saptzae:
It was just intended to be a block diagram to show the major components and
their interaction. I can redraw to show all the wires, but there wasn't
enough space between the cells on the original sketch.
From what we know already, the 787 battery does have separate circuits
for measurement & balancing. Though i'm not advocating it, it is possible
combine both functions by time sharing a single wire. ie: Charge / balance for a
period, then disconnect and measure for a period. Many modern lead acid
chargers work in exactly that way.
We have to agree to differ on the temp sensing . A single cell's temp
gradient would be easier to detect if the sensor were at the cell, rather
than several inches away at the other side of the case. As the temp sensor
is part of an overall feedback loop controlling charge current, the closer
to the cell being monitored, the less phase lag there is in the loop, thus
providing a faster overall response. Ideally, it would be embedded in the
internal folds of the cells during production, but that may be asking too
much.
As for the transient detection, that could be done by sensing cell voltage
dv/dt, either using a fast a/d converter, or much simpler, an analog
differentiator feeding a limit comparator, which then generates an interrupt
to the bms processor. I know it might be more elegant to use an a/d, but it's
not a no cost option as you not only need the fast a/d, but all the software
to drive it as well. Not difficult overall, but who knows what's on those bms
boards already.
If the temp sensor is built into the cell, then why not a pressure sensor as
well ?, though it would increase the cost considerably. Still, for a new
application such as this, perhaps extreme belt and braces will be the only
way to satisfy the regulators until more in service experience is gained...
Last edited by syseng68k; 9th Feb 2013 at 11:50.