I have this type of battery. When it was fully charged I read with the multimeter aroud 38-39 volts. Now I read 21 volts. The question is: When I connect the battery (discharged - 21,22 v) to a bldc motor load, the voltage drops to 0, the motor only spins for 2 seconds every time o plug the battery. Do you have any ideea why the voltage drops to 0 ?
Lithium-polymer batteries don't like being discharged too deeply. Doing so damages them in a way that can cause a fire on subsequent charging.
So nearly all LiPo packs have a protection circuit built into the pack that prevents the battery from being discharged too deeply (by cutting off the current). Some of these will switch off permanently.
The behavior you are seeing is due to that protection circuit.