its pretty hard to hide a significant battery pack... the 48V 18AH (864 watt*hour) pack I put on the hub motor conversion I did for my wife is an 18 lb brick thats 16" long, 3" high, 6" wide, and slots into a rack. now, this can be taken with you, if you don't mind carrying an 18 lb brick, it even has a handle.
thats also quite a lot of power capacity. my next build, I plan on using a 36V system with 350-500W 'mid drive', and figure a 36V 10-12AH battery will be plenty (360-430 watt*hour), these are about the size of a large water bottle.
there's quite a bunch of wiring on an ebike, thats not practical to hide. this conversion, the handlebars have brake switches, throttle, display status panel, and a 3 button control thing, pedal assist would require a cranks sensor, the hub motor itself requires power and control signals, and all these along with the cable from the battery have to run to the controller brick. mid-drives integrate the controller with the crank motor.
I hid the controller in that black trunk bag, which is a mistake, as it overheats in there. I'm taking it out of there and rewiring as soon as I get some power connectors.