New York-based battery management specialist Sendyne has obtained a patent for an invention that dynamically reduces offset error in shunt-based DC current measurement systems.
Offset error – an erroneous value read by the measurement system when the actual current is zero – has a significant impact when Coulomb counting information is utilized for State-of-Charge (SOC) determination. The impact of offset error can be pronounced during repeated charge-discharge cycles such as the ones experienced by hybrid car batteries, resulting in battery operation significantly outside the SOC limits set by cell manufacturers.
As offset error changes dynamically with temperature, it cannot be corrected through a single calibration. The Sendyne invention enables dynamic calibration during all stages of battery operation, reducing error to less than 7 mA in a full scale measurement of 500 A.
Reducing the offset error to near zero enables the use of small-resistance shunts with low output voltages for precisely measuring a wide dynamic range of currents while producing minimal waste heat.
Sendyne’s invention also allows the use of proper EMI filters, completely zeroing out their contribution to offset error. Because current measurement systems are used in noisy environments, such as power systems that convert electrical energy, the ability to use EMI filters can be important.
This invention is used in Sendyne’s SFP family of current, voltage and temperature measurement ICs and modules, which provide this level of accuracy over a temperature range of -40° to 125° C.