better use a mobile battery pack rated 5v, at least 10,000mAh.
with those 1.2v NiMH batteries, you'll need to wire those in series-parallel
example 4 x 1.2v (at 2200mAh each) in parallel getting about 8500mAh total (value reduced intentionally), then 6 x (parallel 1.2v batteries) in series giving you 7.2v. then use a buck converter to step it down to 5.1v
this setup will be bigger, more complex, more expensive, and more troublesome, that the mobile battery mentioned above.
but if you like doing it this way.... then nobody can stop you... the choice is yours!
or you can get a 12v motor cycle battery and use a 3A rated buck converter to step down the voltage from 12v to 5.1v.