July 23, 2012

EPA Technical Memorandum
Subject:  Analysis of Argonne National Laboratory's HEV Data to Evaluate Methods for Determining Nominal Voltage
From:  James Sanchez, U.S. Environmental Protection Agency 
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For the definition of nominal voltage in chapter 5 of the technical support document the follow data analysis was done.  The data used was 10 Hz battery current and voltage data for a 2010 Toyota Prius, 2010 Ford Fusion Hybrid and 2011 Hyundai Sonata Hybrid tested at Argonne National Laboratory.  The two methods for determine nominal voltage were as follows. 
The first method was averaging the battery voltage just after "key-on" with the battery voltage just prior to "key-off.  Ideally the voltage measurement would be taken with no load or zero current to from the battery, but since this is not possible the data was used to determine a reasonable limit for the current during the voltage measurement.  Figure 1 and 2 is a sample of the current and voltage data at the beginning and end of a test over the UDDS.  

Figure 1 - Battery voltage and current as a percent of maximum current at the start of the UDDS cycle.

Figure 2 - Battery voltage and current as a percent of maximum current at the end of the UDDS cycle.
The second method that was evaluated was averaging the battery voltage over the full UDDS when the battery current was less than 3% of the absolute maximum current during the test.   The results turned out to be very similar to the average of the initial and final voltage but were slightly higher due to the voltage roll off after each charging event.  Table 1 is a summary of the two methods.  The column labeled "Avg. OC Voltage" are the results from method 2.
Table 1  -  Summary of nominal voltage results from the two described methods.
                                    Vehicle
                                     Test
                                Initial Voltage
                                 Final Voltage
                           Initial Current % of Max
                            Final Current % of Max
                           Avg. of Initial and Final
                                Avg. OC Voltage
                            % Difference in Methods
                              2011 Fusion Hybrid
                                       1
                                     271.1
                                     272.1
                                     -0.1
                                     -0.2
                                     271.6
                                     275.6
                                      1%

                                       2
                                     270.2
                                     272.1
                                     -0.1
                                      0.0
                                     271.1
                                     276.7
                                      2%

                                       3
                                     270.9
                                     271.8
                                     -0.1
                                      0.0
                                     271.4
                                     276.7
                                      2%

                                       4
                                     270.5
                                     272.2
                                     -0.1
                                      0.0
                                     271.3
                                     277.0
                                      2%
                                  2010 Prius
                                       1
                                     221.8
                                     224.5
                                      1.8
                                      0.9
                                     223.2
                                     228.1
                                      2%

                                       2
                                     223.4
                                     224.8
                                      1.8
                                      0.8
                                     224.1
                                     228.6
                                      2%

                                       3
                                     223.7
                                     224.2
                                      1.9
                                      0.1
                                     224.0
                                     228.4
                                      2%
                              2011 Sonata Hybrid
                                       1
                                     283.0
                                     281.7
                                      2.5
                                      1.4
                                     282.3
                                     284.0
                                      1%

                                       2
                                     280.7
                                     281.8
                                      2.4
                                      1.5
                                     281.3
                                     283.9
                                      1%

                                       3
                                     280.9
                                     282.2
                                      1.9
                                      1.5
                                     281.6
                                     284.2
                                      1%

                                       4
                                     281.4
                                     281.2
                                      1.7
                                      2.2
                                     281.3
                                     283.8
                                      1%

