If you only getting 3v at the anti spark input you battry is the issue/joints plugs up to it. Id expect as soon as you apply a load the v drop. Why you think you’r getting full voltage at the battry.
@RhinoTH Pleas can you confirm your measuring Voltage at the anti spark INPUT/SUPPLY to make sure we don’t going down a rabbit hole
I’m thinking Broken joint/super high resistance between p groups or BMS I don’t see any details on how the battry was built in you build thread only