International Journal of Machine Learning and Cybernetics
1 3
namely, Ada-TargetSOH and Sta-TargetSOH, for robust pre-
diction of lithium-ion battery SOH. To evaluate the efficacy
of the two proposed predictors, we implemented Ada-Tar-
getSOH and Sta-TargetSOH on three types of lithium-ion
battery datasets. The experimental results have shown that
our predictors outperform other existing lithium-ion battery
SOH predictors.
2 Materials andmethods
2.1 Benchmark datasets
The lithium-ion battery experimental data that were used
to evaluate the proposed methods in this research were
obtained from National Aeronautics and Space Adminis-
tration (NASA) Ames Prognostics Center of Excellence
(PCoE) [52]. There were various kinds of lithium-ion battery
datasets in the NASA database, and we chose three types of
lithium-ion batteries (005, 006, and 007) as benchmarks.
For each type, three different operational profiles, namely,
charge process, discharge process and impedance measure-
ment process, were involved as following: firstly, charge was
performed at a constant current level of 1.5A until the bat-
tery voltage reached 4.2V and then continued in a constant
voltage level until the charge current fell to 20mA; next, dis-
charge was carried out in a constant current mode with 2A
until the battery voltage respectively decreased to 2.7, 2.5
and 2.2V for batteries 005, 006 and 007; finally, impedance
measurement was performed by an electrochemical imped-
ance spectroscopy (EIS) frequency sweep from 0.1Hz to
5kHz. In this paper, we focused on the battery discharge
process. For each type of battery, the corresponding dis-
charge process consisted of 168 discharge cycles in constant
current mode. In each cycle, various battery measurements
were listed, such as battery voltage, battery current, and bat-
tery capacity. Among all measurements, the most important
was battery capacity. According to the battery capacity, we
can easily calculate the SOH value of a battery with the fol-
lowing formula:
where
is the initial battery capacity and
is the capac-
ity at the
discharge cycle. Figure1 illustrates the SOH
variation curves of batteries 005, 006, and 007 as functions
of the discharge cycle number. From Fig.1, we can observe
that the SOH shows the degradation trend during discharge
cycles for each type of battery. In addition, it can’t escape
from our notice that there are several spikes in some cycles.
This phenomenon can be explained as follows: in charge/
discharge profiles, the side reactions, which occur between
the electrodes and electrolyte of the battery, make the elec-
trochemical performance of battery decrease; therefore, the
(1)
value of SOH gradually drops on the whole; however, when
the battery rests during charge or discharge processes, the
reaction products may dissipate, which leads to that the elec-
trochemical performance has an opportunity to recover; as a
result, the SOH in current cycle is increased compared to the
former cycle. The above phenomenon is called regeneration
or self-recharge, introduced and analyzed in [53].
In this work, to validate the efficacy of the proposed
methods, we designed two strategies, which are denoted as
DStrategy1 and DStrategy2, for constructing the training
dataset and test dataset for each type of battery. In DStrat-
egy1, the data from cycle 1 to cycle 80 were used as the
training dataset, and the data from the remaining cycles were
selected as the test dataset. In DStrategy2, we chose the data
from cycle 1 to cycle 100 for constructing the prediction
model and utilized the remaining data to evaluate the trained
model.
2.2 Sliding‑window‑based feature extraction
In the discharge process, five measurements, namely, battery
voltage (BV), battery current (BC), load voltage (LV), load
current (LC), and battery temperature (BT), were recorded
for each type of battery in the database. Since the discharge
process is based on constant current mode, all the recorded
values of BC and LC are approximately equal. Therefore,
only BT, BV, and LV are considered in constructing the fea-
ture set in this work. For each type of measurement, numer-
ous values were recorded in each cycle, and the number of
recorded values in the current cycle is different from the
number in the next cycle. Hence, it is difficult to use these
recorded values as features directly.
To solve this problem, the following steps are executed.
Let
={M
, M
, … , M
, … , M
i,j
be the
Fig. 1 The SOH variation curves of three types of batteries vs. dis-
charge cycle number