A big battery will weigh a ton and based on your 3 hrs charge/12 miles it would take a full 24 hrs to charge 100 mile battery. You'd need two at least if you were using it every day
They way the cells can charge in parallel it will be like plugging in more rather than one big battery, I think most ebike battery packs have 20+ cells inside them anyway. The problem is when it pulls so many amps off the plug things start to melt and glow.
A big battery will weigh a ton and based on your 3 hrs charge/12 miles it would take a full 24 hrs to charge 100 mile battery. You'd need two at least if you were using it every day