72v LFP battery, 20s? 24s? Settings?

[Rednecktek]

Math is failing me but I'm thinking about the math on a 72v battery for an ATV conversion. LFP cells are supposedly 3.2v nominal, and Li-Ion are 3.7v, and when I divide 72v by 3.2v I get 22.5 cells. Even if I go by 3.5v that comes out to 20.5 cells which doesn't work either. I've seen videos where they somehow make them out of 20s but the math doesn't jive.

Also, all the 24s BMS's I've seen are set up for Li-Ion, but is there any reason I couldn't use it for LFP by just setting the high voltage at 3.65v like my LFP batteries?

What's the math and BMS on a 72v nominal battery?

 

[sunshine_eggo]

Check the peak voltage of the charger and divide by 3.65V

Sanity check with 12V 4S relationship (it holds almost perfectly for 48V, so should be good for 72V):

72/12 * 4 = 24S

If peak charger voltage / 24 < 3.65, 24S FTW.

 

[koppted]

Check the peak voltage of the charger and divide by 3.65V

Sanity check with 12V 4S relationship (it holds almost perfectly for 48V, so should be good for 72V):

72/12 * 4 = 24S

If peak charger voltage / 24 < 3.65, 24S FTW.

Please correct me if I'm wrong. If he goes for 24s that mean when fully charged the battery voltage would about 87.6v
For 23s max voltage would be 83.9v
For 22s max voltage would be 80.3v
for 21s max voltage would be 76.65v
for 20s max voltage would be 73v

He'd have to decide how many to put in series based on how high a voltage a charger he has or wants to have.

 

[400bird]

Please correct me if I'm wrong. If he goes for 24s that mean when fully charged the battery voltage would about 87.6v
For 23s max voltage would be 83.9v
For 22s max voltage would be 80.3v
for 21s max voltage would be 76.65v
for 20s max voltage would be 73v

He'd have to decide how many to put in series based on how high a voltage a charger he has or wants to have.

That's exactly what sunshine_eggo said, he just came at the math the other way around.
Max charger voltage specification divided by number of cells, the mn round down. At 24s the charger would need to do nearly 88v

So would the inverter/motor controller. Check both the controller and charger specs to make sure you land on a cell count and voltage range that works for all components.

 

[sunshine_eggo]

72V system would typically charge to about:

14.4*6 = 86.4V

86.4V / 3.65 = 23.67 cells

 

[Rednecktek]

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.

 

[sunshine_eggo]

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.

 

[Rednecktek]

Right. I was using that link as an example to get on paper. Most similar kits in that range just say 72v which doesn't really help.

 

[400bird]

I'd find one with a or request a spec sheet.
I read that 48-72 volts as nominal, not max. 48 and 72 are multiples of 12, so nominal makes sense to me. Still, a spec sheet wins.

What are you going to use to charge?

 

[sunshine_eggo]

Right. I was using that link as an example to get on paper. Most similar kits in that range just say 72v which doesn't really help.

Then it sounds like asking for cell count is premature. You don't even know what you're powering yet...

Pretty simple... what is the operating range of your system? What cell count best fits that range? There's your answer.

 

[400bird]

If the math shows 23.63 cells, see if voltage range for 24 or 23 works better.

24s is the common count (if there is one) for 72 volts, so I'd expect better component availability.

 

[Solar Guppy]

Typically 36/48/72 voltages in motive applications are just like ESS voltages

For example:

36V golf cart charges to 44V
48V golf cart charges to 58V
72V golf cart charges to 88V

As and example, the NAVITAS 600A 72V is rated to 100V ( AC drive conversion for golf carts )

 

[OffGridForGood]

There are ATV Electric conversions on YT and EV conversion sites. Ones I have seen mostly use 48 or 96v it seems. And about 13-20kW motors.
If you start with an ATV that has separate engine and transmission parts with pully or chain drive between, then the electric motor can just drive the existing transmission. Some of the ones posted are base on the Polaris 400 Sportsman - if you can find an ATV with blown engine, can be cheap to purchase as a starting point.
I would like to convert my lawn tractor first, as a step to learning the conversion pitfalls. Then do an ATV. Later see how the sodium batteries do in cold for a snow machine conversion future project. If a modular battery system & voltage could be applied, one set of batteries could run all three machines (batteries being the most expensive part of the build).

 

[OffGridForGood]

3HP !! it has been done before !!
and it runs pretty well considering.