Hi everyone, I just finished my first DIY and just barely have started testing it. Part of my buddies driveway is super steep and I wanted to see if my board could haul me up it. It didn’t seem to be able to which surprised me a little bit.
I’m totally get it if it’s just not something a board can do because I’m asking a lot. 230 lbs and the hill was steep enough where you would almost skid down it with the brakes on.
My setup is 12s6p LR21700LA cells. I have maximum pack draw set to 110A per side.
The motors are flipsky 6384 140kv configured in the vesc tool as FOC and 60A per motor.
I have 9" mbs wheels with 15t/72t motor pulley. I think with free spinning my remote shows about 33 mph which is what I was going for in the build because anything faster isn’t super stable on this setup. I wanted torque over top speed.
I am running 5.3 firmware on the flipsky dual 6.7.
So is it simply too steep of a hill and I’m heavy? I kinda figured with this gearing it would spin up the tire before stalling out.
The other things I can think of is it might be setting on the vesc. If I give the board full throttle it is pretty manageable untill 15-20 mph where it really starts to take off. I am assuming the throttle mapping is programmed that way so you don’t get thrown off by blipping the throttle.
The other thing I can think of is I basically started on this hill at 0 mph. Do the motors have a hard time producing torque when they are basically not moving? I know sometimes they want to get caught between poles ( forgive lack of technical term).
Yeah this is your problem. Your battery current is higher than your motor current.
The flipsky 6.7 isn’t a fantastic controller, so try these settings PER SIDE
If your settings actually allowed you to pull 220a from the battery your esc would’ve melted.
Motor current: 80a
Motor current Regen: 65a
Absolute current max: LEAVE STOCK
battery current max: 40a
Battery current Regen: 20a
The motor current per motor will always be higher than the battery current per motor (the esc lowers the battery voltage… higher current and lower voltage on the motor side is the same wattage as the lower current and higher voltage on the battery side)… so you need the motor current limit per motor to be higher than the battery current limit per motor.
You can find the peak battery wattage at 0rpm by multiplying the motor current limit times itself and then times the motor winding resistance.
For example 50a motor * 50a motor * 0.05ohm = 125w (I^2R = W)
same wattage as:
2.74a battery * 45.6v battery = 125w
^In other words if your winding resistance is 0.05ohm, then at 0rpm, 50a motor current equates to 2.74a battery current, assuming your pack is 12S (and the motor current is at 2.5v).
The peak duty cycle at 0rpm (mosfet ON % time) is the above battery current divided by the above motor current so 5.48%. The peak motor side voltage is also 5.48% of the battery voltage at 0rpm with these assumptions — 2.5v motor side voltage is 5.48% of 45.6v battery voltage.
When the motor is turning to get a rough approximation of the battery wattage you take (I^2R) + (torque_of_the_motor_in_Nm * the_rotational_speed_in_radsec)
To get the torque, get the KT = 1/KV where KV is in rad/sec per volt, then multiply the KT (torque Nm per amp) times the motor current.
I have noticed that on my board if I am ridding before hitting a steep hill my board can manage it pretty well considering the lack of torque I have on it. If I have to come to a stop in the middle of the hill and start back up it’ll struggle to get up the hill and sometimes I can’t even get back to the same top speed as before.
I’m sure you can gear a board to try to avoid that and in my case just get more powerful motors but I just figured I’d mention it to answer the question.
At low speeds. Then once you get past 1/3 of your top speed you’ll be pulling less and less motor current until youre only getting 20a at full speed. Basically, you will never pull more than the battery current setting from the battery but if you set the motor current higher you will be able to get more current (without pulling more battery current) while at low speeds)