I am a little concerned having read reports and seen video of total loss of power events caused by the BMS. These have been described by others as “hard cuts.” What appears to occur in these cases is that the voltage of a cell (or cells) sags when power is requested, reaching the low voltage limit, and the BMS abruptly cuts all current. When cells are well balanced, this will occur at a low state of charge. When cells are out of balance, like in the case of a single failing cell with high internal resistance, this could occur at a high state of charge under high load.
I see this exact sort of situation in my 10 year old electric car. My car appears well balanced at full charge, but is widely out of balance when empty and under load.
Individual cell voltages must be reported to the throttle controller. The throttle controller should account for cell voltages and limit current to keep cell voltages above a configurable threshold that is set with a safety factor above the BMS low voltage limit. When the throttle controller is operating in this limited current mode, some feedback should be given to the pilot (perhaps a vibration) to let them know that full power is not available and that they should land soon. My car does this by displaying a turtle shaped warning light on the dash.
This is an issue that should be addressed in the design soonest. Otherwise, I see potential for someone to get hurt by one of these hard cut events at low altitude. As people’s packs age, this will present itself at higher and higher states of charge.
I’m not a programmer, so can’t make the changes myself, but happy to consult further with anyone who can modify the code.
For the time being, the best way to protect yourself is to keep a good eye on the health of your battery. The best way to do this is check cell voltages after flying. They should remain well balanced, even through a rapid discharge. If they appear out of balance at low states of charge, this is the first indication that the lowest voltage cells are starting to fail. An alternative, but less practical method is to look at cell voltages under load. The voltage of the weakest cells will sag the most.