The APU battery is one of two primary batteries in the 787, the other being the main battery in the forward E/E bay. Both are lithium ion units provided by Japan-based battery manufacturer GS Yuasa as part of the Thales supplied electrical power conversion system. The 787 contract, first announced in 2005, marked the first commercial aviation application of Li-ion technology and was selected over contemporary nickel-cadmium because it provided 100% greater energy storage capacity and double the energy from the same sized unit.
In April 2007, however, the FAA issued a notice of proposed special conditions concerning the use of lithium ion batteries on the 787 in which it noted that these types of batteries “are significantly more susceptible to internal failures that can result in self-sustaining increases in temperature and pressure (thermal runaway) than their nickel-cadmium or lead-acid counterparts.” The agency said overcharging, in particular, could result in a “self-sustaining fire or explosion.”
Sinnett says, “Because they’re lithium ion, they contain a lot of energy and can release it quickly. Unless you design it appropriately, that can be a problem. When it’s overcharged it can carry more than it is designed for. It’s designed so you can never overcharge it. We’ve got multiple redundancy built into the system. Two [safeguards] are built directly in the battery, and two are located outside and are independent of the battery. So it is protected with multiple layers. We demonstrated by test and analysis that we are sufficiently safe.”
Presenting other scenarios, Sinnett explains that “there are a number of things that can cause a single cell to overheat, to discharge and then to vent smoke and–if it gets hot enough–to burn. One is over-discharge, in which you let the battery go down too low over successive periods. That can cause damage and lead to a short circuit. So we protect against it by putting in a circuit which protects it from over discharge or over charging.
The only other [possibility] is that over-heating of cells could cause them to vent, or a manufacturing defect could cause a short circuit. But we’ve had 1.3 million hours of operation in flight, and we’re pretty confident in the overall design. However we can’t assume anything, so if there was a manufacturing defect that would lead to a discharge of potential energy we’d expect the battery cell to vent–which looks like smoke. If the system detects smoke, it configures the airflow so that it goes through the E/E bay and goes overboard. If there’s any failures we know of that’s how it would work.”
Read our Jan. 11 story: FAA Launches Boeing 787 Review