Abstract
Research and/or Engineering Questions/Objective
The performance of battery packs on electric vehicles is highly influenced by the inconsistency of cell parameters such as resistance and capacity. In parallel-connected modules, parameter inconsistency results in current, SOC and temperature non-uniformity. However, unlike series-connected modules, it’s practically impossible to monitor each cell’s current information considering cost and energy efficiency. In order to study the relationship between the state inconsistency and parameter inconsistency in parallelconnected modules, it’s necessary to build an electro-thermal model considering cell parameter inconsistency.
Methodology
In this study, an equivalent circuit model based on one-order RC network and a lumped parameter thermal model are used to simulate the electro-thermal behavior of parallel-connected battery modules. To validate the model’s accuracy, several experiments are conducted by using a batch of new NCM lithium-ion batteries with the format of 18650 and rated capacity of 2Ah. The batteries are calibrated and assigned into several modules which are composed of 2 or more batteries according to the degree of parameter inconsistency. A test bench for parallel-connected battery modules is built by using a highspeed DAQ, current sensing resistors, thermocouples, charge/discharge equipment and temperature chambers. The modules’ current distribution and temperature difference is recorded and analyzed to validate the model.
Results
The experimental results show that the current and temperature inconsistency problem both deteriorate as the degree of parameter inconsistency increases. Generally, batteries with smaller resistances have relatively larger currents but have lower temperatures. However, this phenomenon has some exceptions. For example, during charging and discharging process, the cell with larger resistance has a smaller current at the beginning but before the end of the process it will have a larger current instead. The temperature difference is dependent on the rate of total current. Specifically, under small rates, such as 0.5C, the temperature is almost the same for each cell. While under large rates, such as 2C, the ones with larger resistances have higher temperatures. This phenomenon may explain the aging mechanism for parallel-connected modules. Moreover, as the number of module batteries increases, the inconsistency problem appears to be relieved. The simulation results fit well with the experiments.
Limitations of this study
In this work, an important limitation is the lack of time for more experiments concerning the changing trend of parameter inconsistency under long time-scales, because it’s necessary to consider the state of health problem when designing the topology of battery packs.
What does the paper offer that is new in the field including in comparison to other work by the authors?
Most works considering inconsistency problem is focused on the series-connected battery packs. Few studies about the model considering both electrical and thermal problems for parallel-connected battery modules have been done yet.
Conclusions
An electro-thermal model for parallel-connected battery modules considering cell parameter inconsistency is built. Several experiments are carried out and the simulation results fit well with the experiments. The results show that the state non-uniformity deteriorates as the parameter inconsistency and the current rate increases, while as the number of module batteries increases, the state non-uniformity appears to decrease. This phenomenon may help designing the parallel modules for electric vehicles.
Key Words : Electro-Thermal Model; Parallel-connected Modules; Parameter Inconsistency; State Non-uniformity;