texaspyro said:
I've seen optos start conducting at 0.4V in... and that was at room temp.
Yep, the whole circuit's crazy, as designed, as the cut-off voltage could (and probably would) vary to the point where it just couldn't be relied upon to do the job intended without a bit of tweaking. If some of the optos start conducting at 0.4V Vf and other don't start conducting to the spec sheet max of 1.4V Vf, then that's a 1V per cell variation just there.
Anyway, apart from anything else, when you add up all the parts costs, plus the added assembly complexity and the probable need to swap parts around to find ones that work at the right detection point, it isn't a cheap solution, either. A dead simple, ready built and tested, multi-channel LVC that similarly just pulls the ebrake line down with optos, and that has a precise, stable and reliable cell voltage detection point, that handles 8 LiFePO4 cells per board (and can be daisy chained for as many cells as you want) costs just $20 (see this thread for an example: http://endless-sphere.com/forums/viewtopic.php?f=31&t=21695&start=15 ). You could build a simple and reliable LVC with just three or four components per cell, using the same circuit as used on those ready-built boards, at a cost of around $0.80 to $0.90 per cell for the components, plus the cost of the board, connectors, etc, so maybe around $0.10 per cell more expensive than the moronic circuit, assuming that you didn't need to bin any of the components from the the moronic version in order to find ones that were around the middle to lower end of the spec range in order to make it work..
The 100nA current drain when off isn't needed, as the cells have an internal self-discharge current that's massively greater than this. The LVC only needs to be good enough to not significantly increase self discharge, so anything that draws less than maybe 100uA or so will be fine. The circuit mentioned above draws around 1uA, so with no self discharge a single 2650 A123 cell could drive it for more than 250 years.............
Jeremy