Take a look inside your N64. Were the AC comes in, there should be a AC/DC convertor, or a transformer to diodes to convert it to DC. All electronic (besides the power supply) works off DC. The guy who used 8 AA's, worked because he had 12 V going to the circuit. If you want 16 V, you would have to use 10 AA's in series. Plus, you would need to know the Amps of the output DC. And to do that, you would have to connect the batteries in parallel to add the currents.
To better explain, look inside. Find where the AC turns to DC, and bypass that with your battery pack.
Another place you can look would be **broken link removed**. Good luck.