I also did some measurements. It turned out that the N64 controller is
quite power hungry: The wireless circuit draws only 2 mA, but the N64
controller itself draws about 14 mA!!
Under normal circumstances I
would cut the N64 controller's power and only turn it on when I want to
read the current status. Unfortunately if I would do that, the N64
controller would reset the analog stick everytime I'd turn off the
power.
So the controller needs to be turned on the whole time.
16
mA is no problem when you're using 2x AAA (rechargeable or disposable)
batteries within the rumble pack, because the capacity of such batteries
lies between 800 and 1000 mAh usually. This should last a while
You
could also install a large (800++ mAh) LiPo battery inside the rumble
pack, there would be plenty of space if the battery compartment is
removed by using a Dremel.
An all-internal mod inside the N64
controller is possible, too. But you'd need to find a battery with a
decent capacity which still fits inside.