Battery Pack Overcurrent Detection Using Dynamic Thresholds
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Solution Overview
Problem
Existing battery packs using lithium-ion secondary batteries face challenges in accurately detecting overcurrent due to load rush currents, leading to potential damage and increased power consumption, especially in high-power electric tools, as the resistance variation of charge/discharge control FETs results in degraded detection accuracy and erroneous inhibition of charge/discharge.
Innovation Solution
A battery pack design that includes a current detection resistor and N-channel charge/discharge control FETs, with a controller that sets a threshold for overcurrent detection and uses a delay time to differentiate between load rush currents and actual overcurrents, preventing erroneous shutdowns by comparing detected currents with the threshold value and only inhibiting charge/discharge when the current exceeds the threshold for an extended period.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a protection circuit with a predetermined threshold value is used to detect overcurrent, then overcurrent protection is provided, but load rush current is erroneously detected as overcurrent causing erroneous inhibition of charge/discharge
Solution Approach 1:
The patent applies dynamics by making the threshold value variable rather than fixed. The threshold dynamically changes based on the detected current magnitude: it starts high to allow load rush current, then decreases to a lower value for accurate overcurrent detection. This dynamic adjustment resolves the contradiction between providing overcurrent protection and avoiding erroneous detection of load rush current.
Solution Approach 2:
The patent changes the parameter of threshold value over time based on current detection results. When current exceeds a first threshold, the system waits for a predetermined time, then changes to a second (lower) threshold. This parameter change allows the system to distinguish between transient load rush current and sustained overcurrent conditions.
2Power
If the resistance value of charge/discharge control FET is several tens of milliohms to enable high power, then high power capability is achieved, but power consumption in the FET increases significantly during load rush current
Solution Approach 1:
The patent applies preliminary action by detecting the load rush current condition in advance and responding by changing the threshold value before damage can occur. When current exceeds the first threshold, the system immediately initiates the threshold change sequence, preventing the FET from operating in a high-power dissipation state longer than necessary.
Solution Approach 2:
The patent allows the load rush current to pass through quickly by maintaining a high threshold during the brief rush period, then rapidly transitions to a lower threshold. This skipping approach allows the necessary high current to flow briefly without triggering protection, then immediately enforces stricter limits to prevent sustained high power consumption.
3Device complexity
If a current detection method using the on resistance of discharge control FET is used, then circuit complexity is reduced, but detection accuracy degrades due to resistance value variation
Solution Approach 1:
The patent changes the detection parameter from using fixed FET on-resistance to using a programmable threshold value that can be dynamically adjusted. This allows the system to maintain simple circuitry while achieving accurate current detection by adapting the threshold to different operating conditions rather than relying on stable FET resistance characteristics.
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 solution enhances the accuracy of overcurrent detection, preventing damage to the charge/discharge control FETs and reducing power loss by accurately distinguishing between load rush currents and overcurrents, thereby ensuring reliable operation and extending the lifespan of the battery pack.
Implementation Method 1
a charge/discharge current is detected on the basis of a potential difference between both ends of a source-drain resistor of the charge/discharge control FET
Implementation Method 2
A charge/discharge current is detected on the basis of a voltage drop due to the on resistance of the discharge control FET
Data Source
AI summary
A battery pack for an electrically powered tool includes: one or plural secondary batteries connected in series and/or in parallel; a current detection resistor in a current path through which a charge/discharge current flows into each secondary battery; an N-channel charge control FET and an N-channel discharge control FET in the current path so as to control the charge/discharge current; and a controller detecting the charge/discharge current flowing in the current detection resistor and controlling the charge and discharge control FETs based on the detection result. A threshold value indicating an overcurrent detection current with respect to the charge/discharge current is set in advance, and the controller compares the charge/discharge current detected by the current detection resistor with the threshold value, and when the detected charge/discharge current is the threshold value or more, determines that an overcurrent state is detected and turns off the charge and discharge control FETs.