March 14, 2013
Tests of the revised Boeing 787 battery system are expected to be completed “within weeks” and will involve only a single flight test, says VP and 787 Chief Project Engineer Mike Sinnett.
Revealing full design details of the revised lithium ion battery system, Sinnett also discloses that the enclosure system developed for the battery is designed to prevent a fire and not, as widely reported, to contain one. This fundamental fact, unknown until Boeing revealed details of the new system during a March 15 press conference in Tokyo, will be pivotal in the FAA’s coming assessment of whether the modified 787 can be allowed to return to unrestricted flight.
The enclosure is one of several features of what Boeing calls a “comprehensive solution” to provide multiple layers of protection for the battery system. The battery itself is modified with additional spacers and insulation to improve the thermal and electrical isolation, added insulation layers, cells individually wrapped in electrical isolation tape and wire harnesses encased in heat and chafe resistant sleeves.
“We made changes to avoid propagation from one cell to another, better heat resistant sealing and added high temperature dielectric isolators above and below the battery cell. We also added dielectric protection in the spaces between them,” says Sinnett.
The battery also will sit on a redesigned frame containing drain holes to allow moisture to escape. In its testing, Sinnett says Boeing found that moisture paths can lead to short circuits in cells which could lead to “stress in the cell,” or the build up of heat and venting of vaporized electrolytes.
The redesign also addresses the risk of overcharging, which Boeing’s original design work indicated was the only probable cause of a serious battery fire. “At an airplane level, what we are concerned about is a whole battery event that is so energetic it puts the aircraft at risk through heat and flame. The only known ability to create what I just described is overcharging,” says Sinnett.
A revised charger will reduce the maximum charging levels to prevent overcharging and increase the minimum charging level to prevent “deep” discharging. The revised system also will “soften” the wave form in the charging sequence.
Sinnett says that during the two test flights in February that followed the FAA’s initial clearance to fly ZA005 for data collection, “we saw a charging signature which made us ask questions of whether the battery could be seeing stress as a result of that signature. We decided to change the circuitry in the charger so we didn’t see that signature any more, in the case it was a contributor. We don’t know if it was a contributor but we addressed it anyway.”
The large enclosure is designed to eliminate the potential for fire, contain any electrolyte released from the battery, hold heat and pressure and channel vapors and odors overboard via an attached vent line. “We have built an enclosure that will house the battery on the aircraft. This is very important because it eliminates the possibility of fire. This enclosure keeps us from having a fire to begin with. It helps us vent vaporized electrolyte overboard. The gases go immediately overboard. They don’t go into the electrical/electronic equipment bay,” explains Sinnett.