72v motor controller and bms

Good afternoon knowledgeable folks! ^_^

I'm looking at spending my spare time building an electric bike for fun :D

However, general electronics design is most definitely not my forte, and I was wondering if anyone would happen to have a few pointers if I continue down this path.

What I'd like to do: create a battery pack utilizing 420x 18650 cells (20s21p, 3780kWh) to power a 4kW (peak 8kW) motor.

I would like to control the motor with an Arduino (I'm most familiar with the Arduino Due, so I would probably stick with that).

My greatest concern is the monitoring of the cells, and how I balance charge them and protect them during use. If I were to utilize a charger like this 20s 72v charger, my understanding is that this would only monitor a 20s1p battery accurately; I'm trying to figure out how I work in all the parallel cells.

My second biggest concern is how I use the Arduino to control the output voltage to the motor (hardware-wise, not programming-wise).

This project is intended to be an electronics hardware and batteries learning curve for me, and coincidentally I have a bazillion 18650 cells kicking around xD

tl:dr; looking for tips on a balanced BMS (72v) and voltage output control via Arduino

Edit: The goal is to also charge the batteries via a standard 100-120vAC houseplug.

Edit2: On an aside, if this involves me learning how to build 21x small 72v 6A 20S bms's and having them work together, that would be excellent as well. Ha.

Go for ready and tested system parts. Buy a suitable battory, a suitable power controller. This is a task for experienced folks and absolutly not a good project to learn power electronics. It might even get dangerous.

I appreciate your concern, but no worries about my safety; I'm quite conscientious when playing with electricity, and while my coworker (an electrical engineer) might not help me with my side projects, I'm 99.8% sure that he will correct erroneous or dangerous ideas and information that I ask him in hypotheticals :p.

Ignoring the project itself, I guess the first real question is, for completely balanced charging do I really need to have a separate charger for every parallel battery voltage source in the system? I mean, it makes sense, but I feel like I never see batteries with that much hardware on them (or perhaps hardly anything does full balanced charging?).

BMooney:
Ignoring the project itself, I guess the first real question is, for completely balanced charging do I really need to have a separate charger for every parallel battery voltage source in the system? I mean, it makes sense, but I feel like I never see batteries with that much hardware on them (or perhaps hardly anything does full balanced charging?).

Another real question, shouldn’t you already know that before you try to build one yourself?

zoomkat: Another real question, shouldn't you already know that before you try to build one yourself?

That's why I'm in the "Project Guidance" / feasibility forum; as my education isn't electronics based, sometimes determining the correct search term to find the information I'm looking for can be a challenge, and I'm here because I figured there's a ton of people here that might be able to lend their knowledge :)

For example, the question you quoted but did not respond to, was me asking whether many people only go halfway on their balance charging solutions; perhaps when they have many cells they make parallel pairs and only check sets of two, or something similar?

Of course, if nobody here ever goes above the 48v mark, then I guess I just get to go swim up the river on my own ^_^

BMooney: Of course, if nobody here ever goes above the 48v mark, then I guess I just get to go swim up the river on my own ^_^

Sounds good, post updates on what you find.

I shall keep you posted; do feel free to contribute information ^_^

I hope to have the electronics fairly figured out within a few months, but I am only setting aside 1-2 days per weekend, so it might be slow going to start.