Introduction
In recent years, lithium-ion batteries, have been employed and utilized in a vast amount of electronic devices. Lithium batteries can be found in many hand held devices and is expected to play a critical role as energy storage for electric vehicle and on/off-grid electric storage. The reconfigurable series/parallel switching system is capable of efficiently switching between low power-high energy to high power-low energy topologies. This paper provides the techniques and configurations that will establish a series of independent discharging/ charging profiles for each battery in the stack.
Series/Parallel Switching System
Supporting multiple load demands, a BMS is more effective when it can switch independently from series or parallel configurations. The topology developed supports the following configurations, single-multiple series arrangement, single to multiple parallel arrangement, and hybrid single to multiple series/parallel structures.
The pack is capable of operating in the following modes:
- Series set (high power)
- Parallel set (high energy)
- Series/Parallel set (max load)
- Isolation (min load)
- By-pass (safety control)
Fast charging is support through parallel charging, giving priority to lower charged cell.
Experimental Method
The circuit configuration consist of three section: DC-AC inverter using insulated-gate bipolar transistors (IGBTs) and a linear transformer to translate the same output from the primary side to the secondary side and more importantly provide isolation; a full wave rectifier to convert AC-DC; finally a buck converter with a PID controller to maintain the current induced into each battery. Matlab was used to simulate and monitor the battery performance during charging at different conditions of SOCs.
Results
To obtain the appropriate values for the circuit configurations described above, the following parameters were established to ensure that each would operate as intended. The linear transformer has a one-to-one ratio which means that the voltage will be the same on both primary and secondary side. Based on the parameters needed for the battery the voltage had to be stepped down. By using the following equations, the buck converter was configured to meet the battery requirements.
(1)
D is the duty cycle, Vg is the input following the full-wave rectifier and V is the output voltage and is equal to the voltage drop across the sense resistance plus the batteries voltage.
(2)
L is the inductor, Ts is the switching period (50 µs) and is the ripple which was chose to be one percent.
(3)
C is the capacitance, I is the current and is the voltage ripple also chosen to be one percent.
In this process, three 10Ah Lithium-ion batteries are connected in series and have a SOC of 30 %. All batteries are independently charged as indicated in figure 1. In addition, a feedback system (microcontroller) is implemented to maintain the desired current at that output voltage which effects the duty cycle. When the individual batteries are done charging a signal is sent to the feedback enabling a trickle charge. The idea portrayed is an intelligent battery system that has parallel charging capabilities that proposes many advantages versus series charging including the reduction in battery failure and improve cycle life.
Conclusion
Reconfigurable series/parallel switching systems have the ability to outperform traditional singular switching topology, by supporting high energy and power demand systems. In addition, provide a self-reliable topology that can prevent battery malfunction.
Acknowledgement
This work was supported by the FREEDM ERC program of the National Science Foundation under award number EEC-08212121.
References
1. Daowd, M., Antoine, M., Omar, N., Lataire, P., Bossche, P. Van Den, & Mierlo, J. Van. (2014). Converter with the Auxiliary Battery, 2897–2937.
2. Tu, C. H., & Emadi, A. (2012). A novel series parallel reconfigurable hybrid energy storage system for electrified vehicles. 2012 IEEE Transportation Electrification Conference and Expo, ITEC 2012.