Making a Wii Battery Pack?

I've been trying to figure out the best way for making a battery pack for my Nintendo Wii. The Power Supply's output is 12v 3.7a, but I'm not sure what I need to regulate. I'm uming I should find a 12v battery, then create a power plug for the Wii from it, but will I need to control the amperage coming from the battery? If so, what are my options? The power supply takes 60 Watts, but I've heard that the Wii itself takes around 15-20 Watts. Thank you for any help that you can provide!