The long-term consistency of lithium-ion battery cells is a critical challenge in the design of battery packs. This inconsistency isn't just about traditional parameters like capacity and voltage, but also includes factors such as capacity degradation, internal resistance changes, and temperature distribution across the pack. Ideally, batteries from the same batch should perform identically, but manufacturing variances often lead to differences between individual cells.
Battery packs typically consist of hundreds or even thousands of cells connected in series and parallel. The overall performance of the pack is heavily influenced by these inconsistencies, including variations in Coulomb efficiency, self-discharge rates, and internal resistance growth. Studies show that even if a single cell can last over 1000 cycles, the pack's lifespan might be less than 200 cycles due to these discrepancies.
To address this issue, balancing systems are essential. Most current methods involve electronic devices to equalize voltages between cells, but they vary greatly in effectiveness. Recently, Alexander U. Schmid from the University of Stuttgart introduced a novel approach using NiMH and Ni-Zn batteries to achieve electrochemical balance in lithium-ion packs.
Lithium-ion batteries are vulnerable to overcharging, which can cause electrolyte decomposition and lithium plating. In contrast, NiMH batteries handle overcharge well because any gas produced during charging can recombine, forming water again. This makes them ideal for acting as a bypass when lithium-ion cells reach their charge limit.
Schmid used LiFePO4 and Li4Ti5O12 materials in his experiments because they exhibit rapid voltage increases after full delithiation. When connected in parallel with NiMH or Ni-Zn batteries, excess current flows into these secondary cells, preventing overcharging of the lithium-ion cells.
In this system, the NiMH or Ni-Zn battery acts as a shunt once the lithium-ion cell reaches its threshold voltage. This allows all current to flow through the NiMH/Ni-Zn battery instead, protecting the lithium-ion cell from damage. The process ensures consistent charging without the need for complex voltage monitoring systems.
The experiments involved various battery combinations, including LFP/graphite, LMO/LTO, and LFP/LTO with NiMH or Ni-Zn batteries. Results showed that these configurations could effectively manage overcharging while maintaining good performance.
One key advantage of this method is its simplicity and automation. It eliminates the need for continuous voltage monitoring of individual cells, reducing complexity and improving reliability. Additionally, it allows for more efficient use of the battery pack’s total capacity.
Through multiple charge and discharge cycles, the system demonstrated significant improvement in balancing, achieving up to 8% capacity equalization in one cycle. This innovative approach offers a promising solution to the longstanding problem of battery pack inconsistency, paving the way for safer and more reliable energy storage systems.
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