method [16]. The authors in [1] presented a comparative study of the
equivalent circuit model-based SoC estimation approaches algo-
rithms including Luenberger observer, extended Kalman filtering
(EKF) and sigma point Kalman filtering (SPKF) to monitor the SoC
of a LiFePO
4
lithium-ion battery (LiB) cell, and the results showed
that the SPKF was an optimal choice to estimate dynamic SoC behav-
ior. The authors in [2–4] presented an equivalent circuit battery
model-based method for real time battery cell SoC estimation using
linear parameter varying (LPV) system and reduced order EKF tech-
niques. Xiong et al. [5,6] proposed an equivalent circuit model based
SoC estimation method using adaptive Extended Kalman Filter
(AEKF) to estimate battery SoC through the measurements of battery
current and voltage. The authors in [10] proposed a reduced order
electrochemical model to estimate internal battery potentials, con-
centration gradients, and SoC from external current and voltage
measurements. The authors in [11] presented a novel approach
using adaptive artificial neural network and neuron-controller for
online cell SoC determination. The authors in [16] presented v-Sup-
port Vector Regression algorithm based method to estimate the SoC.
A common drawback of the above SoC estimation methods is
that
the model
parameters are identified with offline data or the
training data for ‘‘black-box’’ models is built by previously mea-
surement; as a result, the battery model parameters variances fol-
lowing with its degradation and varied operation conditions are
ignored. Thus, the reliability and applicable of these SoC estimators
were not sufficiently discussed. In order to overcome these draw-
backs, online parameter identification methods were proposed to
track the real-time behavior of the battery. The authors in
[17,18] used the method of recursive least square with an optimal
forgetting factor to carry out the online battery parameters identi-
fication and state estimation. However, both the model parameters
and capacity are important battery parameters, while the above
method fails to estimate the battery capacity and model parame-
ters concurrently. The reliable capacity estimate is indispensable
for an accurate SoC estimate, and which is of paramount impor-
tance for battery State-of-Health (SoH) indication.
A number of research methods have been proposed for estimat-
ing
the cell
capacity and then to calculate the SoH with the estimated
capacity taking the SoH is the ratio of estimated capacity over its
nominal value, most of them are carried out with lumped parameter
models [5,19–26]. The authors in [19] presented a neural-network
model online estimation method for SoH of valve-regulated lead
acid batteries on the basis of the relationship between the estimated
SoC and the battery open circuit voltage. The authors in [20] pre-
sented a probabilistic neural network (PNN) based SoH estimation
method for LiB, where the PNN was trained using 100 pieces of bat-
teries. The authors in [21] presented an experiment data based SoC
and SoH estimation method with fuzzy logic system, while the
experiment data were provided from electrochemical impedance
spectroscopy (EIS) measurements on new and aged cells. However,
for electric vehicles application, it is not easy to obtain the overall
data for training, which leads to the inaccuracy prediction in com-
plex variable practical application. On the contrary, the dynamic
battery model-based method can provide a cheap alternative in esti-
mation or it can be used along with a sensor-based data-driven
scheme to provide some redundancy. The authors in [22–26] pre-
sented battery model-based dual/joint Kalman filters method to
estimate the battery SoC and capacity concurrently. However, their
model parameters were identified by offline data and the influence
of the battery degradation or operation conditions over model
parameters are not discussed, as a result, their performance are
not verified adequately. Furthermore, the authors in [22–25] used
dual EKF or dual SPKF to execute the capacity and SoC joint estima-
tion with one time scale. However, in considering the system param-
eter inclines to change slowly over time while system state is prone
to fast variation over time, it is not an optimal choice to use the same
calculated time scale for battery parameter and state calculation; on
the contrary, it will largely increase the computational burden of
battery
management
system (BMS). Thus the time scale separation
based method, which uses macro scale to calculate the battery
parameter and uses micro scale to calculate the battery state, will
lower the computation cost of BMS. The authors in [26] used the
two time scales to estimate the battery capacity and SoC concur-
rently, and the difference of estimated SoC with dual scales was used
as an innovation to update the Kalman gain to correct the capacity.
But it is very hard to obtain the ‘‘true’’ SoC, thus it is not easy and reli-
able to obtain accurate capacity by the estimated SoC especially
when the estimated SoC is not stable.
The purpose of this paper is to establish general battery param-
eter
and state
dual estimation method using data-driven multi-
scale EKF algorithm, which is a key technique to safeguard the
optimal and safe use of battery – energy source in various electric
vehicles and promote the commercialization of electric vehicles.
The description of the research system and the data-driven mul-
ti-scale EKF algorithm are presented in Section 2. Section 3 de-
scribes the implementation flowchart of the proposed approach.
To evaluate the proposed approach, four different health status of
lithium-ion polymer battery (LiPB) cells are used to carry out the
verification are shown in Section 4. The experiment, simulation re-
sults and evaluation of the proposed method are reported in Sec-
tion 5 before conclusions are drawn in Section 6.
2. Data-driven multi-scale extended Kalman filtering
To make the discussion more convenient, firstly the section con-
structs
a very
general framework for discrete-time lumped dy-
namic system with dual scales. Afterwards, based on the review
of dual EKF, the implementation process of online measured data
driven based multi-scale EKF is built.
2.1. System description
In regarding that the slow-varying characteristic on battery
paramete
r and
fast-varying characteristic on battery state, we
use multi-scale method to construct the discrete time state-space
equation, where the system parameter and system state are pre-
dicted with the macro and micro scale separately. To be more spe-
cific, we consider the problem of learning both the hidden states
v
and parameters h of a very general framework for discrete-time
nonlinear dynamical system as:
v
k;lþ1
¼ Fð
v
k;l
; h
k
; u
k;l
Þþ
x
k;l
; h
kþ1
¼ h
k
þ
q
k
Y
k;l
¼ Gð
v
k;l
; h
k
; u
k;l
Þþ
t
k;l
(
ð1Þ
where
v
k,l
is the system state matrix at the time t
k,l
= t
k,0
+l T
(1 6 l 6 L), herein the T is a fixed sampling interval between two adja-
cent measurement points, k and l being the two time-scales indices
for system parameter with macro scale and system state with micro
scale respectively; u
k,l
is the exogenous input matrix at time t
k,l
; y
k,l
is
the system observation (or measurement) matrix at time t
k,l
;
x
k,l
and
q
k
are the process noise matrix for state and model parameter respec-
tively,
t
k,l
is the measurement noise matrix. Note that L represents the
level of time-scale separation and that
v
k,0
=
v
k1,L
. h
k
is the parame-
ters matrix under the kth macro scale, and h
k
= h
k,0:L1
. With the de-
fined system, we aim at estimating both the system state
v
and
model parameter h from the noisy observations Y.
2.2. Review of dual EKF method
The EKF provides an efficient approach for generating approxi-
mate maximum-likelihood
estimates of the state of a discrete-time
nonlinear dynamical system. However, the prediction precision of
464 R. Xiong et al. / Applied Energy 113 (2014) 463–476