yeah, apply the voltage until it screams " enough!"
No, but seriously, on the subject of applying higher than normal charging voltage to a flat battery, it will actually work if the battery is still serviceable. Twice now, for various reasons, I've had dead flat completely discharged batteries that would not respond to any chargers, smart or old-style. My local auto electrician has an old variable power supply that he uses for this. He'll connect up, crank the output up to, IIRC, 30 volts, then watch for current flow. It usually doesn't take long, as the current starts to rise he'll slowly reduce the voltage until it is back to 14 volts and showing current flow. It can then be placed on any style of charger to complete the job. Obviously cannot resurrect failure like a dead cell, but, for inadvertant complete discharge, it's great.