A battery pack is only as strong as its weakest cell. As battery packs become larger and more integral to the stable operation of devices and systems, the need to monitor battery dependability and performance in real time and without interruption has become more important. When a battery fails, it is typically because it has built up too much internal resistance to supply a useful amount of power to an external load.
It is impossible to directly measure internal resistance of a device connected to a larger system. With conventional circuit measurement methods, resistance is derived by measuring rapid changes in voltage and current. The need for rapid signal changes and an embedded current measurement shunt creates difficulty in adequately assessing impedance during realistic operating conditions.
Active Measurement Cancellation (AMC) has been developed for measuring impedance. This real-time method is effective for measurement of isolated components as well as active, interconnected circuits, allowing impedance measurement in-situ. The AMC circuit is capable of canceling the effects of the interconnected circuit, measuring only the response of the desired component, e.g. a battery cell, thereby allowing accurate, immediate impedance measurements of the components even when in use.
AMC measures impedance without the use of an embedded system shunt; no post-measurement calculations are necessary. It works by connecting a measurement unit to one section of a battery string, and a measurement cancellation unit to a separate section. In an operating battery string, the added effects of the separate positive and negative measurements add up to 0.
The two measurement points can accommodate standard impedance measurement equipment such as a simple handheld digital voltmeter. Cells are measured in place, and the measurement signal sees only the resistance between the measurement terminals. Measurements can occur at any time, whether the battery module is charging, discharging, or resting.