Let's Discuss Battery Max Amps In Depth

[RedBaron]

That’s interesting, I need to learn more about gear ratios. What are you using to make your graphs?

[professor_shartsis]

Esk8 calculator

@b264 how about a “peak loaded velocity & gear ratio calculator” for those uphill races? suppose i’ll be climbing 1.8miles on a steady 5% grade… [image] i have 4x 190kv 0.05ohm motors & 83mm tires, 120a motor current and 60a battery current limit per motor, & 13S (48.1v) what’s my maximum top speed up slope and what gear ratio gives this top speed assuming 0.6m^2 frontal area, 0.75 drag coefficient & 200lbs? assuming battery current limit is 95% or less than the motor current limit, and …

the mosf interesting part is:

8082.063923094534641801 motor rpm @ peak mechanical power

^peak mechanical power is 10746.218459832w @ 8082.063923094534641801 motor rpm

A = meters per second = XX.XXX
B = drag coefficient = 0.75
C = frontal area = 0.6m^2
D = fluid density of air = 1.225kg/m^3
E = wind drag force in watts
F = sine of 5% slope = sin(atan(5/100)) = 0.04993761694389223373491
G = acceleration of gravity = 9.80655m/s^2
H = vehicle mass in kg = 90.7184kg = 200lb / 2.20462lb/kg
I = mechanical watts required for constant speed up slope with no wind drag
J = mechanical watts required for constant speed up slope including wind drag
K = H * G * F
L = (1/2) * D * C * B

E = ((1/2) * D * C * (A^2) * B) * A

I = H * G * A * F

J = E + I

J = (((1/2) * D * C *(A^2) * B) * A) + (H * G * A * F)

J = (1/2) * D * C * B * A^3 + H * G * F * A

J = (L * A^3) + (K * A)

^this can be rearranged to:

A=(sqrt(3) * sqrt(27 * J^2 * L^4 + 4 * K^3 * L^3) + 9 * J * L^2)^(1 / 3) / (2^(1 / 3) * 3^(2 / 3) * L) - ((2 / 3)^(1 / 3) * K) / (sqrt(3) * sqrt(27 * J^2 * L^4 + 4 * K^3 * L^3) + 9 * J * L^2)^(1 / 3)

we know:

J = 10746.218459832w peak mechanical
L = 0.275625 = (1/2) * D * C * B
K = 44.42622815547907982077 = H * G * F

therefore:

A=(sqrt(3) * sqrt(27 * 10746.218459832^2 * 0.275625^4 + 4 * 44.42622815547907982077^3 * 0.275625^3) + 9 * 10746.218459832 * 0.275625^2)^(1 / 3) / (2^(1 / 3) * 3^(2 / 3) * 0.275625) - ((2 / 3)^(1 / 3) * 44.42622815547907982077) / (sqrt(3) * sqrt(27 * 10746.218459832^2 * 0.275625^4 + 4 * 44.42622815547907982077^3 * 0.275625^3) + 9 * 10746.218459832 * 0.275625^2)^(1 / 3)

A=32.32551993764664323864 meters per second

[natch]

@longhairedboy

I run my 12S5Ps on unities at 55 amps, 16/36 on 100mm wheels. 37mph and not slow getting there. Hills aren’t even a question.

ideally you want the max amps just below the BMS cutoff

I have one doubt. In the Focbox UI App, when I limit Max battery amps, is this limit for the whole battery or is it per VESC?

If I set the limit to 35A, would the Unity be draining in some moment 70A (2x35A) or just 35A? My BMS is 60A rated.

[Skatardude10]

I’ll just leave this here as it seems relevant. I was trying to decide between 10S5P and 12S4P. I went 10S5P, I think it just edges out in most categories probably because of the two extra cells total.

[Meeep]

Make sure you are on the latest firmware, it was suspected for a while that is was “per VESC” and later confirmed that it was a bug. The newest firmware fixes this to be battery total.

[longhairedboy]

@natch

In the Focbox UI App, when I limit Max battery amps, is this limit for the whole battery or is it per VESC?

ITs per ESC. So on a unity, you’d limit it to the full amount, and on twin focboxes youd limit it to half the full amount.

[natch]

Ok, I have the last firmware version on my Unity, so should be total the total amount then.

thanks you both!