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Dec 21, 2013, 03:00 AM
Electric baptism 1975
DavidN's Avatar
I concur with desertstalker.
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Dec 21, 2013, 09:42 AM
rip
ripacheco's Avatar
I actually tested scorpion motors with an inline volt meter and indeed the under load volt is lower than the rest volts
This makes Lucien's charts very useful to evaluate the actual performance of these motors
I wish other vendors would take the time to test motors.
With most manufacturers their specs are an act an act of faith until you use it. And most users do not test the motors using good equipment. So you get a review that is at best an empirical rough test.
Dec 21, 2013, 10:24 PM
I am a nice guy! Really!

80% of Battery usage


Just a quick reply to arogantamerikan. If you took the time to learn anything about batteries you would know that their discharge curve vs voltage is anything but linear.

For LiPo batteries the point at which they reach 80% discharge is somewhere around 3.0 volts. Of course this is dependent on a number of factors such as temperature and rate of discharge. Why not do a little research before coming on and taking cheap shots at people. This one makes you look ridiculous.
Dec 22, 2013, 07:13 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
'Joined' dec. 20, two messages, shoots of his mouth, cheap shots, account suspended ... you and I can do the math Mike, this member created a new sock account to show his paranoid biased ignorance. Cowardamerikan instead of arrogantamerikan would have been a more appropriate user name

Vriendelijke groeten Ron van Sommeren
Last edited by Ron van Sommeren; Dec 29, 2013 at 11:18 AM.
Dec 22, 2013, 10:57 AM
I am a nice guy! Really!

Oh! I see.


Quote:
Originally Posted by Ron van Sommeren
'Joined' dec. 20, two messages, shoots of his mouth, cheap shots, account suspended ... you and I can do the math Mike. Coward Amerikan would have been a more appropriate user name

Vriendelijke groeten Ron van Sommeren
Yes. I did not look into his history.
Last edited by Mike Dubovsky; Dec 28, 2013 at 05:11 PM. Reason: Edited to remove statements and heading that may be perceived as "Trolling"
Dec 27, 2013, 07:01 PM
Innov8tive's Avatar
Thread OP
Even though the account has been suspended, the questions posed here by arrogantamerikan are valid, so I will take the time to address them for the education of those that care to read it. I have taken the liberty to correct the misspelled words and punctuation errors in the original post.

Mr. Miller,

Why do scorpion prop data charts show the tested voltage as 7.4v or 11.1v?


Because that is the typical voltage of a Li-Po battery cell when it is being used during a flight at full throttle. They may be 4.2 volts per cell right off the charger, but as soon as you put a 10C to 15C load on the battery, the voltage very quickly drops to about 3.8 volts per cell and slowly tapers off to about 3.6 volts per cell when the battery is about 80% discharged. Over the course of the entire discharge cycle, the battery voltage averages 3.7 volts per cell, and that is why we use 3.7 volts per cell during our tests. It gives a good average power level that you can expect over the entire flight. The actual power you get will be a bit higher at the beginning of a flight, and a bit lower at the end of the flight.

Looking at the graph below from Thunder Power, you can see the blue line showing a 10C discharge rate and the green line showing a 15C discharge rate. If you fly a plane for 6 minutes, you are averaging a little under a 10C discharge rate.





What "informed" person uses a Li-Po cell at 3.7 volts?

Every pilot, whether "informed" or not, uses Li-Po batteries at 3.7 volts. As I said earlier, a Li-Po battery may read 4.2 volts per cell, fresh off the charger, but as soon as it is put to use, the voltage drops down to about 3.9 volts per cell and continues to drop throughout the flight. During the middle of a flight, at full throttle, the battery will be at about 3.6 to 3.7 volts per cell. Towards the end of the flight, the batteries will actually be at about 3.3 volts per flight under a full throttle load, but as soon as you shut the motor off, it climbs back up to a resting voltage of about 3.7 volts per cell. This is why you read 3.7 volts per cell at the end of a flight when you used 80 to 85% of the battery capacity during the flight. If you put a watt meter on the pack at this point and go to full throttle, you will see the pack drop down to about 3.3 volts per cell under load.


Then there is the 80% rule (of thumb) that is talked about by the man who's opinion I trust above all others in this hobby, which raises the lowest usable cell voltage to 3.747v which equals 11.24v as the absolute lowest voltage you should drain your battery down to. I just don't understand why a voltage of 11.1v is shown when the man tells us 11.1v is too low of a voltage for our batteries. what he says makes sense.

You are misinterpreting the "80% Rule" in this case. When I speak of the 80% rule and how it applies to batteries, it has nothing to do with the battery voltage. The main thing you are concerned with is discharge rate and capacity when applying the 80% rule to batteries. If you have a 5000mah pack, you should never use more than 80% of the batteries capacity during a flight, which in this case would be 4000mah. Likewise, if the battery is rated for a 25C discharge rate, I would limit it to 80% of that or 20 amps. Because the battery voltage varies dramatically depending on the discharge rate, it cannot be used as an accurate means of measuring battery usage. When a 3-cell battery is put under a 10C load, during the middle of a flight it will be at approximately 3.7 volts per cell or 11.1 volts, and that is why we test at that voltage.

He uses the 80% rule on everything, esc's, motors, batteries, percentage of alcohol to inert liquid... it's a good rule of thumb. it is also a retail rule, 80% of income is brought in by 20% of your customer base... so i hear.

When I worked at military electronics companies in the past, such as Honeywell and Magnavox, we had to use a 50% de-rating rule for EVERYTHING we did. If a wire could take 20 amps, we could only use it for 10 amps. If a resistor was rated for 1 watt of power, we could use no more than 0.5 watts. If a capacitor was rated for 50 volts, we could use no more than 25 volts and so on. By doing this, the Mean TIme Between Failures (or MTBF) of the components went up to over 50,000 hours of use or more, providing a virtually "failure proof" circuit. The last thing you want to have happen in a battlefield situation is for your targeting computer or communications radio to quit working!

When we talk about consumer electronics, we do not have to use such a large de-rating factor as we do for military electronics, but running an electrical component at its maximum voltage, current or power rating is simply asking for trouble.

To illustrate this effect, take a look at the graph below. This is a chart that shows how the life of electrical components increase as the voltage applied to them decreases or increases with respect to applied voltage. I have put some colored dots on the graph at a few key points for reference. The numbers on this graph have no parameters associated with them because it is a percentage based graph that can be applied to just about anything.

Let's assume for this example that it is a capacitor that is soldered at the end of a speed controller that has 50 volt rating. On the bottom scale 1.0 would be 50 volts, 1.1 would be 55 volts, 0.9 would be 45 volts and 0.8 would be 40 volts. For the left scale, this could be the lifespan of the part listed in 1,000's of hours.

If you look at the black dot, this would recommend the rated life of the part at the rated voltage, so if you ran this part at 50 volts, you could expect to get 1000 hours of use before it failed. If you raise the voltage just 10% and go to the 1.1 point on the graph, you get to the red dot. This would mean that if the part was run at 55 volts, it would only get 0.3 times the life or 300 hours. By raising the voltage just 10% you get less than 1/3 the life from the part!

Now lets take a look at what happens when you reduce the voltage. The green dot shows the point of operation at a 10% reduction in voltage, from 50 volts down to 45 volts. This dot crosses the left side of the graph at the 4 line, so this means that the life of the part would increase from 1000 hours to 4000 hours! By decreasing the applied voltage by just 10%, the life of the part is extended by 4 times!

If we take this one step further and decrease the applied voltage by 20%, from 50 volts to 40 volts, we get to the blue dot. This dot crosses the left side of the graph at about 18, so this means that by reducing the voltage on this part from 50 volts to 40 volts, the life expectancy goes from 1000 hours to 18,000 hours!

This effect applies to all parts that are used in electronics, whether they be resistors, capacitors, diodes, FET transistors, inductors or anything else. Hopefully this sheds some light on why I constantly tell people to never push any of your electronic components to the max ratings, and instead, to de-rate everything to 80% of the max rating. by doing this you greatly increase the life of the parts and make them as trouble free as possible.

(And by the way, an 80% alcohol to inert liquid ratio makes for one fine batch of moonshine!!)





And yes, I know every other prop chart out there shows 10.5v or whatever as tested voltage (which makes no sense) I'm confused.

Many of the older prop charts out there, even the ones that I did 5-6 years ago used 3.5 volts per cell. Back in that time 10C batteries were the norm, and 15C was the hot new thing! These batteries had a higher internal resistance and when loaded to a full throttle condition, they would drop down to about 3.5 volts per cell. Again to be accurate with what you would actually get in a real world situation, this was the correct voltage to test at during that time. Since we have 45C and 65C batteries commonly used today, and these have lower internal resistance, testing at 3.7 volts per cell is more appropriate these days.

What am i missing here? Should I not put so much faith in what this man says... I don't think so.

thanks,
a a



Oh ye of little faith, Of course you should trust me!

You were simply misinterpreting the use of the 80% rule as it applies to batteries, that's all. For clarity, when applying the "80% Rule" to items, this is what you should use it for.

Batteries: Discharge rate and discharge capacity. If you have a 35C battery, do not pull more than (35 x 0.8) 28C from it. If you have a 4000mah battery, never pull more than (4000 x 0.8) 3200mah from it during a flight.

Motors: Max Power ratings and Max current ratings. If you have a motor rated for 100 amps don't pull more than (100 x 0.8) 80 amps from it. If it is rated for 2000 watts, don't run more than (2000 x 0.8) 1600 watts through it.

Speed Controllers: Current Rating. If you have a speed controller rated for 90 amps, don't run more than (90 x 0.8) 72 amps through it. And forget about "Burst Ratings" on speed controllers! If you are even considering that in your ESC calculations use a bigger speed controller!

If you follow these few simple rules, you will be rewarded with thousands of hours of trouble free flying, and guys like me will get a lot less warranty calls!!


Hopefully that clears up this subject.

Have a Happy New Year!

Lucien
Dec 28, 2013, 04:06 PM
Steps? What steps?
zozer's Avatar
An excellent and informative response Lucien. I trust your data, but I'd sure like to meet "the man who's opinion i [AA] trust above all others in this hobby" and hear more about his 'universal 80% rule'...


.
Dec 28, 2013, 05:19 PM
I am a nice guy! Really!

Thanks


Thank you, Lucien, for taking the time to again expand our knowledge of the proper use of these power systems.

I have often wondered why we need batteries with 45 C ratings when even if we limit the discharge to 80% of that, it will only yield a flight of 1-2/3 minutes. It takes me that long just to set up for the landing . I guess it is nice to have that reserve for a few seconds of full power for such things as take-offs and to get yourself out of trouble, but I like to have a flight that lasts at least 5-6 min. with enough left over to miss a landing and go around.

I guess if you are trying to set a airspeed record or something like that you might approach using that capacity though.
Dec 28, 2013, 05:51 PM
rip
ripacheco's Avatar
Excellent post... using the post as a learning tool in a couple of Facebook groups I belong.
Dec 28, 2013, 08:54 PM
KE Spins make me dizzy.
Lucien's response just reinforces the 'oversize your stuff' I learned years ago when I started using the early lipo batteries.

People thought I was crazy when I started using the 6000mah packs in an RC car when most people used 3600-ish packs. They didn't think I was so crazy when I pulled my stuff off a run and it was still dead cool.


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