To compete with conventional vehicles, electric-drive vehicles (EDVs) and their batteries must perform reliably for 10 to 15 years in a variety of climates and duty cycles. NREL researchers use the battery life-predictive model, together with systems-level vehicle thermal design models, to assess lithium-ion (Li-ion) battery:
- Chemical and mechanical degradation caused by environment and cycling
- Performance, lifespan, and cost tradeoffs
- Excess power, energy, and thermal management system requirements
- Warranty, second use, and other business decision factors.
This suite of tools pairs NREL's high-fidelity battery degradation model with electrical and thermal performance models specific to batteries and larger systems, with versions for behind-the-meter (BLAST BTM-Lite), vehicle (BLAST-V), and stationary (BLAST-S) applications.
Predictive models of Li-ion battery reliability must consider the multitude of electrochemical, thermal, and mechanical degradation modes experienced by batteries in application environments. Complicating matters, storage and cycling patterns can trigger varied degradation pathways for Li-ion batteries. Rates of degradation are controlled by factors such as temperatures, electrochemical operating windows, and charge/discharge rates.
Lifetime Prediction Modeling
The NREL lifetime model is the only predictive model of its kind, extending expensive laboratory battery-aging datasets to real-world scenarios. The model captures degradation effects due to both calendar time and cycle aging, including constant discharge/charge cycling, as well as more complex cycling profiles, such as those found in vehicles and grid storage applications.
The model's trial functions are statistically regressed to Li-ion cell life datasets where cells are aged under various conditions. Using this statistical framework, degradation mechanisms, along with temperature and cycling factors, are regressed to produce a lifetime prognostic model.
NREL's lifetime prediction model has been licensed to multiple companies and is integrated into other NREL analysis tools to improve the fidelity of vehicle systems, fleet evaluation, ancillary load, and battery ownership modeling analyses. The model is also applied in real-time control algorithms.
Physics of Degradation
Predictive models are also used to provide feedback during the cell design process. Compared to experimentation, physical models of stress and degradation allow engineers to better understand the impacts of design concepts on lifetime and accelerate the design process. For example, predictive models can enable a designer to reduce stresses in an electrode stack to avert a shortened lifespan.
Multi-physics models of battery degradation include:
- Non-uniform degradation due to temperature and potential imbalance in large cells
- Solid/electrolyte interphase layer formation and growth
- Micro-scale and macro-scale mechanical stress and degradation, coupled with electrochemistry and temperature
NREL's Publications Database offers a wide variety of documents related to the development of batteries and energy storage systems for EDVs. The following publications document project activities in modeling of battery lifetime:
- Predictive Models of Li-ion Battery Lifetime (2014)
- Probing the Thermal Implications in Mechanical Degradation of Lithium-Ion Battery Electrodes (2014)
- Advanced Models and Controls for Prediction and Extension of Battery Lifetime (2014)
- Models for Battery Reliability and Lifetime (2013)
- Battery Wear from Disparate Duty-Cycles: Opportunities for Electric-Drive Vehicle Battery Health Management (2012)
- Variability of Battery Wear in Light Duty Plug-In Electric Vehicles Subject to Ambient Temperature, Battery Size, and Consumer Usage (2012)
- Comparison of Plug-In Hybrid Electric Vehicle Battery Life Across Geographies and Drive Cycles (2012)
- Modeling Detailed Chemistry and Transport for Solid Electrolyte Interface (SEI) Films in Li-ion Batteries (2011)
- Modeling of Nonuniform Degradation in Large-Format Li-ion Batteries (2009)