# Calculating batt current?

So I have read on several occasions that motor amps and batt draw amps are not 1:1. So how do I calculate what the draw on the batter would be? If you’re running two 6374s rated for 85 motor amps and 3500W each, how could I figure out what the draw on the battery would be if running a 12S pack? This would be mostly to determine if a BMS could handle the setup or not.

single cell amp rating * p count = max amp of ur pack
u don’t calculate it based on how much ur motor can handle, but what ur cell is able to pump out

run bypass, no, its not dangerous, trust me, i’ve been thrown off my board because of bms low amp discharge

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No, what i am wondering is how to determine how much current the motors are drawing from the battery, not what the maximum current capacity of the batteries are. I ask this because so many people seem to say motor current and battery draw current are not the same, which makes no sense to me unless the output voltage of the ECU is being stepped up. So if the motors are drawing 200A, how much current is being drawn from the battery? To me, the answer would be 200A (assuming the ECU is 100% efficient which its not), but I am told that is not accurate.

That’s exactly what’s going on. The ESC, in concert with the motor’s inductance, is acting like a DC-DC step-down converter, converting the (relatively) constant voltage from the battery into a varying voltage to feed the motor. The voltage the motor requires changes depending on the load on the motor, and the speed of the motor.

You don’t need to calculate the battery current based on the motor current. It’s irrelevant. You just set the battery and motor current limits appropriately in the vesc tool, and let it work its math-infused magic.

The ESC will draw from the battery only what is required for a given throttle command, up to the battery current limit that has been set. So you set the battery current limit to the maximum the battery can handle. (Or the maximum the ESC can handle, whichever is lower)
That means if the ESC is set to 10 battery amps and 100 motor amps, it will only ever draw up to 10 amps from the battery.

The motors don’t “draw” power. They are dumb, passive hunks of iron, copper, and magnets. All the control is done by the motor controller.

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Go the other way.

• Pick a deck and enclosure that will fit it.
• Then pick a battery size/type/config that will fit that enclosure.
• That will give you a power limit based on the battery’s cells and config.
• From there, you get to pick ESC, ESC settings, motors, drivetrain all to fit the battery’s specs.
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I love you Ryan

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here’s how you set your battery current in settings

the [motors will not pull more than what the esc is telling them to pull], which is equal to however many amps you set them to

so we already figured out that the battery is <= motor current [because they won’t push more than what the esc is pulling, unless you did a boo boo]

now depending on your pack specs, you may have to lower it

if, say, the pack (or bms) can only handle 30A total, you set your battery current settings to 15A for 2wd, since it has to be split between each side

now, if it can handle as much as the motors can go, set the battery amps to 70-80% what you set the motors to (there’s some weird science thing that was explained to me by @DerelictRobot a while back, he can probably fill in if he feels like it)

bam, you have your battery current numbers

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maybe i worded it wrong?

battery will only push what the esc is telling the motors to pull

or maybe i just got the wrong thing goin here

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Yes your wording was shit, not gonna mince words.

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cock

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The ESC is the only “smart” thing in the system.
Battery is dumb, will put out as much power as it can until it explodes.
Motors are dumb, they will take as much power as they are given until they melt.
Only the ESC can control things.

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i think i fixed it

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Sort of… but you still used the word ‘pull’

Although I am quite fond of the word pull, its not an accurate term because the esc is ‘pushing’ to the motors.

either way, if you wanna use a discharge bms (interesting choice), that will be the bottle neck,
seetings = ((battery current limit maybe - some headspace) / 2)

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that’s true,
it pulls from the battery then pulls to the motors tho? like a cunning middle man
it’s6am i should probably stop before i get worse

Think of a small water wheel connected to your faucet.

The municipal water supply is the battery.
The faucet is the ESC.
The water wheel is the motor.

If you broke the faucet off the pipe, water would just blast out - dumb battery.
If you stuck the water wheel straight on that, it would probably explode - dumb motor.
All that’s keeping either of those in check is the “smart” bit. You, with your hand on the faucet.

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if you got the motor current and the duty cycle you can roughly work out the battery amps.
100 motor amps at 60% duty is roughly 60 battery amps
on a side note vesc controls current, things like car escs control speed hence why batteries MUST be overrated for the esc or they will get hot and the esc must be overrated for the motor or it will fail by dumping more amps then it can handle.

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Not when you’re talking about VESC. You can run a 12s1p or a 12s100p battery with any vesc.

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How is everyone misunderstanding OP’s question? It works like this:

motor current = MAX(battery current setting in vesc tool, throttle value (between -1 and 1) X motor current setting in vesc tool X duty cycle (between 0 and 1))

So if you want to drive your motors with 85A all the way up to your max speed, then you need a battery that can supply 85A x2 = 170A or more. If you only care about that acceleration until 50% of your max speed, then your battery only needs to be able to supply 85A to drive the motors at 85A each. If your battery could do only 42A, then you could have full motor torque until 25% of your max speed.

However, remember that having full motor amps at the end of your speed range can suddenly throw you off once you reach max speed. That’s why we set our max duty cycle current limit start to 85%, and max duty cycle at 95% so that the ESC can still break in case we pass that value.

So with that in mind, if you want 85A x2 at 85% duty cycle, that will require a 144.5A draw from the battery. So to answer your question: for an electric skateboard a battery capable of delivering 144A or more will fully satisfy the needs of a dual motor board with each motor running at 85A.

P.S. Remember that voltage sag might not let you reach top speed while pulling that amount of current. So in reality, you might have to over-spec your battery regardless if you want that much power!

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