Power, Plugs and People

Felix Kramer

Sept. 18, 2006: Thinking About Lithium Batteries and Safety

We've recently seen several high-profile cases with videos of lithium-ion laptop computer batteries burning at near-explosion intensely. (One highly publicized picture of a destroyed truck turned out to have a longer story: laptop batteries had actually ignited ammunition which blew up the gas tank. Dell has recalled millions of Li-ion laptop batteries, followed by Apple. What does this mean for Li-ion battery safety in battery electric (EV) and plug-in hybrid (PHEV) vehicles?

I asked CalCars Tech Lead Ron Gremban to put together an overview of some of the main issues, taking into account what's public information on the approaches of some companies using lithium batteries for cars, such as (alphabetically) AC Propulsion, EnergyCS, Hybrids-Plus, Hymotion, Phoenix Motorcars and Tesla Motors. Ron has tried to strike a balance between technical discussions and broad descriptions, sticking primarily to existing products and solutions. We're aware that this is a very complex subject and that technologies are evolving rapidly. We are describing starting points and rasising questions.

First, we need to remind ourselves that the current automotive fuel source is highly flammable and explosive gasoline, stored in a lightweight steel tank. Over time, we've learned how to make this reasonably safe -- and to live with the occasional explosive consequences, especially in crashes. (In the U.S., over 250,000 vehicles of all types caught fire in 2005. This may amount to 1 in every 1,000 vehicles -- far more than one might expect.)

How will Li-ion battery packs compare? The laptop computer fires that prompted the recent recalls are very rare -- on the order of one in 60 million cells --but with hundreds or thousands of cells in each vehicle, the likelihood of failure is both greatly increased and more dangerous.

Though we hope all manufacturers will use best design practices, including safety features detailed below, it's impossible to anticipate all possible modes of failure. Until enough EVs and PHEVs are on the road to gather meaningful statistics, we'll be guessing about what could happen. Even then, some designs will fare better than others, and we will continue to learn from the school of hard knocks.

NOTE: I am not enough of an expert on battery chemistry for this to be used as a design document.

Li-ion batteries' major specific issue is their propensity for thermal runaway. Above a certain temperature, usually 80-150 degrees C, a reaction can occur that produces more heat than can be dissipated. And since each cell contains both fuel and oxidizer, the reaction fuels itself. The temperatures and pressures produced are high enough to melt steel barriers and shoot flames over 20 feet. This makes containment difficult. Additionally, any battery can be shorted, producing an explosive energy discharge if not quickly stopped.

Though proper fusing can greatly reduce the likelihood of serious results from inter-cell shorts, little can be done to save a cell with an internal short, such as occurred in the laptop cells that prompted the recalls. "Rick Clancy, a Sony spokesman, said, the problem appears to have been caused by microscopic metal particles in cavities in the battery cells in each battery pack." (San Francisco Chronicle report) Apparently, the particles occasionally caused an internal short after vibration moved them around.

We can divide available prevention methods into cell chemistry, cell design, electronic control and monitoring, and battery pack design and containment.

CELL CHEMISTRY: Lithium is a very volatile metal that will burn if exposed to air at room temperature. Lithium-ion cells are so named because the lithium in them is not pure but combined into ionic compounds -- a trade-off that increases safety at the cost of reduced specific energy (energy storage per battery weight). Generally, cobalt, manganese, and/or phosphates are used in these compounds for a Li-ion cell's cathode. The anode is usually carbon, and the electrolyte is a flammable organic solvent. If the cell's voltage is allowed to exceed or go below certain values, metallic lithium can "plate out," not only degrading the cell, but making it much more susceptible to future thermal runaway.

Li-ion cobalt batteries, used in laptop computers and cell phones because of their superior specific energy, are the most susceptible to both thermal runaway and the plating-out of metallic lithium. In fact, while reaching a full charge requires a voltage of 4.15V, each cell's voltage must stay under 4.25V to prevent plating! This requires unusually precise electronics on a cell-by-cell (or set of parallel cells) basis (see below).

Manganese is often used in Li-ion cells for power tools because it produces higher specific power (the rate at which energy can be supplied) despite lower specific energy. This chemistry is less susceptible to thermal runaway and plating. Even less susceptible are phosphate-based cells, but they trade off even more specific energy. Three companies -- Valence, Electrovaya, and A123 -- are starting to provide these safer phosphate-based cells for electric vehicles. So far, manganese and phosphate cells have been significantly more expensive than their cobalt brethren. This is mainly due to lower manufacturing volume, but has so far limited their use in cost-effective electric vehicles.

CELL DESIGN: Various fail-safe mechanisms can be, but are often not, built into each Li-ion cell:

* An over-temperature and/or over-pressure cutoff device (usually permanently disabling the cell) can save the cell from thermal runaway if the source of the problem is current into or out of the cell.
* A flame-retardant can be added to minimize the effects of a cell fire. Quallion has developed such an additive. It has not yet been incorporated into consumer product batteries, but it could provide a cost-effective additional layer of safety for EV battery packs.

ELECTRONIC CONTROL AND MONITORING: The tight 4.15-4.25V Li-ion charge voltage tolerance (somewhat different for non-cobalt-based cells) means that each cell (or set of cells in parallel) must have its own precise over-voltage protection circuit. Otherwise, the voltage balance among series strings of cells may vary too much due to variations between cells, or to degradation or failure of a single cell. Though under-voltage protection requires less precision, it is usually included in the same circuits. As protection circuits can occasionally fail and allow the very conditions they were designed to avoid, additional layers of protection are required as well.

The next level of defense is to detect overheating and shut things down before thermal runaway. This, as well as monitoring voltages and temperatures that are still within limits, can catch some battery anomalies that might otherwise eventually turn into catastrophic failures.

BATTERY PACK DESIGN AND CONTAINMENT: A well-designed battery pack will normally prevent cells from heating to unstable temperatures. Even such basic thermal design can be overlooked. And though containment of a multi-cell fire may be difficult, optimal design will prevent a single-cell fire from spreading. Tesla Motors says its testing demonstrates that its liquid cooling and careful design does indeed prevent the spread of a single-cell fire.

Battery containment should be designed to vent fumes from malfunctioning cells rather than send them into the passenger compartment. Packs should be placed beyond crush zones, if possible. For PHEVs they should also be as far as possible from the fuel tank.

The: US Advanced Battery Consortium (USABC) focuses on specifications for batteries used in vehicles. Its Abuse Test Procedures Manual includes mechanical (shock, drop, penetration, crush, etc.), thermal, and electrical abuse tests. This test regime provides an objective standard with which to judge battery packs for safety. The major automobile manufacturers will no doubt (for very good reasons) insist on their vehicles' battery packs passing the USABC tests.

Increased public awareness will no doubt put a spotlight on companies building or modifying vehicles into EVs and PHEVs, to explain their approaches to battery safety. Whether or not they submit their packs to formal USABC testing, the USABC safety criteria must be addressed. Tesla Motors Co-Founder Martin Eberhard, for example, responded to questions in a Technology Review Forum on August 4 and then in a Battery White Paper, Tesla delineated the layers of safety features in its battery pack.

Of course, experimenters must also be cognizant of battery safety issues. Though they may not have the resources to validate their designs through destructive testing, they still need to design for both thermal and electrical safety.

Though safety issues must be addressed in vehicle and battery pack designs, we believe well-designed EV and PHEV vehicles with Li-ion battery packs can be as safe as the gasoline vehicles they replace.