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Old 02-07-2010, 05:20 PM   #3 (permalink)
Join Date: Feb 2009
Location: phx
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i was curious...

seems that a major concern with bms modules is that they fail and drain the cell that they're monitoring. One failure mode is the mosfet failing on - then, the module can't turn it off and the thing drains the cell to destruction.

IIRC, you're planning on using PWM to control the shunt current. Could the shunt resistor and fuse be sized such that a low PWM duty is required? Then, if the fet fails on, it's like a PWM duty of 1, and the increase in current causes the fuse to blow. When the fuse blows, the module is not longer powered, the main system is not receiving a signal from it anymore, and shuts everything down.

Maybe you've already considered such a case and have come up with a better solution. BMS modules seem like a good thing, but (from what I've read) they have also caused cell failures. The function of a BMS is very simple, but I think benign failure modes will be the key to a successful unit.

I plan on upgrading to Li when my pack dies - maybe in a year or so, so I'll be following closely!
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