As far as my knowledge goes, the non-backlit Portables must use 7V 1.5A power supply which is designed to soft limit the current at 1.5 amps. So if the computer will try to take more than 1.5A, the power supply will lower the voltage to keep it happy. So when the current will hit 1.5A, the voltage will be lowered. Imagine something with a preset wattage, but with fixed current and adjustable voltage. Also that model needs battery to be present all the times to serve as a buffer. Newer styled power bricks will provide too much current and will also not regulate the voltage if the current exceeds maximum - they just shut down.
But the backlit Portables are completely different story. Their logic board is completely different with much newer styled circuits. They can use higher amp units with no damage. They can be even used without a battery, but might have problems starting up, especially if the original HD is hooked up. I used my friend's unit with SDtoSCSI module and it always started without battery present.