That's because they're turned on by the very battery you're trying to charge, meaning the battery has to be at over 6V - 8V. It's a safety feature, to prevent sparks if the AC is plugged in before the clamps are attached. Some people have modified the circuitry by tracing it out and finding another 6V - 8V voltage source inside the charger.
When I ran my battery down to 1.5V, I hooked up a desktop PC power supply's +12V output to the battery, with a 100W, 8 ohm resistor in series, then when the battery voltage got high enough I used my regular charger. You should be able to use a laptop AC adapter the same way, if it can put out at least 3 amps, but the resistor will get hotter because most laptop AC adapters put out 19.5V, meaning the resistor can dissipate up to 50 watts, and you always want the resistor to be rated for at least double the actual watts.
When I ran my battery down to 1.5V, I hooked up a desktop PC power supply's +12V output to the battery, with a 100W, 8 ohm resistor in series, then when the battery voltage got high enough I used my regular charger. You should be able to use a laptop AC adapter the same way, if it can put out at least 3 amps, but the resistor will get hotter because most laptop AC adapters put out 19.5V, meaning the resistor can dissipate up to 50 watts, and you always want the resistor to be rated for at least double the actual watts.
