Download Sendyne Corp. Impact of Current Measurement Offset Error in SOC

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