Stationary NiMH Battery SOC Equalization Control
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Solution Overview
Problem
Stationary storage batteries with nickel metal hydride secondary cells experience variations in state of charge (SOC) due to differences in charging efficiency, leading to reduced performance and potential inability to reuse battery packs from vehicles, as they cannot maintain a stable electric power supply over a wide SOC range.
Innovation Solution
A controller is used to manage charging and discharging of nickel metal hydride secondary batteries by stopping operations when specific SOC thresholds are reached and implementing additional charging to equalize the SOC of all cells to 100%, thereby reducing SOC variations and maintaining battery performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If the stationary storage battery uses a wide charged level range for operation, then the battery can supply electric power for a longer duration, but variations in SOC among cells increase due to differences in charging efficiency
Solution Approach 1:
The controller performs preliminary equalizing charging before the main charging operation. When a cell's SOC reaches the upper limit first, the controller continues charging other cells at a reduced rate until all cells reach the upper limit, preventing SOC variation accumulation and enabling long-duration operation with maintained uniformity.
2Adaptability or versatility
If the battery pack is reused from vehicle applications, then resource utilization improves, but cell degradation causes differences in charging efficiency that increase SOC variations
Solution Approach 1:
The controller applies different charging rates to different cells based on their individual SOC states. When some cells reach the upper limit before others, the controller reduces the charging rate for those cells or stops charging them individually, allowing other cells to catch up. This localized control approach accommodates degraded cells while maintaining overall pack reliability.
3Reliability
If the controller stops charging when any cell reaches the upper SOC limit, then overcharging is prevented, but other cells may not be fully charged due to SOC variations
Solution Approach 1:
The controller dynamically adjusts the charging rate based on real-time SOC monitoring of individual cells. When a cell reaches the upper limit, the controller reduces or stops charging for that specific cell while continuing to charge other cells at the normal rate. This dynamic adjustment allows the system to prevent overcharging while maximizing the charging of all cells, improving overall charging productivity.
Data Source
AI summary
A stationary electrical storage system includes: a stationary storage battery including nickel metal hydride secondary batteries; and a controller repeatedly carrying out charging with an external power supply and discharging to supply electric power to a load, stopping discharging to the load when an SOC of any secondary battery has become a first threshold, and stopping charging from the external power supply when an SOC of any secondary battery has become a second threshold. The controller additionally charges the whole stationary storage battery such that the secondary battery other than the secondary battery having the lowest SOC is allowed to be charged to a fully charged level beyond the second threshold by not stopping charging when the SOC of the any secondary battery has become the second threshold and the SOC of the secondary battery having the lowest SOC becomes higher than or equal to the second threshold.