Another DIY 6384 Foil Assist Build from Germany (with 2.2 kW power limit)

Hello everyone,
first of all, thank you for all your contributions on the forums! It helped a lot to plan my own build.

I started wing foiling and dock starting / pump foiling early this year. My main motivation to build a foil assist is to help me progress faster in pump foiling on flat water and eliminating the need to swim back to the dock each time. Maybe I will also use it for surf foiling on the Baltic Sea now and then.

My build requirements are:

  • max. 2.2 kW of power output (due to German regulations)
  • at every state of charge the power of 2.2 kW should be reached
  • getting up on flat water with a 10 liter pump board (I hope this is achievable, otherwise I need a larger board :see_no_evil:)

My main parts:

  • Motor: Alienpower 6384S 130kV
  • VESC: Flipsky 75200 pro
  • Remote: Flipsky VX3
  • Battery Cells: 36x Molicel 21700 p42a
  • Case: Camdenboss CHDX8-222

The VESC might be overkill for my build. Depending on how things will be going I might switch to an ESC. Probably the Flycolor XCross HV3. But I am not sure if I can limit the power to 2.2 kW on these. Maybe someone of you knows?

Battery size
I have purchased 36 battery cells. This gives me a few options on the battery size:

  • 8s3p
  • 10s2p
  • 10s3p
  • 12s2p
  • 12s3p

At every state of charge I would like to be able to max out the 2.2 kW of power. Due to the motor having a max. amp of 80A the 8s3p will not meet this requirement when almost empty:

8s almost empty:
2200W / (8 x 2.8V) = 98.21A > 80A

and 8s state of charge when power drops below 2.2kW
(2200W / 80A) / 8 = 3.44V
→ this means when being half way through the batteries energy the power drops below 2.2kW.


10s almost empty:
2200W / (10 x 2.8V) = 78.57A < 80A


12s almost empty:
2200W / (12 x 2.8V) = 65.48A < 80A

The 12s3p and 10s3p might also be not possible due space limitations and the already big VESC. So currently my options are 10s2p and 12s2p. The 12s2p should be the best option giving me more energy
and running cooler than the 10s2p. If there will be enough space in the case I will consider 10s3p again.

12s means more voltage and less current. The higher voltage results in higher RPM. The lower current might result in less torque. I need as much torque as possible and don’t care about the max speed. For now I have went with a 130kV motor due to fast availability. Let’s see how it turns out. 100kV might have been a better pick.

Currently I am waiting for all my parts to arrive.

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don’t go for the VESC based ESC. Get this instead and a 5V BEC.

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Hey @okp, thanks for your recommendation. But your url is not working for me. Seems like a url which leads to a user area.

A problem with ESC is, that I can not limit the current depending on voltage as the voltage changes over the batteries state of charge.

12s battery full:
2200W / (12 x 4.2V) = 43.65A

12s battery almost empty:
2200W / (12 x 2.8V) = 65.48A

If I set the current limit on the ESC to 65.48A I will receive more than 2.2 kW when the battery is full. When I set the max current to 43.65A I will not max out the power when having a lower state of charge.

The benefit of a VESC is that you can set all max values and also limit the power. The VESC will do the calculation on how much current it can draw depending on available voltage and the defined power output.

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yes, I’ve updated the link. I tested yesterday the VESC with good cooling and compared it back to back with this HV3… it’s night and day. I know we can setup VESC for much more torque, but I’m not sure this is achievable with FOC. Maybe a VESC with the good old BLDC would do the deal.

I’ll test this but I’m not sure FOC is the way to go on the VESC for us.

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All the theory in the first post has the assumption that the motor current is the same as the current that gets drawn from the battery.

By thinking twice about it, this assumption does not make sense, because it would mean that also the voltage of the battery is applied to the motor. But the motors RPM is controlled by the voltage. The motor voltage is variable and not constant.

There has to happen a transformation of current to voltage by the VESC/ESC. This means with rising discharge the current drawn from the battery will rise to achieve the equal power at the motor.

Pmot = Pbat
Umot x Imot = RPMmot x Mmot = Ubat x Ibat

Umot ≠ Ubat
Imot ≠ Ibat

A 12s2p and a 8s3p battery configuration should perform identical. Do I have a misconception?

How will they know and be able to test for higher power? Also I hope you are light as I found I needed 3.5kw if I really wanted a good push start on the assist.

In the end I will have to make them believe that the power is at max. 2.2 kW due to being it a self-build. As long as I can’t they will treat me as if I am above 2.2 kW. So I will have to show them my settings I guess.

Really, I would not care if they would not control it. But they are after it here… Last weekend we have been dock starting and they checked on us if we have motors…

Another option would be getting a license plate and it would be legal riding above 2.2kW. But then I can’t just swap it to another board…

Maybe you can tell me about some additional number in your setup when taking off so I can reverse engineer some numbers for my setup?

What is your weight?
How big is your board?
What is your RPM at take off?
What is your voltage at take off?
Which motor do you have and what is the kV rating?
Which propeller are you using?

What happens if you keep the the registration on the mast and not on the board?

Well, good question. I would have to ask them. When you see a boat, the registration is on the boat, not on the drive unit. Even when the drive unit is detachable. I guess they would apply the same.

What is your weight? ±95kg
How big is your board? 4’6" 24L Slingshot WF1
What is your RPM at take off? ±3000
What is your voltage at take off? I run 10S
Which motor do you have and what is the kV rating? 120-140Kv
Which propeller are you using? Efoil 6384/63100 120kv-140kv folding prop by ThermikDreher - Thingiverse


Thanks! I am at ±75kg but board at 10L.

So it might work out with a bigger board.

Assembled my spot welder today and began building the battery. I have decided to go with 12s2p.


looks good, what is the welder power supply?

It is a 3s1p LiPo with 5000mAh and 65C to 130C from SLS. I have changed the connectors to AS150.

The idea is to weld nickel copper sandwiches. Let‘s see if it works out.


Alright I have been doing some welding tests today.

I was able to weld 0.1 mm nickel strip + 0.15 mm copper strip at 90.0 J to 100.0 J. Below 90.0 J only the negative electrode was welding.

I also was testing 0.1 mm nickel + 0.2 mm copper. The problem was that it welds only the negative electrode side to the battery and I went up to 170.0 J.
I was thinking about going higher with the Joules. But the LiPo became warm, so I have aborted at this point.

Regardless the strip thickness I have observed that the negative electrode always welds better to the battery. Is that normal?

Edit: I get max. 1300 A out of the battery. This might be the issue. With pure nickel I don‘t have this problem. I can weld 2x 0.1 mm pure nickel strips at 35 J and both electrodes weld well.
I have found out that I can overcome this issue for nickel copper sandwiches when I weld the spot of the positive electrode again with the negative electrode. I can do this at 40 J with 0.1 mm nickel + 0.15 copper while the LiPo battery stays cooler.

I think the registration is like 15 bucks without any running costs. No big deal getting one for each Board.

Yes, I will go for it if the power will not be sufficient.