PickyPerkins, #1009:
That's an interesting post, thanks, though are there typos in the battery
load timeline ?:
Initially about 9.6kW (32V at 300A)
Then 8.7kW
Then 10kW for 10 seconds
Then 4kW
10.3kW for about 20 seconds
Then zero for 47 seconds
Then charging at 46A.
If you have an accurate timeline, then you can calculate the total
watts hours needed to start the apu. It's quite easy to have profiled
current limiting in the apu starter electronics, since it's inverter
driven and start time trades off against max starter current. The
20v minimum seems a bit iffy, in that each cell's voltage will be 2.5v,
at that point. So, what will the cell voltages be when a start has
completed from 20v ?. Less than a min safe voltage, or what ?. The final
point being that the cell data sheet only lists discharge characteristics
up to 250 amps, with nothing after that, yet they are driving these cells
at 300 amps ?. My take on it is that they skimped on the cell spec and
should have used cells of much higher ampere hour capacity for this
application. The weight saving obsession must have it's limits at some
point.
Aviation kit is usually designed to have a higher margin of safety than it's
commercial equivalent. I would have thought that they would at least have
doubled the cell capacity in relation to max expected load. To do this would
mean larger cells, but perhaps longer service life and less prone to
catastrophic failure. Running components well within ratings improves
reliability, all else being equal, but these batteries are being pushed hard.
High current to start the apu, then fast charging within short timescales,
both of which will cause significant temperature rise and perhaps hot spots
due to varying resistive paths within the cell.
Let's hope they get it right, but the proposed solution seems light on
detail in terms of what changes will be made to battery management and
charging. Afaics, there is still only a single current sensor. There's
no mention of improved temp sensing, non volatile logging for the cell
voltages, contactor fail safe or other items, though some of this was
hinted at online a couple of weeks ago.
As for the strengthened enclosure, reminds me of the old black joke about
fusing: If the fuse blows, put in thicker wire and if that also blows, put
in a paperclip
...