The main reasons for the increased PACK voltage difference caused by the low frequency of battery cell use are related to the self-discharge characteristics of the battery cell, the failure of the BMS equalization mechanism, and the accumulation of polarization effects.
1. Amplification of differences in cell self-discharge rate
The root cause is the inherent difference in the self-discharge rate (K-value) of the battery cells. During high-frequency use, the BMS's equalization function can dynamically compensate for this difference; however, during long-term rest, the self-discharge difference will continue to accumulate.
Cells with high self-discharge rates experience a faster voltage drop. Cells with low self-discharge rates maintain a higher voltage, leading to a gradual widening of the voltage difference within the module.
2. BMS load balancing function malfunction.
Passive equalization limitations: The passive equalization of the BMS only operates during charging and discharging. At low frequencies:
During the resting period, the equalization circuit is not activated and cannot eliminate the voltage difference. After a long period of rest, the voltage difference may exceed the compensation capability of the equalization circuit (e.g., the equalization current is only 50mA, which cannot compensate for a large voltage difference).
Active balancing failure: Some systems will shut down the active balancing module when idle in order to save energy, further losing the ability to compensate.
3. Accumulation of polarization effects
Increased concentration polarization: Long-term stagnation leads to uneven distribution of lithium ions in the electrolyte. Upon resuming use:
Cells with different degrees of polarization have different voltage response speeds, and the voltage difference increases significantly at the end of charge and discharge. After being left to stand, the battery's internal resistance increases, exacerbating voltage divergence at the end of charge and discharge. 4. The influence of ambient temperature.
Temperature accelerates self-discharge: The self-discharge rate increases significantly under high-temperature conditions. If a temperature gradient exists in the PACK: The cell voltage decays faster in high-temperature regions, further widening the voltage difference.
5. Suggestions for Improvement
Regular maintenance charging and discharging: Perform small current charging and discharging (e.g., 0.05C) on the PACK used at low frequencies periodically to activate the BMS equalization function.
Temperature uniformity management: Ensure that the temperature difference within the module is ≤3℃.
Upgrade BMS strategy: Add intermittent balancing function during rest periods, or adopt Coulomb efficiency compensation algorithm.
6. Conclusion
Infrequent cell usage leads to increased PACK voltage difference, essentially due to the uncontrollable accumulation of self-discharge differences during the resting period, compounded by BMS equalization failure and the influence of ambient temperature. This issue is supported by multiple case studies in the documentation (such as module voltage difference testing and K-value screening data), and requires proactive maintenance and system optimization to mitigate.
The above content is based on my daily work, communication, and literature review. Due to my limited abilities, there may be omissions in the viewpoints presented. I welcome colleagues in the industry to actively exchange ideas and make progress together!
