Looks like a great project bike for someone who can already wrench a bit and wants to learn electric-bike from the inside.
For starters, replacing the batteries, of course.
Playing around with some numbers a little. Assuming high-discharge batteries, like LG HG2. Advertised with 20A max continuous discharge. String up (or whatever you call that) 20 of those in line for 72V, assuming the nominal battery voltage of 3.7V. At full charge, that would be 84V, of course, so I wonder if 72V is the absolute maximum or the nominal for the existing controller and motor.
If it is the maximum, let's put only 17 in line, for 71.4V.
At 71.4V, a 30kW motor would yank a hair over 420 Amps (nice)
, requiring a hair over 21 such string of 17 batteries in line. Assuming one would want a safety margin and draw, say, only 15A from each string, that would be 28 such, for 476 batteries.
Would be tight, but I think it would still leave some useful volume for cargo space.
Looking at the smaller end of standard dimensions for car batteries, and the way those are arranged here.
18650 batteries oriented vertical, you could just fit three layers, accounting only for the height of these lead-acid blocks.
On each level, from left to right, you could fit 12 rows, and those could extend 21 batteries deep, front to back.
That would be 756 18650s max, and even leave just a little breathing room.
Just on the lower level here.
At 3,000 mAh per battery, or 3 Ah, that would be 1,428 Amp hours total charge.
Even when discharging at "only" 15A, I'm guessing some heat-management would be required.
At 4A max charging current per battery, distributed over 28 parallel strings, it'd take an unfortunate 12 hours and 45 minutes for a full charge (disregarding thermal losses).
Now, when I try and convert those 1,428Ah to kWh, I'm getting a number that just seems way, way too big, 101 and change.
Who can spot the elemental and trivial error I made?
As for the brushed motor. I'd imagine one might want to first convert the batteries, and then ride it a bit to see if the motor is too much, too little, etc. So that, when the brushes need replacing, one has a good idea what brushless motor one might want to install in its stead.
Of course, one could probably convert some cheap bike with a busted IC engine, and the effort would not be much greater. But I felt like playing with the numbers.
If there were absolutely anything to be afraid of, don't you think I would have worn pants?
I said I have a big stick.