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Sendyne Corp. Impact of Current Measurement Offset Error in SOC monitoring of HEVs Monitoring of State of Charge (SOC) in Hybrid Electric Vehicle (HEV) batteries is important for maintaining operation of the battery within the manufacturer’s specifications for Depth of Discharge (DOD). Adherence to such specifications is important for the longevity and health of the battery pack as well as for the fuel consumption of an HEV. Because HEV batteries operate in a narrow SOC window (typically between 40-60% SOC), differences in voltage in this window can be small, making it difficult to use voltage measurement for determination of SOC with a reasonable level of accuracy. Further, voltage measurements cannot provide trustworthy information while the vehicle is in operation. Therefore, the battery management system (BMS) must rely on Coulomb counting through the current measurement system to determine SOC. Besides gain error, offset error of the current measurement system has a significant impact on the accuracy of the SOC estimation. Offset error is the erroneous value of current read by the measurement system when the actual current is zero. The offset error changes dynamically with temperature and cannot be corrected through a single calibration. To appreciate the contribution of offset error in SOC determination, a simulation was performed of a driving cycle of an actual HEV, based on data provided by the Argonne National Laboratory Downloadable Dynamometer Database. The simulation showed that even during a 45 minute drive, a low accuracy current measurement system may introduce more than 4% error in SOC estimation. The impact of this error in estimation is that the battery may be operating up to 20% outside of the manufacturer warrantied range. The Sendyne SFP10x family of current measurement ICs exhibit an offset error of less than 14 ppm (parts per million), which translates to an error of less than 7 mA in a full scale of 500 A over a temperature range of -40 oC to +125 oC. Confidential Internal Use Only ©2015 Sendyne Corp. 1 Accumulation of error of State of Charge (SOC) estimation during a 45 minute drive 200 2013 Ford Cmax Hybrid UDDS CS+HS 1.4 kWh/5.0 Ah 150 100 55 48.9% 50 50 0 45 47.3% 40 SOC with 100 mA offset error 35 0 500 1000 SOC actual 1500 Load Current 2000 2500 Time (s) “This data is from the Downloadable Dynamometer Database (http://www.transportation.anl.gov/D3/) and was generated at the Advanced Powertrain Research Facility (APRF) at Argonne National Laboratory under the funding and guidance of the U.S. Department of Energy (DOE)” Effect of offset error on State of Charge (SOC) error 5 4.61 ΔSOC Error during a 45' UDDS drive cycle 4 SOC Error (%) SOC (%) 60 3.08 3 2 1.54 0.77 1 Sendyne SFP10X 0.11 0 0 50 100 150 200 Offset error (mA) 250 300 350 -50 -100 -150 Current (A) 65