermal cutoff, pyro fuse and circuit breaker. BMS

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