Careful about building your own pack with used cells. The cells will not be balanced as well and when when goes flat, the charger will still try to charge the pack to the full voltage,
very legit concern, but it is the first thing that beginners and non-professionals (unlike you and me) learn to overcome, and probably half of this forum and others (enless sphere, eevblog, reddit, youtube, and more engineering and electronics forums) is all about solutions to that specific fear that is heavily used for marketing (showing fires and explosions just like the chinese propaganda of actors walking on the streets and just falling dead because of a "virus" that was designed specifically to be ani-trump and anti-trump-tariffs). The method I used was using the busbar concept where single cells are only connected to the rest of the battery via tiny fusing connectors that connect a cell to a thick copper conductor that acts as a busbar that collects the current from all. In addition to that, checkout batteryhookup.com and learn what kinds of "used" stuff they have that people probably on your neck of EE invent arbitrary rules of when they have to be replaced (probably 2 years even though they were never cycled). I try to get more than one cell connected in series or parallel as modules (e.g. 7s3p) to save time working on putting them together.
unless you have quite a smart battery management system. That can lead to fires. I'd recommend at least getting a new commercially-made battery pack where the cells start out very close to matched. Even if it's a Chinese one.
"Chinese" is so old fashioned because everything is made in china. It's an obvious worldwide conspiracy. But if the thing that is made in china is branded sony or panasonic and you get it from a trusted source like the warehouse i mensioned above which is fortunatelt lovcated in my state, then it's good.
For the "management" circuit, I think it is another commercial gimmick for the unknowing. You can use a balance charger, or some diy microcotroller/arduino balancer/charger. The internet is full of free and open-source code, schematics, and even pcb layouts. Personally, I simply measure the voltage of every seriesed group of paralleled cells once every few cycles and I never noticed more a new mV's in over 2300 miles because all the cells came from the same "modules". A future project that I will probably never get to is to put together a small circuit that sends the voltage of each to a phone/computer wirelessly at all times (e.g. through bluetooth) to minimize the effort.
Finally, I made a battery that cost me about $40 that gives me 20 miles that is a very well made. I can make a guess that if something similar exists, it may cost me at least 10 times as much. All the "ready made" options are very low quality and probably more dangerous and do not include the joy of achievement.
Also be aware that the US power limit for road legal eBikes is 750 W, so if you were ever challenged by Officer Friendly, it might come out badly.
About that, in my county and probably most counties in this country (at least the rational ones that are still American), there is no limit as long as you don't use the road part that is used by motor vehicles. I talked to my Sheriff dept and local dmv and no one cares as long as you are on the shoulders in our farm roads and on bicycle dedicated routes in town.
How did you connect the battery cells together? Do you have straps and a spot welder or did you somehow solder them?
That's the best question and it is what I feel I invented. All the videos and pics you find on the internet are all of weirdos taking cells outside of series/parallel modules where cells were already connected (welded, soldered, ...etc) which is insane. I only solder wires to the wires or connection points on modules that are already factory connected (usually series, so you can test charge them before adding them). For the cover, try to visit the cheapest stores in your area (walmart, goodwill, dollar store, ..etc) to find plastic boxes/containers of all shapes and sizes and for the straps, get as many dollar-a-piece little dog leashes that look like backpack hardware and build your own thing.
Also remember that regular bikes aren't built for eBike speeds, in terms of brakes, spoke strength, and frame strength.
If I think like that, I will never be able to get anywhere and will probably be homeless soon and then starve to death.
But thank you for all your concerns and points that look like a bourgeois attitude of a rich guy, lol.
cheers!
