The closest most electrical engineers come to chemistry is probably through courses they took in college. Right now they are probably glad they went into the electronics rather than chemistry because chemists haven’t been able to solve current problems with lithium-ion batteries. These problems started with a fire on a Japan Airlines 787 in Boston. A similar situation occurred a week before when smoke poured from the same battery system on an All Nippon Airways plane, forcing it to make an emergency landing.
Following these events, the U.S. Federal Aviation Administration (FAA) issued an emergency directive, grounding all Boeing 787s operated by American airlines until Boeing can prove the batteries are safe. Other country’s regulators then followed suit.
“The battery failures resulted in release of flammable electrolytes, heat damage and smoke on two 787 airplanes,” the FAA said. “These conditions, if not corrected, could result in damage to critical systems and structures, and the potential for fire in the electrical compartment.”
The National Transportation Safety Board (NTSB) is examining the lithium-ion battery from the 787 that caught fire in Boston, along with “black box” data from the airplane. NTSB currently believes the battery did not suffer an overcharging. Japanese investigators concluded the same thing after looking at the 787 that made an emergency landing in Japan.
The Dreamliner is the first airliner to rely on lithium-ion batteries for electrical power. It has two of these, about 10 in wide, 14 in long and 8 in high; they weigh 63 pounds. One is located in the rear electrical equipment bay, near the wings. It is used to start the auxiliary power generator, a small engine that is primarily used to power the plane when it is on the ground. The second battery powers up the pilot’s computer displays and serves as a back-up for flight systems. Other airliners use traditional mechanical and hydraulic systems that divert power from the engines to run electrical equipment. Where batteries are used, they have been traditional ones such as lead-acid or nickel-cadmium. Compared with the larger 777, the 787’s electrical system is five times more powerful and can produce enough electricity to power 500 homes.
Dr. Peter Harrop, Chairman, IDTechEx said the larger a lithium-ion battery is, the more there is to go wrong. Those making safe small versions for phones or tablets cannot necessarily make safe big ones. To some extent, improved temperature performance from different cathodes can correlate with improved safety, though no lithium-ion cell is inherently safe.
Lithium-based batteries are popular because lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density for weight. The energy density of lithium-ion is typically twice that of the standard nickel-cadmium cell. The load characteristics are reasonably good and behave similarly to nickel-cadmium in terms of discharge. Li-ion’s 3.6 V/cell allows battery packs with only one cell. Most of today’s mobile phones run on a single cell.
Li-ion has no memory (like NiCad) and no scheduled cycling required to prolong battery’s life. And, its self-discharge is less than half of nickel-cadmium.
Despite its advantages, lithium-ion has drawbacks. It is fragile and requires a protection circuit to limit the peak voltage of each cell during charge and prevent the voltage from dropping too low on discharge. In addition, cell temperature is monitored to prevent temperature extremes.
Aging is a concern with most Li-ion batteries. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years. Other chemistries also have age-related degenerative effects, including NiMH when exposed to high ambient temperatures.
Storage in a cool place slows the aging of Li-ion cells. Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should be partially charged during storage; the recommendation is a 40% charge.
If the chemists can’t get it right, maybe electrical engineers can come up with a foolproof technique that safeguards the batteries and the associated equipment.