08-16-2017, 10:16 PM
Battery Technology Life Verification Test Manual
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INTRODUCTION
This manual has been prepared to guide battery developers in their effort to successfully
commercialize advanced batteries for automotive applications. The manual includes criteria for
design of a battery life test matrix, specific life test procedures, and requirements for test data
analysis and reporting. Several appendices are provided to document the bases for the procedures
specified in the manual. Previous battery life test procedures published in FreedomCAR battery
test manuals (References 8 through 11) are superceded by this present Technology Life
Verification Test (TLVT) manual.
This introduction presents the FreedomCAR battery life goals and life verification
objectives, along with the general approaches for life test matrix design, reference performance
testing, and life test data analysis. Organization of the manual is then summarized.
1.1 FreedomCAR Battery Life Goals
FreedomCAR battery life goals for two representative power-assist hybrid-electric vehicle
(HEV) applications are presented in Table 1 of Reference 8. The calendar life goals are the same
for all automotive applications 15 years in service. The cycle life goals depend on the powerassist
ratings 240,000 cycles at 60% of rated power, plus 45,000 cycles at 80% of rated power,
plus 15,000 cycles at 95% of rated power. A cycle consists of a power profile that includes the
vehicle operations of engine-off, launch, cruise, and regenerative braking. This set of three
operating conditions corresponds to the 90th percentile of automotive customer requirements.
Similar requirements have been specified by FreedomCAR for 42-volt applications (Reference 9),
fuel cell powered vehicles (Reference 10) and ultracapacitors (Reference 11).
1.2 Battery Technology Life Verification Objectives
Commercialization of advanced batteries for automotive applications requires verification
of battery life capability in two distinct stages. The first stage, addressed in this manual,
demonstrates the battery technology s readiness for transition to production. The primary
objective is to verify that the battery is capable of at least a 15-year, 150,000-mile life at a 90%
confidence level. An important secondary objective is to provide data for optimization of the
battery product design and usage. These objectives need to be met with minimum cost and time
expended for life testing. This implies careful use of accelerated life testing at elevated levels of
key stress factors. For promising technologies, it is expected that the life verification costs will
be shared by the developer and FreedomCAR. Test articles will be prototypical battery cells.
The second stage of life verification is an integral part of product design verification,
conducted jointly by a production battery supplier and an automotive original equipment
manufacturer (OEM). The objectives are to (1) demonstrate that the complete battery system
meets the life target for its intended usage by the 90th percentile customer, and (2) confirm
product warranty policy and projected warranty costs. Detailed requirements for this stage of life
verification are subject to OEM/supplier negotiation, under timing and budget constraints for
vehicle development. Testing of full battery systems from production-capable facilities is
generally required.
Prerequisites for battery technology life verification testing are as follows:
1. The development status of a candidate technology must be such that its key materials and
fabrication processes are stable and completely traceable.
2. A high percentage of cells produced must represent the best of the technology.
3. Life-limiting wearout mechanisms must be identified and characterized by physical
diagnostic tools.
4. Battery life models should be available, calibrated by special short-term test results as
appropriate.
5. Parallel evaluation of alternative cell designs, materials, and fabrication processes should
be completed.
6. Detailed cell production planning should be in progress.
1.3 Battery Life Test Matrix Design Approach
Battery technology life verification testing includes a range of stress factors appropriate to
achieving high, but relevant, acceleration factors. The goal is to verify (with 90% confidence)
that the battery life is at least 15 years by using only one to two years of accelerated life testing.
Thus, an acceleration factor of at least 7.5 is desired at the highest level of combined stress
factors. To be relevant, an elevated stress factor must induce a wearout failure mode that truly
represents the failure modes that will occur in normal service. Selection of specific stress factors
and levels must be based on a thorough understanding of the relevant wearout modes for the
candidate technology. Stress factors that should be considered include (a) temperature, (b) state
of charge (SOC), © rate of discharge energy throughput, and (d) discharge and regenerative
pulse power levels. Each combination of stress factors must correspond to a known (or
estimated) acceleration factor.
Design of the life test matrix should be based on established design-of-experiment
principles (e.g., Reference 5). This will minimize life test program cost and maximize confidence
in the resulting life projections. Although test efficiency is desired, the life test matrix must also
reflect known or suspected interactions of the stress factors. Confounding of effects for critical
stress factor interactions must be avoided.
Reference Performance Testing Approach
At fixed time intervals during life testing, each cell will be subjected to a reference
performance test (RPT) to measure its cumulative deterioration at its specified stress levels. In
contrast to previous life test protocols, the specified RPT procedure minimizes the time spent offtest
and possible reduction in life due to irrelevant stresses induced by the RPT. More extensive
RPTs may be conducted on supplemental life test cells (outside the core life test matrix) to assess
deterioration in such performance parameters as rated capacity and cold-cranking power. The
minimum RPT primarily will measure power at a reference temperature (30 C) and specified
minimum operating SOC. Power at the maximum operating SOC, as well as capacity from the
maximum SOC to the minimum SOC, will also be measured. The RPT power measurements will
be adjusted to account for measured cell temperatures that differ from the reference temperature.
Life Test Data Analysis Approach
An empirical procedure for estimating battery life from RPT data has been developed.
Assuming a general model of ASI versus time (see Appendix A), it uses a proven data analysis
method [Reference 4] that minimizes the effect of test measurement noise on the life projections.
The assumption is that power fade mechanisms are the dominant mode of battery wearout.
Reliable projection of battery life requires estimating the power fade rate at each stress level in
the life test matrix. These deterioration rate estimates are then extrapolated back to the lower
stress levels expected in normal vehicle usage.
The effects on battery life of stress factors included in the scope of the supplemental life
test matrix will be estimated by comparing those factors with the results from the core life test
matrix, using standard statistical methods.
Organization of the Manual
This manual is organized into three major sections, plus references and appendices, as
follows. Section 2 contains requirements for design and verification of the life test experiment,
including (a) characterization of battery failure modes, (b) selection of stress factors and stress
levels, © design and verification of the core life test matrix, and (d) design of a supplemental
life test matrix. Section 3 contains specific life test procedures for (a) initial characterization of
all cells, plus supplemental characterization of selected cells, (b) stand (i.e., nonoperating) test
and cycling test of cells in the core life test matrix, and © special tests for cells in the
supplemental life test matrix.