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 )
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.
vs
10s almost empty:
2200W / (10 x 2.8V) = 78.57A < 80A
vs
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.
Motor
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.
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.
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.
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
while
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…
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?
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
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.
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