787 Batteries and Chargers - Part 1

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Failure scenario

There three main management issues.

a) Cell production quality
b) Cell operational management
c) Cell failure handling.

It was reported that a cell was found shorted. We do not know whether this was the initial failure, or whether due to a) or b). It does not really matter because c) should have worked better, and may be what did the real damage.

If short was the initial failure, from an electrical perspective:

1. A cell fails and shorts - (reason either a) or b) above
2. Cell voltage drops, removing (most) of its 4V share of the 32V
3. Bus voltage remains essentially the same at 32V
4. Other cells take up 4V / 7, thus are subjected to severest over-charge in terms of voltage and current
5. More cells fail in a cascading manner
6. Combined per-cell energy and the bus feed battery destruction

Shorting

It is not going to be a solid short from one moment to the next. The energy is just too great. Cell could not ever dissipate it's 200 - 300 Watt hours quietly. It would short, arc, burn out the short and do more damage along the way. There would be over pressure, relieve valves opening, electrolyte blown out. More shorting as separators fail by the heat. The whole cycle continues until energy is dissipated and some spot shortened can not be burned out.

Throughout all this, terminal voltage will fluctuate and other cells be subjected to voltage and current spikes, until the next one fails and the cycle repeats itself even faster.