maxwell wrote:Because I have two packs when a cell goes lame only one pack would trip, then I can find the naughty one and replace it, having only 4 in parallel makes this a lot easier (and the trip more likely). Thats why I have spent a bit of time designing the BMS.
You know electronics better than me to design a BMS from scratch.
There's a lot of variance with the PTCs on these that may be a consideration too. When I ruptured a couple from overdischarge (I was just below 3.7v/cell open-circuit voltage and pulling 35 amps from 12-cells in parallel when it happened), it seemed the PTCs caused a cascade failure throughout two parallel subpacks by some cutting out their cells earlier than others, increasing the load on the remaining cells. Perhaps your BMS can prevent this from happening. Regardless, there's very little capacity left below 3.7v resting voltage, and that very little capacity dumps very quickly, leading to overdischarge occuring very easily. So I'd suggest a 3.7v cut-off voltage even if that sounds a little high compared to the specs for single cells.
I must admire your bravery Xyster running all those cells in series without a BMS in a wooden box near your crown jewels!
I have no interest in fathering children; those "jewels" just kind of hang around, doing nothing much for me anyway...
Mine are next to my back and in an aluminium box, not worried a bit (tremble tremble). Only time will tell.
I'll be watching your results closely, especially as I'm interested in doing the 60-amp controller mod w/variable current limit. If it's safe, I'd love to squeeze another 5 amps from my 18650s for 40 amps total, then add a 6s boost pack of emoli's or A123's on a relay for 96 volts. 4000 watts ought to get me up some of the hills around here much better than my present 2500 watts, which is kind of marginal with my hubmotor for sprightly climbing steep (>15%) inclines. I'm also hesitant to change a system otherwise working so well.