Boeing had fair warning that there could be problems with lithium-ion (Li-ion) batteries but went ahead and used them in its 787 Dreamliner. Now the fleet is grounded because at least one Li-ion battery has caught fire.
The warning came from Cessna and its CJ4 business jet. The CJ4, which is the biggest, fastest and longest range member of the Citation CJ family of light business jets, was the first to use a Li-ion battery as the main ships battery when the airplane entered service in 2010.
The CJ4 has one of the most advanced electrical systems of any light business jet because it has four power sources instead of the normal two. There are the primary starter-generators, one on each engine, and either one can carry the entire electrical load. Each engine also has a small wild frequency alternator that provides power for anti-ice heating elements. That is not unusual in jets, but what is different is that Cessna installed rectifiers that can convert the AC output of either small alternator into enough DC power to keep all essential equipment operating if both primary generators fail.
On top of that, the Li-ion battery seemed like magic. The battery weighed about 30 pounds less than a conventional battery, but had enough capacity to power all equipment necessary for night IFR flight for 45 minutes instead of the required 30 minutes.
The CJ4 is a terrific performer and a delight to fly. It’s hard to believe that the little original CJ could grow into a jet of such speed, range and cabin comfort. And I remember the Cessna people telling me when I first flew it how well the Li-ion battery was performing and how extensively it had been tested to earn certification.
Being first with the Li-ion battery the FAA issued what it calls special conditions for CJ4 certification. The battery had to be heated and abused more than a normal nicad or lead acid battery, and it had to be treated as a potential fire hazard because under rare conditions the electrolyte can burn. Cessna went well beyond the FAA requirements and the Li-ion battery performed flawlessly in testing and certification flying.
But when the CJ4 entered service there were soon problems with the Li-ion battery. It wasn’t performing as it had during certification testing. And at least one Li-ion battery overheated to the point of smoking, if not actually on fire. The FAA issued an AD requiring all of the Li-ion batteries to be replaced by a nicad or lead acid.
I’m sure Boeing engineers feel at least a little superior to little old Cessna so they may not have paid much attention to the CJ4 Li-ion battery experience. And the source of problems in the CJ4 is believed to be ground power units set for the wrong voltage. Large airplanes like the 787 have APUs that provide ground electrical power and air conditioning so they are almost never plugged into ground power carts during normal operations.
It’s been something like 50 years since new battery technology was introduced for aircraft and that was the nicad. And it didn’t go well at first. Like the Li-ion, the nicad was smaller and lighter than a lead acid battery of the same capacity. And the nicad has lower internal resistance so it can pump out more amps quickly than a lead acid battery, and that is very important during a turbine engine start when the engine must accelerate rapidly or melt.
But the lower internal resistance of a nicad also makes it more susceptible to a thermal runaway than a lead acid battery. Pilot legend has it that at least a few nicad batteries suffered thermal runaways and melted their way right through the battery mount and fell out of the airplane.
The concern over nicad overheating is so great that all nicads have temperature monitoring. If the battery heats beyond a certain threshold—typically around 160 degrees F—it becomes an emergency and the battery is switched out of the charging system by the pilots. If the battery temperature doesn’t start down once the charge is removed the checklist calls for an emergency landing as soon as possible.
There have been improvements in lead acid battery technology to the point that the nicad’s advantages have diminished. Many turbine aircraft operators have switched to lead acid because the cost is much lower than for a nicad, and no special maintenance is required for the lead acid as for the nicad. The lead acid battery doesn’t last as long between replacements, but the cost difference is enough that lifecycle cost is typically lower for the lead acid. And you don’t need to worry about thermal runaway with the lead acid.
It took several years, but the early problems with nicad batteries have been resolved. So will it just take time to get the Li-ion battery right, or is it simply the wrong technology for aircraft. The solution in the CJ4 was straightforward with a return to conventional batteries. Will Boeing do the same thing? Or are the characteristics of the Li-ion so fundamental to 787 operation that the big jet couldn’t function with the lower performance of a nicad or lead acid. We’ll see.