Battery Controller Current Balancing via OCV Difference
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Solution Overview
Problem
In battery systems with secondary batteries connected in parallel, the variation in current and temperature among batteries leads to uneven heat generation, accelerating battery deterioration, as the current is temporarily concentrated on batteries with higher internal resistance during charging and discharging.
Innovation Solution
A controller calculates the difference in open circuit voltages of the batteries and restricts currents when this difference is large, using detectors to monitor voltages and currents, and estimates the charged state to accurately control current flow, thereby reducing heat generation in batteries with higher internal resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If current restriction control is not implemented, then productivity is improved (continuous charging/discharging can proceed without current limits), but battery deterioration is accelerated due to heat generation in high internal resistance batteries
Solution Approach 1:
The control device implements feedback control by continuously monitoring the difference between maximum and minimum open circuit voltages of parallel-connected batteries, and adjusting the current limit based on this feedback. When the voltage difference (indicating current imbalance) exceeds a threshold, the current limit is reduced to prevent excessive heat generation in high internal resistance batteries, thus resolving the contradiction between maintaining productivity and ensuring battery reliability.
Solution Approach 2:
The system dynamically changes the current parameter based on the state of battery imbalance. By calculating the difference in open circuit voltages and comparing it to a threshold, the system adjusts the current limit parameter in real-time, allowing high current operation when batteries are balanced (maintaining productivity) and reducing current when imbalance occurs (protecting battery lifespan).
2Reliability
If current is restricted to prevent heat generation, then battery deterioration is suppressed, but productivity decreases (charging and discharging speed is limited)
Solution Approach 1:
The current limit is made dynamic rather than fixed. The control device continuously adjusts the current limit based on the real-time voltage difference between batteries. When voltage difference is small (batteries are balanced), the current limit is high, allowing fast charging/discharging. When voltage difference exceeds the threshold (imbalance detected), the current limit is reduced to prevent heat generation. This dynamic adjustment resolves the contradiction by adapting the current restriction to the actual battery state.
Solution Approach 2:
The system performs preliminary detection of battery imbalance through open circuit voltage comparison before excessive heat generation occurs. By detecting the voltage difference in advance and proactively adjusting the current limit, the system prevents the harmful effects of current concentration in high internal resistance batteries while maximizing productivity during balanced states.
3Measurement precision
If open circuit voltage difference is used as the control parameter, then measurement precision is improved (voltage is easier to measure accurately than current distribution), but device complexity increases (requires continuous voltage monitoring and calculation)
Solution Approach 1:
The open circuit voltage difference serves as an intermediary parameter that indirectly reflects the current distribution state among parallel batteries. Instead of directly measuring difficult-to-obtain current distribution data, the system measures the easily obtainable open circuit voltages, calculates their difference, and uses this as a proxy to control the current limit. This intermediary approach achieves accurate current distribution monitoring with simpler measurement equipment.
Solution Approach 2:
The system replaces direct current measurement (which would require complex current sensors and distribution monitoring for each battery) with voltage measurement and calculation. By substituting the mechanical/electrical measurement of current with electrical measurement of voltage followed by computational processing, the system achieves the same control objective with simpler hardware and more flexible implementation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively suppresses heat generation and subsequent battery deterioration by ensuring balanced current distribution among batteries, extending their lifespan.
Implementation Method 1
a controller that controls currents that flow through the plurality of secondary batteries based on an open circuit voltage of each of the plurality of secondary batteries
Implementation Method 2
heat generation by the battery relatively high in internal resistance among the plurality of secondary batteries which results in a temperature of the battery at which deterioration of the battery is accelerated
Data Source
AI summary
An ECU performs processing including obtaining a current in a battery assembly, calculating a current in each battery, calculating an SOC of each battery, calculating an OCV of each battery, calculating ΔOCV, calculating an average value Ave of ΔOCVs, carrying out current restriction control when the average value Ave exceeds a first range and exceeds a second range, providing a warning signal when the average value Ave does not exceed the second range, and carrying out normal current control when the average value Ave does not exceed the first range.