So maybe it would help if I linked an example. I'm looking at variations of a
72v 3kw motor kit and converting an ATV over. I think it would be fun to build a battery but doing 300 little 18650's isn't my style and I was hoping to use good old prismatic LFP cells.
But it serms there's still confusion about if I need 20s, 24s, or whatever. The math still isn't clear because I'm still seeing partial cell counts here.
You will almost always see partial cell counts. You have to understand the results and make a decision. Let's look at 12, 24 and 48V:
12V/3.2V/cell = 3.75 cells
24V/3.2V/cell = 7.50 cells
48V/3.2V/cell = 15 cell - but wait!!!! Then why to we use 16!!!???
We use 16 because the operating voltage range of LFP better matches the operating voltage range of lead-acid batteries.
So you need to consider the operating voltage range of any application when sizing the battery. You're essentially designing a propulsion system. You need to have an essentially perfect understanding of all the components and their parameters.
Is 4 hp enough for you?
Looks like you've been misled:
3000W Electric Motor Kit
High Speed
Kunray's 72V 3000W brushless motor has a rated speed of 4900rpm/min and a maximum speed of 6700rpm/min.
- Rate Voltage: 48 - 72V
- Rate Power: 3000W (4hp)
- Rate Current: 45A
- Rate Speed: 4900rpm/min
- Max Speed: 6700rpm/min
- Torque: 5.4N.m
- Load Weight: about 100 - 150kg (220 - 330lb)
Based on the above, it looks like 72V is a max, not a nominal. It is possible that they mean that as a nominal voltage range, but I would not assume that.
That means:
72V/3.65V/cell = 19.72 cells
72V/20 cells = 3.6V charge
Based on the data available, 20 cells would be appropriate, but it's critical they never be charged above 72V
I would hesitate to design a system based on amazon specifications and a "testing report" that's not readable. I would seek out manufacturer's specifications of both the controller and the motor.