Hi,
RetiredBA/BY and HeadingSouth:
Still putting my money on a single cell failure and the charger trying to bring the pack upto full voltage, but isn't that what a balancer circuit is supposed to do ?
Most batteries are "series charged" i.e. the charger current flows through all cells from a DC source applying current to itīs plus terminal. (opposite as the current the battery delivers when supplying current to a load. E.g. the 787 APU starter system).
Lithium batteries are not charged directly from a bus. The designers prefer to charge them separately. In 787 the MAIN battery are kept charged separated from the bus by a diode: A "high speed switch" that is controlled by the bus voltage. When the bus drops below (estimated) 30 V the battery (if it is ON) supply current to the bus to maintain itīs voltage at required values. A low voltage in the DC bus would make the loads (electronics) fail.
Well, how Li ion (MAIN and APU) is being charged in 787? From the info we have, serially. I.e. the battery chargers are connected to itīs extreme (minus and plus terminals) and the cells are being monitored to avoid the DANGER of exceeding itīs voltage during the charging. Why? Simply if the current is the same in all cells (series connected) and the cells are different between each other (mismatches due several factors) the voltage in each one could be different, charging it (each cell) not equally. This is normally not a big problem in Lead Acid or even Ni Cd. In Lithium ion this is dangerous.
So the 787 designers (certainly) adopted the cell balancing scheme. This explain the harness over the cells. The several thin wires we imagine were used for individual cell voltage monitoring and to "bypass currents" if and when a given cell starts to present a growing voltage.
This approach requires software algorithms and is used for most of chargers probably also because is cost effective. But there are other ways to charge SAFELY the cells:
Imagine you have one (smaller) independent charger for each cell. This (1/8) little charger could easily (without software algorithms) charge each cell to itīs safe limit AND SIMULTANEOUSLY verify if the cell is healthy. A sick cell can be e.g. one that despite you pass through it the maximum current (no bypass at all) itīs voltage doesnīt shows the normal envelope (by different internal characteristics due ANY kind of fault)
Question:
The charger that were used in 787 fleet until itīs grounding is able to verify this cell condition? And proceed accordingly in order to avoid a thermal runaway?
IMO the best approach would be using a parallel charging scheme. Since ANA incident i am designing a solution (not only the charger) in order to eliminate (or reduce) most of the danger in using these wonderful cells.
The charger is part series charger (high current) and parallel charger (low current) and is being currently used (and tested). The software algorithms being used mainly to check if cells are healthy. Continuous cell characterization. (IMO an essential safety function). And to compare, track and record their electrical characteristics throughout useful life.
PS
Two CPT flying RC (carrying potential fire bombs) has a privileged view of the issues.
