VESC Motor Current Max vs Battery Current Max

ESK8 Electronics
1

I am new to ESK8 build and have spent many hours reading before posting. I have some basic questions I am hoping an expert can quickly answer or point me to the right thread. This site has been inspirational and hope to eventually build my own monster off-road board. But for now, I am trying to learn by tweaking an existing board.

I bought a Propel Endeavor 2 Pro board and it simply doesn’t have enough low end torque and I am trying to increase better grass hill climbing capability. It uses a Flipsky VESC and I have been working with Propel’s support which has been good but wanted to get expert opinion on this site for education and verification.

Please kindly allow me to provide some background info on the setup:

A - Battery Pack: Samsung INR21700-50G. 12S5P. Total output 44.4V. Continuous Amp 25A. 1110wH. The individual Samsung battery spec shows battery can provide PEAK current discharge at 14,700mA each which equates to 14.7A x 5 series = 73.5 amp PEAK output

B - The board has a Flipsky Dual FSEC6.7 Pro 140A base on VESC 6.6. The VESC specs are = Continuous current 140A, instantaneous current: 400A

C - Remote = Flipsky VX2

D - Motor is 6374. Max amp / KV unknown (waiting to find out exact specs)

E - BMS specs unknown.

F - 15 teeth on motor with 68 teeth on Wheel. The wheel is 8"x2" pneumatic (800mm x 200mm)

Here is my setting for my VESC picture below.

IMG_2246
IMG_22461920×1440 269 KB

Questions:

  1. In the current factory setting, the “Battery Current Max” is set to 25A, which makes sense since the 5P series can provide 25A continuous to each motor. Question is … how does the 25A from the battery provide enough power to both motors which has factory setting of “Motor Current Max” = 50 A each. So the battery has to provide 50A x 2 motors = 100 A. How does the 25A setting allow 50A output to each motor?

  2. Is it correct to set the “Battery Current Max” to the continuous battery specs? The battery pack peak Amp output is 73.5 A. (I read somewhere these peak output can only be sustain for a few milliseconds).

  3. Since my battery pack can provide peak current output 74 amp and the factory default for “Motor Current Max” is at 50.00 A, Is it OK to set my “Motor Current Max” at 60 A permanently? I did set it to 60A each motor to test some short run and the low end torque was better for grass hill climb. At this setting will I damage the battery long term?

  4. At “Motor Current Max” at 60A setting, how long can I do a full output for hill climb and be safe? Is two minutes of full throttle OK for the battery’s health?

  5. In the “Motor Current Max” of 60A setting, I did notice a pretty significant battery sag. I was on a paved hill, and battery was at shows 57% at rest on my remote. I did a full throttle and the battery capacity dropped down to 30% for a split second and recovered back to 56% at rest once I reached the top of the hill. Is this acceptable battery sag? My main concern is not damaging the battery long term.

  6. What is the largest value you would recommend for the “Motor Current Max” and “Battery Current Max” without damaging the current battery pack?

Thank you in advance for everyone’s patience as I am a newbie and really trying to learn and understanding.

2
  1. So, lets see for your battery INR21700-50G specs: comtinious current discharge is 9.8a. You have 5p battery. This makes max battery output as 49A. So its max your battery can output. 2. Battery current max 49A as pes spec of 9.8 a for cell. 3-4.I dont see reason to put more than 24.5 a on each motor, battery wont gove more than restricted by max battery output anyway. 5.sag may be verified with moosh battery tests if it has for this battery. But cells sag whrnon on maximun current for them. This is ok probably

Hope this is clear now. Ask if not

3

Just personal thoughts: i would never consider peak current in esk8. Also, cells with 9.8A current are on low side. For future projects check milocel 21700 P42a or even newer p42b. Both of tgem has continious current of 35a with susteinable 45a if consider tempreture monitoring. To not damage battery - do not store fully charged or fully discharged. Do not try to discharge with higher current than it can output as per spec ang you would be ok. Also good thing to check your battery health (p groups voltages from time to time)

4

@OleksiiF Thank you so much for the prompt reply. It’s very kind of you to take the time to help out a newbie. What you say makes complete logical sense and it’s what I would have concluded too. Thanks for clarifying for me that the Battery Current Max (continuous) should be no more than 49A. I am assuming Propel rounded the 24.5 up and just allocated “Battery Current Max” 25A for each motor (25x2 = 50).

The weird thing is that Propel support said that the factory default for “Motor maximum current” should be 50A and when I didn’t get enough low end torque they told me to dial it up to 60A. I have a specific hill I tested the board on and at 50A, the board would not make it up the hill… but at 60A it made it up without stalling.

That’s why I am so confused as I would have thought that by setting the “Battery Current Max” to 25A, there is no reason to set the “Motor Current Max” any higher than 25A. However in real life it’s making a difference in how the board’s provide torque and power for motor value of 50A vs 60A which both motor values are way above the battery value of 25A.

Still unclear why it makes a difference on the higher setting of “Motor Current Max” and in real life. Any additional insight?

1 Like
5

I looked up Molicel P42A… wow… max continuous current is 45A. Which means 5P = 225A. That’s crazy. It does seem like the storage capacity is about 14% lower than the Samsung… but the cost isn’t any more than Samsung. Maybe when these original factory battery wears out… I will replace it with Molicel batteries instead.

Thanks again for your help.

6

Well, to say thuth, its kind of unexpected to me that increasing motor max without increasing battery max made any difference. There is always place for knowlage improvement, right? I would advuse to check telemetry if you have one for current in such load(or if you dont have one, make test with laptop connected or somth) just to make sure what are current drain on battery. Wish you a good luck.

1 Like
7

If your battery pack is 70A.

Battery Current Max:

  • 70A / 2 motors = 35 amp
  • To be safe, keep it at 30 amp

Battery Current Max Regen:

  • (5P * 4A) and divided by 2 motors = 10 amp.
  • Because recommended charging for lithium battery is between 2A or 4A max.

The correlations between Motor current and Battery current changes based on the percent of motor power. Example: 6355 Motor is rated at 2500 Watts and 80 Amps.

So if you are pulling 1250W of power with a 44v battery.
That is 50% rated of the motor’s power.

The amps on the Battery side would be 1250W / 44v = 28.4amp.
Then your Motor current is at 56A = 1250W /(44v * 50%).

But now consider this, 36v is minimum voltage for 12s battery pack.

  • 1250W / 36v = 34.72amp
  • Your Battery Current is 35 amp (rounded)
  • Your Motor Current is 70amp = 1250W / (36 * 50%)
8

How you calculated bat current max?

9

It depends on how many battery is in parallel and what is your constant current discharge of the battery.

I have 12S6P Panasonic NCR18650B that has max recommended discharge current at 6.8A

6P * 6.8A = 40.8A

Then my Battery Current Max is 40A.
And if I am running dual VESC. Then (40A / 2 VESC) = 20A is my Battery Current Max setting.

1 Like
10

@Deakbannok - I hope you don’t mind me running questions and calc by you.

Thanks for the clear equations. Your examples and calculations really helped me understand better. Just to clarify - when setting the Battery Current Max, you are using the “max continuous discharge” value for the battery - right? Not the peak discharge value?

So to extrapolate your equations based on the below battery specs, my batteries are Samsung INR21700-50G has a “max continuous discharge” of 9.8A. So with my 5P configuration, it would be 9.8A * 5P = 49A (rounding it up to 50A) for ease. So using your equation, Battery Current Max should be 50A / 2 motors = 25 amps. That is exactly what the factory setting was. Are my calc correct?

image
image885×827 89.2 KB
image
image897×423 28.6 KB

Regarding the Battery Current Max Regen… I am not sure why Propel has it set at -25A (per my original post screenshot). Don’t even understand what the “minus” sign is all about.

The motor I am using is a 6374 rated at 3000W motor each. The Propel’s factory original setting for Motor Current Max is 50A each. Using your equation, that would mean 50A to motor at 44V = 2200W or 73% of motor max rating. By increasing to 60A * 44V = 2640W or 88% of motor rating. I am still unclear why Propel asked me to set Motor Current Max so much higher than the Battery Current Max. Is your understanding the same as OleksiiF that Motor Current Max should not be higher than Battery Current Max?

Thank you in advance.

11

“When setting the Battery Current Max, you are using the max continuous discharge value for the battery - right?”

Yes, or little less is better for your battery.

9.8A * 5P = 49A (rounding it up to 50A) for ease. So using your equation, Battery Current Max should be 50A / 2 motors = 25 amps. That is exactly what the factory setting was. Are my calc correct?488

Yes.

Regarding the Battery Current Max Regen … I am not sure why Propel has it set at -25A (per my original post screenshot). Don’t even understand what the “minus” sign is all about.

“minus” -25A is the return current to the battery.
How fast can the battery be recharge?
Usually, max recharge current for the Lithium is rated for 4A. (Your cell is rated at 4.9A for 1 hour charging) But I prefer to keep it at 4A or less to prolong life energy of the batteries.

So 5P * 4.9A = 24.5A (round it down to 24A).
24A / divided by 2 VESC = 12A.
So your Battery Current Max Regen is “-12A”

Depends on what type of the ride are you planning to. hills, mountains, commute roads, or offroad.

why Propel asked me to set Motor Current Max so much higher than the Battery Current Max . Is your understanding the same as OleksiiF that Motor Current Max should not be higher than Battery Current Max ?

Generally, you want to keep electrical components cool.
Because all the waste current will be dissipating into thermal heat.
If you have a high output battery pack like Lipo, then you want to calculate your motor current lower.
If you have a low output battery pack, then your motor current will be higher.

Because over a longer period of time, random temperature fluctuations, stray magnetic fields and mechanical movement will cause magnetic properties to decay

Last note: VESC operates rather like a transformer (using the motor’s inductance) to deliver all the power from the battery to the motor.
Also motor current limiting is to protect the MOSFETs.

12

Thank you for the info.

I ride my board usually on trails, bike path, off roads, and parks. I don’t commute with it. I do need the power of Motor Current Max = 60A when off-roading on hilly grass once in a while. Not often.

Is it correct to assume -12A would also affect the braking characteristic meaning much less aggressive if set at -25? I like the current aggressive braking characteristic but Propel’s default factory setting of -25 really leaves me in a quandary as it sounds like it may damage the battery long term. I might try to set it -12 but wanted to understand if it affects the braking characteristic before doing it as the process of updating the VESC is rather involved.

Since I have a Samsung Lithium Ion battery back, is it correct to assume that would be consider low output battery pack? I really don’t ride my board very hard and usually keep it around 20 mph max (my board can hit up to 34mph. But once in a while, when I need to do grassy hill climb, I will need do full throttle. So hopefully I won’t generate too much heat in general. My biggest concern is to NOT damage the battery long term.

Thanks once again for your help. You have been very kind in providing the info.

13

Is it correct to assume -12A would also affect the braking characteristic meaning much less aggressive if set at -25?

Yes, the braking time and distance will be extended a little.

But here are the reasons why… I will try to keep it simple.
Your motor can act as a capacitor or an inductor. It will store the current and dissipates somewhere.
Even you have a motor that handles 80A. You cannot set motor current max brake to -80A.
Because this current has to go somewhere.
This is why we separate them partial to the motor and the battery.
To reduce inrush current to the MOSFETs and give sometimes for the VESC to absorb.

For your battery, if you have BMS and you are charging it at 25A+.
The battery protection might cutoff and VESC will thinks that charging voltage is not enough and will try to increase the charging voltage and burns something.

Example:
Motor Current Max Brake : -50.00 A
Battery Current Max Regen : -12.00A
(50 - 12) = remain 38A is a short-circuit current.
This short-circuit current will be dissipate by whatever current the MOSFET can absorb.

Overall, If you bypass BMS or don’t have one.
For non-sustain braking (like 1-3 second), you can try to set battery brake current equal or less than the motor brake current.

14

To simplify a little, the reason why battery amps can be lower than motor amps is because the voltage on the battery side of the vesc is constant, whereas phase voltage is speed dependant.