EVSTF-06-07e
State of Charge Adjustment
Preamble/Justification
The EVS-GTR specifies test methodology for evaluating propulsion battery performance in vehicle crash
tests and in various abuse conditions. In most of these tests, the performance criteria are that no fire or
explosion shall occur.
Li-ion based rechargeable electrical energy storage systems (REESSs) have demonstrated significantly
decreased tolerance for abuse as state of charge (SOC) is increased. In regards to safety, selfpropagating thermal heating response is the most relevant failure mode and is strongly affected by the
REESS SOC. As described in Figure 1, the peak heating rate of a cell in a thermal runaway event for
REESS systems is stable at low SOC, but increases in a non-linear exponential rate as SOC nears 100%.
Peak Heating Rate (W)
700
600
500
400
300
200
100
0
0
50
100
150
State of Charge (%)
Figure 1. Peak heating rate (W) versus state of charge (Data acquired from 16 Ah production level
pouch type LMO/NMC cells by Sandia National Laboratory for a NHTSA sponsored “Stranded Energy
Project”)
Experimental data shows that the onset temperature of thermal runaway of cells in a Li-ion based REESS
is lower and enthalpy, and heat release rate in the event of a fault is greater at higher levels of SOC. For
this reason, standardizing the SOC during safety testing is critical for relevant, reliable and repeatable
test responses. Generally, Li-ion based REESS in electric vehicles operate in the stable range of SOC
(where peak heating rates are low). However, there could be REESS designs with cells operating at
higher range of SOC to increase energy capacity. Ideally, all testing should be conducted at the
maximum SOC under normal use as specified by the manufacturer to fully evaluate the safety
implications of the test. However, due to practical limitations for conducting the test, batteries and
battery electric vehicles may have to be tested at a lower SOC.
Though NHTSA recognizes that open circuit voltage is not a direct measure of SOC in Li-ion based REESS,
for NHTSA’s crash tests of electric vehicles (EVs) and plug in hybrid electric vehicles (PHEVs); prior to
testing the agency sought input from the manufacturer on the voltage at maximum SOC under normal
use or the range of normal operating voltage. Table 1 shows the open circuit battery voltage measured
following a completed charge cycle as specified under normal use by the manufacturer and the
measured voltage at the time of conducting the crash test. These tests were conducted in 2011-2012
and no attempt was made to maintain a certain level of SOC. Nevertheless, the drop in voltage at the
time of conducting the test from the manufacturer’s specified voltage at maximum SOC is less than 5 %.
Table 1: Voltage at maximum SOC under normal use, pre-test SOC, and percentage drop in voltage of
EVs and PEVs crash tested under 2011-2013 NCAP.
Vehicle
2013 Ford C-Max Energi
SEL
2013 Ford C-Max Energi
SEL
2013 Ford C-Max Energi
SEL
2013 Nissan Leaf
Body Style
Engine Type
Max SOC
(OEM) V
Pre-Test
SOC V
% drop in
voltage
345
337.4
2.2
345
340.7
1.2
345
334.7
3.0
5-Dr Hatchback
Gas-Plug-In Electric
Hybrid
Gas-Plug-In Electric
Hybrid
Gas-Plug-In Electric
Hybrid
Electric
400
393.8
1.6
2013 Nissan Leaf
5-Dr Hatchback
Electric
400
393.9
1.5
2013 Nissan Leaf
5-Dr Hatchback
400
393.5
1.6
2013 Ford Fusion SE Energi
4-Dr Sedan
345
338.5
1.9
2013 Ford Fusion SE Energi
4-Dr Sedan
345
338.7
1.8
2013 Ford Fusion SE Energi
4-Dr Sedan
345
330.5
4.2
2012 Mitsubishi i-MiEV
5-Dr Hatchback
Electric
Gas-Plug-In Electric
Hybrid
Gas-Plug-In Electric
Hybrid
Gas-Plug-In Electric
Hybrid
Electric
360
359.4
0.2
2012 Mitsubishi i-MiEV
5-Dr Hatchback
Electric
360
359.4
0.2
2012 Mitsubishi i-MiEV
5-Dr Hatchback
Electric
360
357.7
0.6
2012 Ford Focus Electric
5-Dr Hatchback
Electric
350
349.3
0.2
2012 Toyota RAV4 EV
5-Dr SUV
Electric
386
380.2
1.5
2012 Chevrolet Volt
2012 Mitsubishi i-MiEV
Hybrid
2012 Chevrolet Volt Hybrid
5-Dr Hatchback
Electric
390
389
0.3
5-Dr Hatchback
Electric
360
359
0.3
5-Dr Hatchback
Electric
390
379.5
2.7
2011 Chevrolet Volt
5-Dr Hatchback
Electric
390
389.1
0.2
2011 Nissan Leaf
5-Dr Hatchback
Electric
400
387
3.3
5-Dr Hatchback
5-Dr Hatchback
5-Dr Hatchback
Independent contracted testing laboratories which have conducted New Car Assessment Program
testing for NHTSA have indicated the relative simplicity in attaining and maintaining SOC at 95% of the
maximum SOC under normal use as specified by the manufacturer for electric vehicles (EVs) and plug in
hybrid electric vehicles (PHEVs)by charging the vehicle just prior to testing. For this reason, NHTSA
recommends that for a vehicle with a REESS designed to be externally charged, the SOC when the
vehicle is tested should not be less than 95% of the maximum SOC under normal use as specified by the
manufacturer.
For vehicles with no external charging capability such as hybrid electric vehicles (HEVs) and hydrogen
fuel cell vehicles (HFCVs), the adjustment of SOC immediately before the crash test will not be possible
as the fuel will be replaced with non-flammable materials. Therefore, for vehicles with a REESS designed
to be charged only by an energy source on the vehicle, we agree with the Task Force 6 recommendation
that the SOC when the vehicle is tested shall be no less than 90% of the highest SOC achievable under
normal operation of the vehicle (as specified by the manufacturer).
For component tests of REESS of HEVs, it may not be possible to charge the REESS when outside the
vehicle. Therefore, we agree with the Task Force 6 recommendation that the initial SOC of the REESS
which cannot control SOC by itself shall be no less than 95% of the highest SOC which is achievable
under normal operation of the vehicle and that the SOC when the REESS component test is conducted
shall not be less than 90% of its initial SOC. For other REESSs, we agree with Task Force 6 that the SOC
when the component test is conducted shall be no less than 95 % of the highest SOC in accordance with
the procedure specified by the manufacturer for normal use.
We understand that determining SOC is specific to the REESS and may be dependent on a number of
measured parameters, including temperature, voltage, current, and method of charging. For some
REESSs, there may not be a direct relationship between open circuit voltage and SOC and so it may not
be possible to use OCV-SOC curves for determining SOC. As a result, the manufacturer may need to
provide a method of reading SOC from the CAN bus or provide levels of measured parameters and
associated tables to determine maximum SOC in normal use, and pre-test SOC. NHTSA recommends
Task force 6 to consider the feasibility of a common/standard method of SOC calculation or CAN
reporting for determining pre-test SOC.
NHTSA does not agree with the Task Force 6 recommendation to specify a time limit for when the test
should be conducted (48 hours from the time of initial SOC measurement). While this specified time
period may not be sufficient to significantly reduce SOC in some vehicles, it may be long enough to
significantly lower SOC values in others below the minimum threshold. Additionally, for vehicles that
can maintain SOC above minimum threshold levels for a long period of time (greater than 48 hours), the
specified testing time may be overly restrictive for conducting the test. Therefore, we recommend not
specifying a time limit for conducting the test. Instead, we recommend obtaining information from the
manufacturer to determine maximum SOC under normal use and pre-test SOC.
A battery pack maximum SOC depends on various factors including temperature, and type of charging
and therefore need to be specified. Experimental data from a 6 Ah Li-ion battery tested at C/5 rate
(Figure 2) shows that for a temperature range between 10 oC and 30 oC (20 oC ± 10 oC), the variation in
maximum capacity in ampere-hour (Ah) is ±5%.1
1
From a JRC document – Initial condition of SOC in EVS-GTR test procedures.
Figure 2. Maximum capacity versus temperature for a 6Ah lithium ion battery tested at C/5 rate.
In order to avoid discrepancies due to differences in SOC at different temperatures, we agree with JRC’s
proposal to use a narrow temperature range for testing REESS while taking into consideration
practicality of conducting the tests. Since vehicle crash tests are often conducted outdoors where
control of ambient temperature is not possible, a reasonable temperature variation of ±10 oC is
recommended. On the other hand, for component tests (which can be conducted under controlled
environmental conditions), a narrower temperature range of ±5 oC is recommended.
Therefore, we agree with the Task Force 6 recommendation for conducting SOC adjustment at an
ambient temperature of 20 ± 10 °C for vehicle-based tests and 22 ± 5°C for component-based tests. We
also agree with the Task Force 6 recommendation that where different charging procedures are
possible, the REESS shall be charged using the procedure which yields the highest SOC.
Based on the above evaluation, NHTSA offers the following changes to the regulatory text developed by
Task Force 6 for consideration.
II. TEXT OF REGULATION
The paragraph 6.1.5.2.1., amend to read
6.1.5.1.2.1.
The SOC of the REESS shall be adjusted in accordance with the
paragraph 6.2.1.2..
Insert a new paragraph 6.2.1
6.2.1.
General procedures
Renumber former paragraphs 6.2.1. to 6.2.1.1.
6.2.1.1.
Procedure for conducting a standard cycle
Insert new paragraphs 6.2.1.2-6.2.1.2.3., to read
6.2.1.2.
Procedures for SOC adjustment
6.2.1.2.1.
The adjustment of SOC shall be conducted at an ambient temperature
of 20 ± 10 °C for vehicle-based tests [and 22 ± 5°C for component-based
tests.]
6.2.1.2.2.
The SOC of the tested-device shall be adjusted according to one of the
following procedures as applicable. Where different charging procedures
are possible, the REESS shall be charged using the procedure which yields
the highest SOC.
(a) For a vehicle with a REESS designed to be externally charged, the
REESS shall be charged to the highest SOC in accordance with the
procedure specified by the manufacturer for normal use until the
charging process is normally terminated.
(b) For a vehicle with a REESS designed to be charged only by an
energy source on the vehicle, the REESS shall be charged to the
highest SOC which is achievable with normal operation of the
vehicle. The manufacturer shall advise on the vehicle operation
mode to achieve this SOC.
(c) In case that the REESS or REESS sub-system is used as the testeddevice, the tested-device shall be charged to the highest SOC in
accordance with the procedure specified by the manufacturer for
normal use until the charging process is normally terminated.
Procedures specified by the manufacturer for manufacturing, service
or maintenance may be considered as appropriate if they achieve an
equivalent SOC as for normal use. In case the tested-device does not
control SOC by itself, the SOC shall be charged to not less than
[95 %] of the maximum normal operating SOC defined by the
manufacturer for the specific configuration of the tested-device.
6.2.1.2.3.
6.2.1.2.3
After the adjustment of the SOC according to paragraphs 6.2.1.2.1.
and 6.2.1.2.2. above, the test shall be started within 48 hours subject to
taking all reasonable and practical steps to ensure the initial SOC (i.e. the
SOC achieved at the end of the adjustment according to the paragraphs
6.2.1.2.1. and 6.2.1.2.2. above) is maintained, otherwise an appropriate
verification shall be made to confirm that the SOC is maintained not less
than 90 % of the initial SOC e.g. by checking the voltage-drop and using
the OCV-SOC curve of the REESS, etc.
When the vehicle or REESS subsystem is tested, the SOC shall be no less
than 95 % of the SOC according to paragraphs 6.2.1.2.1. and 6.2.1.2.2. for
REESS designed to be externally charged and shall be no less than 90 % of
SOC according to paragraphs 6.2.1.2.1. and 6.2.1.2.2. for REESS designed
to be charged only by an energy source on the vehicle.
(Subject to confirmation by relevant TFs)
The paragraphs 6.2.2.3.1.(b), 6.2.3.3.1.(b), 6.2.4.3.1.(b), 6.2.5.3.1.(b), 6.2.6.3.1.(b), 6.2.7.3.1.(b),
amend to read
6.2.2.#.1.
General test conditions:
(b)
at the beginning of the test, the SOC shall be adjusted in accordance
with the paragraph 6.2.1.2.