Because of the size of the starter/generator units fitted to the APU, a NiCad cannot produce sufficient current, no matter how big it is.
@ pub user,- I'm calling bull**** on that one!
current -demand can be met ,simply by having lots more smaller cells.
Why do you think the present lithium configuration has 3 sub-cells in parallell.? much easier to make one big one, but the path across the plates to the terminal-post will be far longer.
The advantage of lithium technology is 3-fold
1- shape of cell is not as constrained as is the case with others.
2 higher energy-density. (stores more power in the same space)
3 higher discharge/charge current-capability.
The downaide is the need to very carefully monitor and control EVERY cell's operation.
this is still not being done monitoring batches of 3 is where I think they came unstuck and history will repeat until they treat the technology with due caution. a bigger,stronger firebox is just a big lashup kludge.
IMHO, the software in the battery controller should
automatically start the APU before the point at which doing-so would over-discharge the battery (IE when about 1/3 of the USABLE capacity has been drawn) this would go some way to ensuring that batteries remained serviceable, charged and rechargeable and met their primary function as reserve supply for ETOPS operation. thus , delays due to batteries not meeting despatch criteria, would be virtually eliminated.
A big tin box with a sewage-drain is not going to achieve that!