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332 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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4
NCA
LFP
3.5
Voltage (V) 3

2.5
0 20 40 60 80 100
DOD (%)

FIGURE 13.14 Open-circuit voltage (OCV) versus DOD for NCA and LFP types of Li-ion batteries.

The extended Kalman filter (EKF) is widely used for estimating SOC. The EKF approach is
highly sensitive to the accuracy of the battery model and parameter values, and therefore, special
care should be taken in order to avoid significant error and divergence [43, 44]. To reduce the sen-
sitivity to the model parameters, an adaptive EKF was proposed in [45]. Several other methods,
including robust Η ∞ and sliding mode observers, and support vector machine techniques have been
employed to estimate the SOC of batteries [46–48].

13.4.3 State of Health (SOH)

The SOH has several definitions, such as the maximum charge that can be released after the bat-
tery has been fully charged [49], or the battery’s capacity of storing energy and preserving charge
for long periods [50, 51], or the remaining battery capacity for the current cycle as compared to
the original battery capacity [52]. The SOH can also be defined as a set of indicators or diagnostic
flags, which reflect the health status and physical condition of the battery, such as loss of rated
capacity [53].
The value of SOH is beneficial for applications like HEV and EV, where it is used as an indica-
tion of specified power or to estimate the driving range. Similar to the SOC problem, several tech-
niques have been developed for SOH estimation, including EKF [53], adaptive observer [47], and
probabilistic neural networks [49]. Measuring the internal equivalent DC resistance of a cell, which
increases with capacity degradation, is another characterization tool for SOH [52].

13.4.4 State of Life (SOL)
The SOL is defined as the remaining useful life (RUL) of a battery or as the time when a battery
should be replaced [39]. This indicator is considered from the design stage in order to plan ahead
maintenance and replacement schedules, prevent failures during operation, and increase the reliabil-
ity and availability of an ESS. Several methods have been published for RUL estimation [54–56].

13.4.5 Cell Balancing Systems
In order to achieve higher voltage and current, a battery pack consists of several cells, which are
connected in series and parallel layouts, respectively. The cells in a string could have different SOC
levels due to several internal and external sources of unbalancing, which may result in different
capacity fading rates between cells. Internal imbalances include different self-discharging resistance
and impedance, and external causes may include thermal variation across the string [57, 58]. During
charging or discharging, imbalances in between cells may lead to extreme voltages and hence severe