Battery Pack Voltage Synchronization for Cross Current Prevention
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Solution Overview
Problem
Power storage systems using lithium ion secondary batteries face cross currents due to voltage imbalances between batteries, which can lead to system failure, especially when new and old batteries or batteries in different deterioration states are used together, making it challenging to operate them safely and efficiently.
Innovation Solution
A power storage system with an upper controller and secondary battery packs connected in parallel, where the controller measures and balances inter-terminal voltages by controlling discharge and charge switches to ensure synchronized discharge and charge start times based on impedance calculations, preventing cross currents by matching voltage levels and impedance between battery packs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple secondary battery packs are connected in parallel to increase power storage capacity, then the power storage capacity is improved, but cross currents occur due to voltage imbalances between battery packs
Solution Approach 1:
The system performs preliminary voltage measurement and impedance calculation for each battery pack before discharge begins. The control device calculates the expected voltage change rate based on impedance values and pre-determines the discharge start timing for each pack, ensuring that voltage imbalances are compensated before cross currents can occur.
Solution Approach 2:
The control device continuously monitors the actual voltage of each battery pack during discharge and compares it with the expected voltage based on capacity and impedance. When the actual voltage deviates from the expected voltage, the control device adjusts the discharge timing dynamically to maintain voltage balance and prevent cross currents.
2Adaptability or versatility
If batteries with different deterioration states are used together to reduce costs, then cost efficiency is improved, but voltage imbalances cause cross currents that reduce system reliability
Solution Approach 1:
The system measures the internal impedance of each battery pack, which varies with deterioration state, and uses this parameter to calculate the expected voltage change rate. By incorporating impedance-based predictions, the system can accommodate batteries with different deterioration states while maintaining voltage balance through adjusted discharge timing.
3Object-generated harmful factors
If discharge starts from the battery pack with the highest inter-terminal voltage to reduce cross current, then cross current is reduced, but the inter-terminal voltage rises in already discharging packs causing new cross currents
Solution Approach 1:
The control device calculates the expected voltage change rate for each battery pack based on its capacity and impedance before discharge begins. This preliminary calculation allows the system to predict how each pack's voltage will change over time and determine the optimal discharge start timing to maintain voltage balance throughout the discharge process.
Data Source
AI summary
A power storage system includes a plurality of secondary battery packs and an upper controller. When causing an additional discharge secondary battery pack that is not being discharged to start being discharged by connecting the additional discharge secondary battery pack in parallel with a discharging secondary battery pack that is being discharged, in a state in which some of the secondary battery packs are being discharged, the upper controller predicts discharge start timing at which inter-terminal voltages of the additional discharge secondary battery pack and the discharging secondary battery pack correspond to each other. Subsequently, the upper controller causes the additional discharge secondary battery pack to start being discharged at the discharge start timing.