vwhammer said:
Is it as simple as adding a switch to the BMS?
The BMS is not the place for it, because of they way they are designed. The ones I have seen simply manage charging, balancing cells and discharging the battery. They don't control the power using a control signal, rather they automatically sense charging voltage, cell voltage and current, and do controlled discharges.
However the motor controller should be able to turn power off, using the brake switch or key switch signal inputs. Sounds great, assuming the controller never fails, which of course it can. The kill switch is a backup.
The original poster wanted a way to disconnect power absolutely, separately from the controller. Frankly a relay contactor is probably the least expensive device for it. It seems archaic to depend on an old fashioned relay, but there are reasons to do so. The electronic shutoff should work, but if it fails, very bad things can happen and a backup kill switch is a wise investment.
Top of the head, there are a couple of arguments for it. First, for safety and theft prevention these bikes should have a keyed on off switch. Second, if the controller fails, you need a kill switch to prevent motor runaway.
In the first case, thieves, children, drunks, and idiots can't seem to keep their hands off electric bikes. So it is prudent to have a key lock as the master switch for power, as you would for a car or a gun. This key lock would logically control the contactor to apply power to the controller and motor. I think the original poster meant an additional kill switch like those found on industrial equipment, typically a big red stop button. Can't hurt to have one. Cars don't, though, probably because the makers don't want to suggest their car might need one. I think they are required on larger industrial machines though.
In the second case, electronics do fail. Bicycles are in a very unfriendly environment for electronics. Our controllers, motors and batteries are exposed to sun, rain, overheating and vibration, all of which can cause electronic gadgets to fail. Because all controllers use NMOS FETs as power switches, which
most often fail shorted, the electronic off signals to the controller may do absolutely nothing. Lithium batteries do not like shorts, motors would run full speed -- you get the picture.
Of course a fuse or breaker should prevent fires and motor runaway. But still if all else fails, it's prudent to have a reliable way to shut everything down. Many's the time I have smelled insulation burning before the fuse blew. Time to turn off the key switch, or hit the kill switch.
