Is my math correct?

Batteries, Chargers, and Battery Management Systems.
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MarkL   1 mW

1 mW
Posts: 13
Joined: May 03 2020 11:16am

Is my math correct?

Post by MarkL » May 17 2020 10:49am


Just waiting for my motor to come by.

1) Please confirm my pack design
2) Please suggest optimizations (design/ cell technology)

Hub nominal rating: 144V 120A

Be X the voltage of my cells when full.
Be Y their Ah when full.
Be Z their max discharge amperage.

I may go for LifePo4 (IFR) 18650 cells.
Should I maximize the nominal output of my motor, please confirm the design of my battery pack:

Serie connections:
144 / X.

If X = 4 -> 144 / 4 = 36.

120/ Z.

If Z = 10 -> 120 / 10 = 12 cells.
If Y = 1.2 -> 12 * 1.2 = 14.4Ah ( 14.4 / 120 x 60= 7min of use)

So, I am good for a 36S12P pack, or (36 * 12) = 432 cells ($$).
That seems like a lot of cells to me.

Math correct? Which technology would you recommend?

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serious_sam   10 kW

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Joined: Mar 05 2017 8:07am
Location: Australia

Re: Is my math correct?

Post by serious_sam » May 18 2020 6:06am

I would say there's no problem with the algebra.

But motor current and battery current are different parameters, so using motor current to calculate battery current requirements is not correct.

What controller are you going to use? What is the motor driving? Why have you chosen LifePo4 (about 1/3 the energy density of li-ion)?
Last edited by serious_sam on May 18 2020 6:33am, edited 1 time in total.

obcd   100 W

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Joined: Jul 16 2019 3:45pm

Re: Is my math correct?

Post by obcd » May 18 2020 6:29am

Besides that, they normally don't take the full charged voltage as cell voltage but an average. For instance, a Li ion cell can usually be used safe between 3V and 4.15V. Normally, 3.6V is chosen as cell voltage. So a 36V battery has 10 cell groups in serie. When being fully charged, such a pack will measure 41V
LifePo4 has a lower cell voltage. If I remember well, 3.2V is used as nominal cell voltage.

MarkL   1 mW

1 mW
Posts: 13
Joined: May 03 2020 11:16am

Re: Is my math correct?

Post by MarkL » May 18 2020 8:58am

I chose LifePo4 as a security measure (<-> less chance of it taking fire). But now I feel wrong.

Right now, my controller is a generic 24mosfet HY3210 (<-> 120A) with caps rated at 100V.
At the final stage, the motor will be controlled by a KLS14201-8080I ... 0V1.10.pdf
It is given for 92% efficiency. Right now, mine is given @84% efficiency.

The motor will be driving a 60kg motorbike prototype. I am VERY space constrained.

I also saw Leaf G1 modules, but same thing, it takes a lot of space.

Can you please correct me?

Yeah, LifePo4, even if safer, seems like an odd choice when space is restricted.

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