Secondary Battery Resistance Estimation and Current Control
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Solution Overview
Problem
Lithium ion batteries experience capacity degradation and increased direct current resistance due to charge-discharge cycles, especially at low temperatures or low State of Charge (SOC), leading to abrupt performance drops when voltage exceeds maximum or falls below minimum allowable values during charging and discharging.
Innovation Solution
Estimating battery resistance based on SOC and temperature, and adjusting charge current to prevent voltage from exceeding maximum allowable levels, thereby maintaining battery performance and extending battery life.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If charge-discharge control is employed to maintain battery capacity, then battery performance is preserved, but direct current resistance increases at low temperature or low SOC
Solution Approach 1:
The system performs preliminary estimation of battery resistance based on SOC and temperature before charging begins. This allows the control device to pre-determine appropriate charge current limits that prevent voltage excursions, rather than reacting after the problem occurs. The resistance estimation table is prepared in advance through experiments, enabling proactive control adjustments.
Solution Approach 2:
The charge current is dynamically adjusted based on real-time SOC and temperature conditions. The control device continuously monitors battery state and modifies the charge current limit according to the estimated resistance at each operating point, rather than using a fixed current limit. This dynamic adaptation prevents voltage overshoot while maximizing charging efficiency at each moment.
2Reliability
If upper and lower limit values of charge and discharge current are set by detecting temperature, then battery safety is improved, but voltage may instantaneously exceed maximum allowable voltage at low SOC or low temperature
Solution Approach 1:
The control strategy transitions from using only temperature as a control parameter to using a combination of SOC and temperature. By incorporating SOC into the resistance estimation, the system captures the non-linear resistance behavior that occurs at different charge states. This multi-parameter approach enables more precise voltage control, especially in the critical low SOC region where temperature-only control fails.
Solution Approach 2:
The system implements a feedback mechanism where the estimated resistance (based on SOC and temperature) continuously informs the charge current limit settings. The control device monitors actual battery voltage and SOC, updates the resistance estimation accordingly, and adjusts the charge current limit in real-time. This closed-loop control prevents voltage excursions by continuously adapting to changing battery conditions.
3Duration of action of stationary object
If charge current is limited to prevent voltage exceeding maximum allowable voltage, then battery life is extended, but charging time increases
Solution Approach 1:
The system applies partial current limiting only when necessary - specifically when SOC and temperature conditions indicate high resistance that could cause voltage overshoot. When conditions are favorable (moderate temperature and appropriate SOC range), the system allows higher charge currents without limitation. This selective application of current limits prevents unnecessary charging time extension while still protecting against voltage excursions.
Solution Approach 2:
The charge current limit is dynamically adjusted based on real-time SOC and temperature conditions rather than applying a conservative fixed limit throughout charging. As the battery transitions through different operating states, the system optimizes the current limit accordingly, allowing maximum safe charging speed at each moment. This dynamic approach minimizes charging time while maintaining battery life protection.
Data Source
AI summary
Since in existent charge-discharge control the battery direct current resistance of a lithium ion battery increases at a low temperature or in a low state of charge (SOC), the battery voltage may possibly exceed greatly the maximum allowable voltage instantaneously due to over-voltage during charging or may possibly decrease greatly to less than the minimum allowable voltage during discharging. If the lithium ion battery continuously lies in such a state the battery performance is degraded abruptly. The battery resistance of a lithium ion battery module (41) is estimated in accordance with SOC and temperature of the lithium ion battery module (41), and current to charge is set properly based on the estimated battery resistance.