Calculating unloaded Motor RPM with Lipo's

[BLACKJACK_101]

What is the correct Lipo cell voltage to use when calculating unloaded motor rpm? I have been using a nominal voltage of 4.0volts per cell multiplied by the the KV of the motor to give the unloaded RPM, as an example a 2150KV motor motor on 3S multiplied by 12volts = 25800RPM, 11.1volts = 23865RPM and 4.2 per cell = 27090RPM, any thoughts on the best way of doing this.

[tagasagwan]

Hi Bro!

Well, there's nothing wrong with the way you calculate the unloaded motor rpm. If you want to base it on the nominal lipo voltage then use 3.7v per cell multiplied by the moto's KV. if you want, you may also get the exact voltage of you lipo pack by using a lipo checker, multimeter etc. then multiply it by the motor's kv.

Example: 4S1P lipo and a 2,200kv motor. nominal voltage is 14.8V x 2,200kv = 32,560rpm unloaded.

hope it helps!

Stephen

[BLACKJACK_101]

Thanks Stephen,

I suppose as long as we get the RPM range somewhere in the ball park for a particular application using 3.7volts-4.2 volts as a starter then that should be enough, be interesting to know what the RPM/voltage ratio is once the boat is on the water, telemetry here we come!

[tagasagwan]

Thanks Stephen,

I suppose as long as we get the RPM range somewhere in the ball park for a particular application using 3.7volts-4.2 volts as a starter then that should be enough, be interesting to know what the RPM/voltage ratio is once the boat is on the water, telemetry here we come!

That's true! Hehehe! You may use a eagle-tree or the Spektrum radio with telemetry. I use the later but then, to many wires and sensors to think about! Now, I just use the simpler spektrum MR200 and MR3000 rx. Works great so far!

[BLACKJACK_101]

That's true! Hehehe! You may use a eagle-tree or the Spektrum radio with telemetry. I use the later but then, to many wires and sensors to think about! Now, I just use the simpler spektrum MR200 and MR3000 rx. Works great so far!

I've used the Spektrum radio for quite a while now in my boats with the MR3000, no problems there, what Spektrum telemetry add ons do you use in the hull to run with your MR3000, I do have a Spektrum Tx with inbuilt telemetry.

[Fluid]

You don't need telemetry, a data logger will do. Depending on the load, starting pack voltage, the cell C rating/internal resistance and how long into the run you are, on-the-water voltage for a 4S pack can vary from over 4 volts to less than 3.4 volts per cell in reasonable use. For example, my FE-30 P-LSH racer with 4S1P/5000 pack averages 4 bolts per cell over a race, starting at 4.2 and ending at 3.85. My record-setting P-Offshore cat running 4S1P/5000 at 82 mph started the run at 4.15 volts, averaged 3.15 for the run and recovered to 3.8 volts on the way back to the beach.






.

[siberianhusky]

Don't waste your time with the Spektrum telemetry, thought it might be good, LOL bought an eagletree. At least the Spektrum sensors work with the eagletree so it's not a total waste.
Could never seem to get a reliable rpm reading out of it, and the telemetry range is far to short, It would tell me the info if I brought the boat in close, otherwise it always tells you no telemetry connection.
Probably works fine is a car where you aren't usually too far from it, useless in a boat.

[tagasagwan]

I use Spektrum DX3S tx on all my boats. I don't use the telemetry anymore. Like I said, too many wires to think about! hehehe! I agree with Fluid and Siberianhusky! I just run for fun since we don't have a governing body in Manila to sanction races. Sometimes we do have fun race with our boating buddies!