Battery SOC Estimation Using Instantaneous Voltage Under Load
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Solution Overview
Problem
Existing methods for estimating the state-of-charge (SOC) of batteries fail to accurately account for the effects of temperature and current, especially at high currents, leading to inaccuracies in determining the available SOC.
Innovation Solution
A method that determines the stored SOC using open-circuit voltage and temperature, measures instantaneous voltage, and calculates available SOC based on the difference between instantaneous and minimum acceptable voltages, using a Kalman observer for estimation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If coulombmeter counting is used to estimate state-of-charge, then the stored capacity information is obtained, but the available state-of-charge becomes inaccurate at low temperature and high current due to internal impedance variations
Solution Approach 1:
The patent changes the estimation parameter from stored state-of-charge to available state-of-charge by incorporating voltage measurements and compensation factors that account for temperature and current effects. The available state-of-charge is calculated as SOC_available = SOC_stored - ΔSOC(T, I), where ΔSOC compensates for impedance variations under different operating conditions.
Solution Approach 2:
The patent introduces voltage measurement as an intermediary parameter to bridge the gap between stored charge and available charge. By measuring terminal voltage and comparing it with open-circuit voltage, the system detects the voltage drop caused by internal impedance and uses this information to correct the state-of-charge estimation.
2Measurement precision
If charging and discharge tests are performed at low current to determine temperature-dependent capacity limits, then temperature compensation data is obtained, but the method cannot be applied to study current dependence because strong currents heat up the battery
Solution Approach 1:
The patent segments the state-of-charge estimation into two independent components: stored state-of-charge (determined by coulombmeter counting) and available state-of-charge (determined by voltage-based compensation). This segmentation allows temperature compensation to be developed separately at low currents while current compensation is applied through voltage measurements during high-current operation.
Solution Approach 2:
The patent performs preliminary charging and discharge tests at low currents to establish temperature-dependent capacity limits and compensation factors. These pre-determined compensation factors are then stored and applied during actual operation to correct state-of-charge estimates without requiring repeated low-current testing.
3Reliability
If the battery discharge stops when the lowest acceptable voltage is reached, then the discharge cutoff is protected, but the available state-of-charge is lower than stored state-of-charge at high current or low temperature
Solution Approach 1:
The patent implements feedback by continuously monitoring terminal voltage and comparing it with the minimum acceptable voltage threshold. The voltage-based state-of-charge estimation provides real-time feedback on available capacity, allowing the system to adjust discharge operations and prevent voltage from dropping below the acceptable limit.
Data Source
AI summary
A method for estimating the state of charge of a battery, the method comprising the following steps: determining the stored state of charge; determining the open-circuit instantaneous voltage of the battery according to a predetermined map and the stored state of charge; measuring the instantaneous voltage across the terminals of the battery; determining the available state of charge according to the stored state of charge, the instantaneous voltage, the open-circuit instantaneous voltage and the minimum acceptable voltage; and the minimum acceptable voltage being predetermined by design.