Engineering On The Edge

At Engineering on the Edge, we are accustomed to looking far into the future, but for this post, we decided to look into some news that could affect future technologies. The acceptance of current technologies, such as hybrid and electric vehicles, can have a dramatic impact on technologies of tomorrow that build on what companies are producing today. With headlines trumpeting the potential for battery fires in GM’s Chevy Volts, the company has announced engineering changes to the structure around the battery pack as well as the addition of a sensor to observe battery coolant levels.

GM has decided to roll out changes to the structure around the Chevy Volt battery pack. Image courtesy of GM.

These enhancements and modifications will address the concerns raised by the severe crash tests. There are no changes to the Volt battery pack or cell chemistry as a result of these actions. We have tested the Volt’s battery system for more than 285,000 hours, or 25 years, of operation. We’re as confident as ever that the cell design is among the safest on the market. — Mary Barra, GM senior vice president of Global Product Development

No Volts have caught fire outside of testing by the National Highway Traffic Safety Administration, and GM has not issued a recall. The fires occurred days or weeks later in testing, and could pose a hazard for technicians working on the cars who might not follow proper safety procedures. The enhancements are being incorporated into the Volt manufacturing process as production resumes this month. The design changes can also be requested by current Volt owners, who would like peace of mind beyond the 1 million miles of testing and 20 million customer miles that have been driven in Volts.

Because no manufacturer wants to see their product’s safety questioned, DE asked Sandeep Sovani, manager of Global Automotive Strategy, at ANSYS, Inc., how simulation and testing might be used to avoid such problems before production. Here’s what he had to say:

Battery makers are concerned about the potential for fires in the lithium ion batteries used in the latest electric and hybrid vehicles. Two typical causes have been identified that can lead to a cascading reaction known as thermal runaway, which in turn can cause fires. The first is a short in a cell, which may be caused by a crash impact or by an impurity. The second is a blockage or other malfunction in the cooling system that cools the cells by running coolant through microchannels in the battery.

There are hundreds of different variables involved in battery design that interact in complex ways that can affect the potential for thermal runaway as well as having an important effect on the efficiency of the battery. Cell and pack makers perform a considerable amount of testing during the development process to investigate the impact of these variables, but there is never enough time to come anywhere close to investigating the complete design space. Physical testing is also quite limited in its ability to evaluate battery performance under extreme conditions such as vehicle crashes.

Simulation with tools such as computational fluid dynamics (CFD) and thermal analysis can fill the gap by enabling battery makers to evaluate a large combination of design variables under extreme conditions to determine their propensity to produce thermal runaway. For example, at the cell level engineers can simulate a short and determine how much heating will occur, how the electrochemistry in the regions of the cell surrounding the short will be affected, and whether phase change will create gases that may be trapped creating an explosive high pressure. Simulation can be run at the package level to ensure the cooling system is adequate to handle the heat generated by a short and prevent thermal runaway from occurring. With simulation, the cooling system can be extensively characterized over a wide range of scenarios so that the battery management system can be programmed to take effective countermeasures for any scenario.

For more information on hybrid and electric vehicle simulation, read the feature article Sovani co-wrote with Scott Stanton, technical manager at ANSYS, in DE’s January issue.