Voltage Compensated Active Cell Balancing for Battery Stacks
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Solution Overview
Problem
Battery stack monitors face challenges in accurately measuring cell voltages during balancing due to IR drops from cable and connector impedances, leading to measurement errors and reduced balancing efficiency, with no known methods to compensate for these drops during cell voltage measurement.
Innovation Solution
A monitoring device that measures cell voltage, calculates open cell voltage by adjusting for voltage drops, and balances the battery stack concurrently, using a processor and memory to store instructions for measuring voltage drops and balancing currents, and includes features like an integrated power switch and current sense amplifier to avoid overcharging or undercharging cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If passive balancing via MOSFETs and resistors is used to balance cell voltage, then cell voltage balancing is achieved, but measurement accuracy deteriorates due to IR drops in cable-impedances, interconnect-impedances, and cell impedance
Solution Approach 1:
The system performs preliminary action by measuring the balancing current and calculating the expected IR voltage drop before it significantly affects the measurement. The BSM measures the balancing current flowing through the cable and interconnect impedances, calculates the voltage drop using Ohm's law (V=IR), and compensates for this drop in advance when determining the cell voltage, thereby maintaining measurement accuracy during active balancing operations
Solution Approach 2:
The system implements feedback by continuously monitoring the balancing current and using this information to dynamically adjust the voltage measurement compensation. The BSM measures the actual balancing current, calculates the corresponding IR drop, and feeds this compensation value back into the voltage measurement calculation, creating a closed-loop system that maintains accuracy throughout the balancing process
2Productivity
If cell balancing is performed during battery cell voltage measurement by the BSM, then balancing operation is maintained, but measurement accuracy deteriorates due to significant reading errors from IR drops
Solution Approach 1:
The system introduces an intermediary calculation step that measures the balancing current and uses it to compute the voltage drop caused by cable and interconnect impedances. This intermediary current measurement acts as a mediator between the balancing operation and the voltage measurement, allowing the system to separate the effects of balancing current from the actual cell voltage and thereby maintain accurate readings during continuous balancing
Solution Approach 2:
The system replaces the mechanical approach of stopping balancing to take measurements with an electrical calculation approach. Instead of interrupting the balancing current flow to eliminate IR drops, the system substitutes the physical interruption with an electrical calculation that computes and compensates for the voltage drop, allowing continuous balancing operation without sacrificing measurement accuracy
3Measurement precision
If balancing is disabled during voltage measurement to avoid measurement errors, then measurement accuracy is improved, but balancing efficiency deteriorates due to reduced duty cycle
Solution Approach 1:
The system changes the parameter being measured from raw voltage to compensated voltage. By measuring the balancing current and using it to calculate the voltage drop parameter, the system transforms the measurement parameter to account for the IR drop effect, thereby maintaining accuracy without needing to stop the balancing operation and thus preserving high balancing duty cycle
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
Enables accurate cell voltage measurement and increased balancing duty cycle, optimizing the size and cost of the balancing circuit while ensuring safe operation during charge and discharge cycles.
Implementation Method 1
measure a voltage drop associated with a measured balancing current
Data Source
AI summary
A monitoring device includes an input terminal configured to receive an input signal from a battery system management (BSM); an output terminal configured to output cell parameters used to determine an open cell voltage associated with one of a plurality of cells within the battery stack connected to the monitoring circuit based on the input signal received from the BSM; a processor; and a memory storing executable instructions for causing the processor to: measure a cell voltage associated with the one of the plurality of cells within the battery stack; measure a voltage drop associated with a measured balancing current; calculate the open cell voltage by adjusting the measured cell voltage based on the measured voltage drop; and balance the battery stack based on the calculated open cell voltage, wherein balancing and calculating the open cell voltage are performed concurrently.