I finally had the opportunity yesterday to fly with my new amp meter.
Based on my amp and volt readings during flight it takes about 1500 watts (per motor) to maintain level flight. Watts=Amps*Volts so with 50V the amps at level flight is 30A. When the batteries discharge down to 42V the amps go up to 36A which is still 1500 watts. So in this scenario amps increase when voltage drops.
Full throttle is a whole different scenario because as the voltage drops the RPMs drop and therefore watts drop. So full throttle with depleted batteries does not give you the same climb rate. In this case the amperage actually decreases as the voltage decreases and my testing confirms this. The relationship between volts and amps at full throttle is linear with the following data points:
Notice we are going over the 80 amp rating of the ESCs. I havenāt had any trouble with mine nor have I seen any complaints about them burning up. Perhaps this is due to good airflow because of the ideal placement on the arms.
Using the above equation, if we use a 14S battery battery without changing the Kv of the motor we would expect to see the amps go up to 100A at each ESC. However, if we also change the motor from 180Kv to 154Kv then we will see fewer amps: 74A instead of 87A per ESC. Thatās how we achieve lower amps with the same power output.
Thatās a lot of amps! Appreciate you doing this test for us all. Do you need full throttle to climb at a decent rate, or is there enough power at say 75% throttle to climb? If so what is the climb rate at a lower throttle setting vs full throttle?
The OpenPPG has plenty of power so there is really no need to ever push the amps above 80. Itās only the first few minutes of flight when the batteries are fully charged that it will even go above 80. My son and I have climbed between 250 to 300 feet per minute with it. He weighs a little less than I do and can climb a little faster with the same wing. Here is an example from one of my flight logs of a 1500 foot climb in 6 minutes followed by a 6 minute glide back down. Notice the later climbs with low batteries are equally as steep:
The esc are rated for 100A burst and 80A continuous draw. I do have some 160kv motors if people want to try them or convert to 14s. Its just 7s batteries are a bit more rare.
I gathered more data for voltage vs amperage at full throttle and modeled it in excel. I found the relationship to be a power curve. I recorded the voltage in two ways (Open Circuit Voltage and Load Voltage). I performed measurements with 2 and 4 Bonkas (12S1P and 12S2P). Hereās a chart of the data after which I will explain what it all means and how I collected it:
First we need to understand that a battery is NOT an ideal voltage source. The voltage at the terminals will drop when a load is applied. This drop in voltage occurs because of the resistance within the battery itself. Here is a link if you want to learn more: Measuring Internal Resistance of Batteries - SparkFun Learn
Those who own an OpenPPG will be very familiar with the fact that increasing the throttle will drop the voltage (and the battery percentage). But did you know that doubling the number of batteries in parallel will cut the resistance in half resulting in a smaller voltage drop? This actually increases the voltage to the ESCs and motors and gives you more power (watts). This increase in watts comes from an increase in both volts and amps. The ESC is rated for 80 amps so we should be careful not to run higher than that.
Those who are flying with 8 batteries need to be careful at full throttle because that resistance will be cut in half yet again and supply even higher volts resulting in even higher amps. Iām already drawing over 80 amps with 4 batteries while with 2 batteries it barely reaches 80 amps. Remember though, that flying with fewer batteries puts more load on the batteries which can be dangerous. So more batteries is better as long as you are careful at the top end with the throttle as to not draw too many amps.
Back to my data. āOpen circuit voltageā is the voltage of the battery before applying a load. I recorded the voltage, then applied full throttle and recorded the amps. You can see a clear difference in amps when using 2 vs 4 Bonkas because of the way the voltage drops more under load when fewer batteries are used (Fewer batteries=more resistance=more drop in voltage=less amps).
āLoad voltageā is the voltage during throttle up. To collect this I recorded a video of my amp meter and voltage meter (throttle controller). Then I stepped through the video and recorded the values.
According to this data model, if you actually delivered 50 volts to the ESC you would draw 110 amps! However, because of the resistance of the batteries you would never get there but high quality packs with low resistance and/or multiple packs in parallel could put you well over 80 amps which is already possible with 4 Bonkas.
Thx for all your diligence on data collection - very much appreciated
So can we set a max current draw on the escās when we know we are going to fly 4 or more bats. Full throttle = 90% for 4 bats ā 80% for 8 bats.
I know I can program my GoldenMotor bike motor to set max current draw! Not easily but I never change the packs.
I figure I would be doing 4 max on foot launches , 8 or 12 on a trike. So having a fast way to set current limiting on the escās would be nice!!!
Chis said that their ESCs are fully programmable and can be set to a max current. They are much more expensive though and we would need four of them. If you do it based on current you wouldnāt need to adjust it between battery types or quantities. I donāt think ours can be programmed like that. However, my son and I are planning on tweaking the PWM code in the throttle controller to adjust throttle percentage to allow us to convert to 14S without getting new motors. It wonāt be based on amps though but thatās ok because we can use my amp meter to get it tuned just right.
We could limit the physical travel of the controller but I donāt want to lose that much resolution. Specially since it would need to be limited by about 70% to convert to 14S and would make the throttle very sensitive and touchy. But adjusting the PWM would make it feel and respond the same as it does now.
For just flying with 8 Bonkas you could install an amp meter like I did and just keep an eye on amps when the batteries are full. Then when they are low you would still have access to full throttle.
Do you know how much more expensive? Great work on the tests, graph etc. Iām wondering if using 6awg wire would help to reduce the voltage sag, would be less resistance. Are you using 8awg? Anyone out there build theres using 6awg?
Perhaps Chris can chime in on the escās. Cost as well as if they would be suitable for this project? If they were not much more expensive might be good alternative for some.
Let me reiterate that voltage sag is reducing the voltage getting to the motors which is reducing the amps. We donāt want higher amps.
If you manage to get more voltage to the ESC, by reducing voltage sag, then you will draw more amps and other actions will need to be made to reduce amps. Options include: changing the PWM output in the throttle controller programming like I mentioned, or lower Kv motors, or even smaller propellers. The most expensive option would be new speed controllers that can limit amps. I think Chris said his are over 500 dollars each.
Most of that voltage sag is coming from the batteries as is evident by the significant difference between 2 and 4 Bonkas. Itās normal for a battery to output less voltage under load. Higher quality batteries will sag less. More batteries in parallel will sag less. Poor quality batteries will have higher resistance and create more heat within the batteries as well as output less voltage.
Iām using 8awg until it branches out to the 4 ESCs where I am using 10awg. The ESCs themselves come with only 12awg. None of my wires or connections get warm at all.
Thanks glidedpilot. Iāll probably use 6awg on a single lithium ion pack since there will be more amps flowing vs a 4 bonka setup, from battery to ESCs.
Throttle code change sounds better if they are that much per esc.
Good plan. Plus you may need another type of connector too. Another option is to double up the 8awg which is essentially what we have now when we run two sets of batteries.
Iāve been reading about the idea of programming the PWM code in the throttle controller as a means of controlling amps at the ESC. I am finding itās not a good idea. The ESC needs to be rated high enough to handle full throttle amps of the given motor Kv and prop size. The ESC reduces throttle by pulsating the power to the motor on and off. While the average current may be lower than the ESC rating, the āon currentā will be over the limit. You may argue that the heat will be based on the average and therefore itās ok to handle power this way but the ESC is not as efficient at partial throttle as it is at full throttle.
So, my plan to convert to 14S wouldnāt be a good idea simply by changing the throttle code. I would need to change the motor Kv or propeller size instead.
I dont agree with that idea, it goes against all my RC and ESC knowledge I have. All you are doing is lowering your throttle correct? That startup current might be slightly change but its going to be insignificant, plus thatās why ESC have burst currents and youāre well well within the range. Its so small I dont think its worth thinking about. I dont see reason why it shouldnāt work.
Are you saying it would be fine to lower the throttle just a little with 12S or more significantly with 14S? Do you think there would be a benefit of going to 14S if I have to limit the throttle more than 70 percent?
