EV Battery SOH Estimation Using Relaxed Open-Circuit Voltage
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Solution Overview
Problem
Current methods for estimating the state of health (SOH) of batteries in electric or hybrid vehicles lack precision, especially when estimating SOH beyond a certain threshold, and often require removing the battery from the vehicle for direct capacity measurement.
Innovation Solution
A method that involves charging the battery to maximum state of charge, followed by relaxation periods, discharging the battery in running cycles until a target state of charge is reached, and then estimating SOH based on the total capacity discharged and initial discharge capacity determined from open-circuit voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If direct capacity measurement is used to determine SOH, then measurement precision is improved, but ease of operation deteriorates due to battery removal requirement
Solution Approach 1:
The patent uses open-circuit voltage as an intermediary parameter to indirectly determine battery capacity and SOH. Instead of directly measuring capacity through battery removal and discharge tests, the method measures the open-circuit voltage after relaxation periods and uses pre-established voltage-capacity correlations to infer the SOH, thereby avoiding the need to remove the battery from the vehicle.
Solution Approach 2:
The patent replaces the mechanical approach of physical battery removal and direct capacity testing with an electrical measurement approach. By measuring electrical parameters (open-circuit voltage) and using computational correlations, the system substitutes the need for physical manipulation and direct capacity discharge measurements.
2Ease of operation
If empirical degradation models are used for online SOH estimation, then ease of operation is improved, but measurement precision deteriorates especially beyond SOH of 70%
Solution Approach 1:
The patent changes the measurement parameter from direct capacity measurement or empirical model estimation to open-circuit voltage measurement after controlled relaxation periods. This parameter change allows for more accurate SOH determination across the entire range, particularly improving precision for degraded batteries (SOH < 70%) where empirical models fail.
Solution Approach 2:
The patent implements preliminary relaxation periods after charging and before measurement to allow the battery to reach a stable open-circuit voltage state. This preliminary action ensures that the voltage measurement accurately reflects the battery's true state of charge and capacity, eliminating transient effects that would otherwise compromise measurement precision.
3Measurement precision
If battery capacity is measured by discharging to determine SOH, then measurement precision is improved, but loss of time increases due to multiple running cycles required
Solution Approach 1:
The patent substitutes the time-consuming mechanical process of discharging the battery through multiple running cycles with a rapid electrical measurement of open-circuit voltage. After a single relaxation period following charging, the voltage measurement and subsequent correlation calculation provide SOH information without requiring extended discharge cycles.
Solution Approach 2:
The patent uses open-circuit voltage as an intermediary that can be measured rapidly without depleting battery capacity or requiring multiple running cycles. This intermediary measurement provides the necessary information to calculate SOH through pre-established correlations, avoiding the time loss associated with direct capacity discharge measurements.
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 method allows for precise estimation of battery SOH without removing the battery from the vehicle, maintaining vehicle operation, and providing accurate estimates across various usage conditions.
Implementation Method 1
a step of charging the battery until the battery reaches a maximum state of charge SOCmax
Implementation Method 2
a step of determining a state of charge SOCOCV of the battery as a function of the open-circuit voltage, with the battery not being used
Data Source
AI summary
A method for estimating the state of health of an electric or hybrid vehicle battery that can be carried out while the vehicle is running includes: (a) a full charge step; (b) a first relaxation step; (c) a step of discharging to a state of charge SOC≤SOCtarget carried out in k vehicle driving cycles, for each of which the capacity Qn discharged by the battery is determined; (d) a second relaxation step at the end of the cycle k; (e) a step of determining a state of charge SOCOCV as a function of the open circuit voltage; (f) a step of determining an initial discharge capacity Qi of the new battery from SOCOCV; and (g) a step of estimating the state of health SOH of the battery from the total capacity discharged by the battery during step (c) and from Qi.