A trickle charger will take at least a week to fully charge a dead battery.
My Black & Decker smart charger goes up over 16 volts when it senses a battery below 12 volts. I don't know if that is potentially harmful to engine electronics but I do disconnect a battery cable when seriously charging so I don't have to worry about killing a $1000 vehicle computer. It only takes a minute to disconnect and another to set the clock after charging.
How can you tell it is fully charged? A few hours after charging has ceased the battery voltage will be 12.6 or higher.
