Abstract
Research and Engineering Questions/Objective
Charging for lithium ion battery under low temperature results in lithium dendrite, which may make the battery unsafe. AC heating is a safe and effective method to warm battery up before charging. And battery temperature accurate estimation is essential to avoid thermal runaway when heating. Thus, a temperature estimation method based on battery impedance measurement is investigated and a porotype integrated into on-board charger with such function is designed.
Methodology
Two series of tests to uncover EIS (electrochemical impedance spectroscopy)-SOC (state of charge) relationship under certain equilibrium temperature and EIS-T (equilibrium cell temperature) relationship under certain SOC were done upon 8Ah prismatic lithium iron phosphate battery. The relationship between EIS and T was analyzed and represented with a linear fit based on least square method. Then a temperature estimation method was raised. To measure impedance on board, a prototype divided into two parts, excitation module and measure module, was developed. SPWM (sinusoidal pulse width modulation) is used to generate different frequency perturbing signal in the excitation module and digital lock-in amplifier to detect low SR (signal-noise ratio) response signal in the measurement module. The impedance measurement prototype shares the main power circuits with the charger to reduce size and cost making the traditional charger an intelligent on-board charger.
Results
Results of the tests showed that impedance at frequency higher than 10Hz is insensitive to SOC but varies with temperature. And impedance phase at 79.4Hz changes most significantly with temperature, which can be adopted to estimate temperature. Based on the plot of impedance phase of 79.4Hz at different temperature, a linear fitting polynomial was derived. The intelligent on-board charger consisting of impedance measure prototype and charger has a relative impedance phase measurement relative error, about 10% from 1Hz to 100Hz leading to 5 Celsius degree temperature estimation error. The error is not vital, for charging under 20 or 25 Celsius degree has little different effect on battery safety performance. Limitations of this study The aforementioned impedance prototype can only measure impedance in a several-series-one-parallel pack, because the perturbation current through every branch is unknown in a several-parallel pack. And the AC heating process have to be interrupted when temperature estimation is carried out. Further study is needed to solve the problems and fulfil the function of the intelligent charger.
What does the paper offer that is new in the field in comparison to other works of the author?
Highlights of the paper are shown below.
a) Integrate impedance measurement prototype into on-board charger to ensure capability and low cost.
b) Estimate temperature with designed prototype to assure safety when heating before charging.
Conclusions
To estimate cell temperature when AC heating is used before charging, a cell temperature estimation method based on impedance phase at 79.4Hz is adopted. And an intelligent charger with impedance measurement and charging function is developed. The results show that the designed novel charger has good performance to estimate temperature with accepted size and cost.
KEYWORDS – lithium ion battery, electrochemical impedance spectroscopy, temperature estimation, AC heating, intelligent charger