Actually, some of the new chargers uses a technology called PWM which uses power much more efficiently. Say if you're charging a battery (say around 9V when full) less than your power supply voltage (say 13.8V) at 5amps. The charger will actually draw about 4 - 4.5amps.
So if you plan on charging a 7.2V pack at a maximum of 8amps then a power supply of 8 or more amps will be more than enough.
For older technology the charger actually dissapates the excess votlage as heat so it's less efficient. In that case then you would require a power supply with output more than 9-10 amp to charge any pack at 8amps.
I believe the APS Dyno-Charge is one of the newer chargers to use the PWM technology. That's why it's so lightweight and compact and it doesn't require a fan to keep it cool.
I just got mine a yesterday!! It works great and it looks even better!!!