The problem is, once the cell voltage gets below about 4.5 volts, most basic battery chargers won't flow current. According to Hot Rod Mag, who say they got this tip from Optima's engineers, this is what you do. Get a battery with good charge, and put it in parallel with the dead optima, using jumpers. Charge the optima as normal. The good battery fools your charger into thinking there is enough volts to charge, while the dead battery provides the sump for the current.