BlueSeas said:
The new BMS I'm testing as part of my current build thread requires a charger as part of the process to get it to turn on. Wire it all up, and it's dead to the world until it senses reverse current from a charger.
I'm not keen on this feature. I quess it's OK if the firmware is rock solid. But if the damn thing turns itself off, say due to the low voltage shutdown setting, I don't want to pedal home this 120 lb monster.
Well, it *is* a good safety feature, in that it helps prevent damage to the cells that some people have done by riding a pack to "empty" and then waiting till it recovers enough to turn back on, then doing it again and again until the cells pass out from exhaustion.
Does bypassing the BMS B- and BMS Load B-, with a switch, essentially shorting these connections hurt the electronic switch in the BMS?
Assuming B- is the actual battery negative, and Load B- is the actual discharge output negative, then you could short across those (switch, breaker, jumper cable, etc) to bypass the BMS, without hurting the BMS itself. However, you then risk damaging the cells from overdischarge (or whatever it was the BMS was trying to protect the pack from), and then whatever future risks that damage then incurs.
However, if I were to need to do this sort of thing, what I would probably do is not to directly jumper across the FETs, but rather to put a "bypass" switch into the logic lines that turn on the gate signal to the FETs, so that I could just turn the FETs back on. It might require cutting a trace on the PCB to put the switch in there, depending on the design.
If the logic basically grounds a pullup that turns on the gate drive, then the switch just has to disconnect the logic from the gate drive so the pullup does the work of re-enabling the FETs, so you just need a simple SPST toggle (or slide or push) switch).
If the logic goes to 5v (or whatever) to turn on the gate drive, and grounds it to turn it off, then the switch has to disconnect the logic *and* provide a 5v (or whatever) to the gate drive, so you can use an SPDT that uses the common to conect to the gate drive, one pole to connect ot the 5v (or whatever), and one pole to connect to the logic output.
Etc.
If you use a relay to do this, then you cna put the switch anywhere you like (even on the handlebars) and the relay inside the battery.