The entire fleet of Boeing Dreamliner aircrafts is grounded, because of a fire in a Lithium-ion battery. What do we know about the cause of that fire?
- LPV65, made by GS Yuasa.
- Small prismatic, metal case, safety vents on the side
- Standard Li-ion (LiCoO2) cell, which may suffer from thermal runaway
- Specs: 65 Ah, 3.7 V nominal, 4.0 V at end of charge
- Very High power handling: a short discharge time of 41 s places it in the top 20 % for Li-ion cells
- Metal case
- Approx 28 x 28 x 25 cm (11 x 11 x 14″)
- Approx 28 kg (63 lb)
- 2-pole blind mate power connector, Anderson power
- Circular screw-in multi-pole control connector
- The case does not seem to provide any space for the cell vents, should they open: the vents of at least 1/2 of the cells appear to be right against the outer walls
Electrically, power circuit:
- 8S1P arrangement
- 29.6 V nominal, 32 V max
- 65 Ah, 1.9 kWh
- 580 A peak, 17 kW peak
- 5 mOhm internal resistance
- Fuse between 2 and 3rd cell
- No switch: the battery output is always live, and the BMS cannot directly protect the battery
- Centralized BMS, in two boards
- Multiple wires to 8 cells, to sense cell voltages, with redundant connections
- Multiple thermistors to sense cell temperatures
- Hall Effect current sensor on board, output bus bar through it
- The damage occurred inside the case and was contained by it
- The cells and all other battery components are relatively intact, though blackened and covered with the remains of burned materials
- The only component that actually burned may be the translucent plastic sheet placed above the cells
- The most evident effect may have been just the white smoke that is typically released by abused Li-ion cells’ electrolyte
- The investigators report that the battery voltage at the time was 32 V.
1) Over charge due to unbalance
The battery was not top balanced. Therefore, the 32 V was not equally divided into the 8 cells, meaning that some cells were at higher than 4 V, and therefore over-charged. LiCoO2 cells are prone to thermal runaway if overcharged.
2) Charging over current
The charger is capable of sourcing a very high current (on the order of 1000 Amps). The battery was discharged when the charger was turned on. The charger was able to overcome the very low resistance of this battery, and pump a very high current into it, at 32 V.
For example, it the battery was at a discharged voltage of 25 V (3.2 V / cell), then the current would be (32 – 25) / 5 mOhm = 1400 A.
The cells overheated and went into thermal runaway.
The underlying problem
- Either the BMS did not detect the over temperature, over voltage on a cell, or over current
- Or, the BMS detected the problem, but did not report it to the aircraft; the BMS had no recourse, as the battery does not have a switch that the BMS could turn off
- Or, the BMS did report the problem to the aircraft, but the aircraft did not obey the BMS and shut off the current
- Or, the BMS did report the problem to the aircraft, but the aircraft did acknowledge the problem, but it had no recourse, as it does not have a switch that it could turn off