I've had some luck bringing both flooded and AGM batteries back to life.
Typically, the battery is "sulfated", where the sulfur in the electrolyte has come out of solution and deposited on the plates, sort of an electroplating process. If this can be reversed, the battery can sometimes be saved: the trick is getting the chemical reaction started...
Yes, it's OK to jump the bike battery from a car battery, as long as you match the polarity. In order to get any current flow in a sufated battery, you will need more than the voltage from a sitting battery, so start the car. The car's charging system won't allow the voltage to get so high it damages the battery, unless the battery has shorted cells, in which case it's already toast. Once the battery starts to read voltage after disconnecting from whatever charging source, connect a load across the battery ( I generally use a 60 watt headlight) until the voltage drops off a fair amount, then re-charge and repeat. Each cycle should increase the current draw during charge cycles and time to discharge when under load. After only one or two cycles with the jump from the car, your "smart charger" may work?
