Just like the calm after a storm, lithium-ion batteries exhibit a fascinating behavior known as 'Open Circuit Voltage Relaxation'. This phenomenon, like the gradual settling of disturbed water back to a serene state, plays an important role in determining the health & efficiency of batteries.
It is a process where the voltage of a battery stabilizes after being disconnected from a load or charger. This process is not instantaneous but evolves over time, reflecting the complex electrochemical reactions.
The image below shows this behavior for 3 different chemistries over a 24-hour period. The NMC & NCA batteries were charged to an SOC of 45%, and the LFP to 65%. What follows is a period of open circuit, where the batteries are left undisturbed, allowing to observe the natural stabilization of the voltage.
The initial phase of Voltage Relaxation (VR) is a swift change in voltage, occurring within the first few minutes post-charge. To capture these details, a logarithmic time scale is used, revealing the exponential nature of the voltage reduction during this phase. In the 2nd phase, the voltage exhibits a linear decline in the logarithmic plot. Eventually, they approach a plateau, indicating steady-state open-circuit voltage (SS-OCV).
The study# shows that NCA & NMC chemistries, sharing a nickel-based electrode material, start with similar voltage levels. The NCA voltage soon converges with the NMC voltage as time progresses. The NMC battery distinguishes itself by reaching 98% of its SS-OCV within approximately 2 hours & achieving a steady state by the 5-hour mark, a characteristic particularly advantageous for quick charge & discharge cycles.
The LFP chemistry, while initially following a similar VR trajectory to the nickel-based chemistries, soon diverges. It lacks the inflection points present in NCA & NMC, indicating a more straightforward relaxation pattern. This simplicity suggests that a simpler equivalent circuit model may be all that's needed for LFP's accurate simulations.
The key takeaway from the study is that a 3-hour relaxation period is optimal for capturing more than 98% of the SS-OCV for SOC estimation in NMC & LFP batteries. However, the NCA batteries continue to evolve without reaching a steady state even after 24 hours, signaling the need for further investigation into their long-term behavior.
The impact of the relaxation period on the precision of SoC measurements by BMS is very clear. A thorough understanding of the VR characteristics is essential for optimizing SoC estimation algorithms. By allowing the necessary time for batteries to reach their equilibrium state post-charge or discharge, BMS can provide more accurate & reliable SoC values.
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