Hi!
For cell testing I use a small “bms” that connects to the balancing connector on my 6S pack. If a individual cell goes below the programmed LVC it will trigger a loud beep.
My question is if anyone seen a similar solution equipped with a relay/switch instead of a buzzer?
It won’t be to hard to rebuild the one I have but time is money.
The idea is to let the bms trigger a signal to the Vesc that disables it limits the current draw.
A picture of the beeping version:
Why not use the in build voltage cut off of the VESC?
That’s extra made for exact that purpose.
Does it work on a per cell level?
No, it measures overall voltage. But your cells should stay roughly balanced (<0.2V diff) anyway. Otherwise you have a whole lot of other problems.
That was my understanding as well.
You wont see a 0.2V diff under high load on normal RC-cells. A simple and small bms will increase the battery life span. Especially if you tend to run long laps with high amp spikes.
How should that work? What should the BMS do to expand life span?
The BMS should balance the cells and keep them from getting over discharged…
the role of the BMS is to handle both HCV and LCV. it is just as bad to drain a cells as it is to over charge.
A BMS itself can not really handle LCV, it can only cut off. So VESC setting and throtteling power upon certain input voltage makes much more sense.
HCV protection is only if the charger fails. Usually you use a CC/CV charger which tops out at right voltage.
The cut off is the handling.
throtteling power upon certain input voltage makes sense in theory but you will get better protection if each cell is monitored.
Well, I am running it with vesc throtteling since 3 years at different skateboards/mountainboards and never had an issue. In which way should the cell level protection be better?
When getting to ~15% Battery it throttles power to keep the pack at 3.7V/cell. You just have to remember to unplug the battery when storing the board.
Thats quit a lot more comfortable then running 30km/h and suddenly lose control because the BMS cuts off.
If you keep within margins you are safe. If you push the limits you will need better protection.
Cut off is not the only option. My guess is that VESC can interpret a analog or digital input and tigger a throttle cut back based on it.
If you are satisfied with your setup you should keep on using it.
Why 3.7v? This is with lithium Ion cells? With the Samsung 30Qs that’s still 20% battery remaining. That’s an additional 10 minutes ride time remaining. It that your reserve, to get back in?
Samsung says don’t go below 2.5, but I use 3.0v setting on my alarm on my “simple BMS” …which by the way is a terrible thing to call it. 
More accurate would be “cell low voltage alarm”
I haven’t figured out the 75/300 VESC settings yet. How do you set the low voltage throttle back, to a user defined value?
Thats in my skateboard, using Lithium Polymere cells. They “fall” quite quickly after 3.7V. First that is not really healthy for the cells and second it’s only ~15% and I see it more like a “reserve fuel”. If I need it (because I miscalculated my tour/range) I can change the settings by phone and still dont have to walk 
Motor Config-> General-> Voltage:
Cut off start for start throttling voltage
Cut off end for absolute minimum voltage where power is cut.
Between there is a linear throttling. So minimum voltage/cut off end voltage should be never reached.
