If you have a 3-cell Lipo pack that reads 6volts then you have a bad pack. It's ruined if it's that low. Cell voltage isn't supposed to go under 3v/cell so it should never read below 9v under a load. Smart ESCs will prevent that from happening. It won't charge because your charger has "smart" circuitry in there to prevent things that could cause explosions/fires... like charging a pack that's too low of voltage.