docw009 said:
Once I thought I could get the BMS to shut off if I put a switch in one of the balance wires and opened it up. It didn't work
most of the common cheap bms designs don't check for a floating cell input, or for a voltage that's far enough below the lvc cutoff point for a cell. in both of those cases, the bms just operates normally as if a cell was connected to that input, and was still within the normal range between lvc and hvc. (there have been a number of instances of packs that failed in one way or another because of this problem, where a broken balance wire eventually led to overcharge of a cell group, or a cell group that discharged way too low during a deep discharge, and the bms can't see that, because its too low for it to detect.
)
if it were a well-designed bms that checks for open-circuit error conditions, and shuts down the output based on that, it'd work to put a switch in the balance lead.
while that will shut off the output. which appears to be the point of this particular thread, it doens't actually shutdown the bms, which a bms with an on/off switch may respond to by going into a low-power state for storage, in case that's a requirement.
if it helps, there's a thread aroudn here with a circuit diagram / analysis of the common cheap bms design. sorry i can't remember who posted it, but the title is similar to the description.
