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Overpowered Mig3 and the case of the puffy batteries

#1
I decided to go big or go home with my Mig3 and took the power setup from the Strix Alatus and put it on the power pod. It flew pretty well on a very windy maiden but both packs I ran through it came down moderately/a-little puffed. I am not sure quite what happened and could really use some input on where I went wrong.

The setup:
Cobra 2217/08 2300kv Motor (Alatus branded)
Cobra 60A ESC (Alatus branded)
1800mah 4S 60C RMRC Battery
6x4E APC Prop

This is exactly the setup I use on my Alatus, in fact the battery I bought just for that plane but borrowed it for this. I did two flights and both came back with slightly puffy batteries.

Flight 1:
5 minutes
Fully charged battery
End voltage 14.60V (moderate puffy)
Temperature: 65F
Mix of throttle ranges with a few bursts at 100% up to maybe 20-30 seconds at once

Flight 2:
6 minutes
Fully charged battery
End voltage 14.81V (slightly puffy)
Temperature: 65F
50% except at launch where I might have hit 60%

It seems like I should be well within spec _except_ I've overpropped the motor for 4S. This is the exact power train the Alatus ships with and that flies fine with much higher AUW. I've never seen puffing on that. Overpropping the motor shouldn't do anything but overheat it, right? Worst case some fractional increase in amp draw due to friction? I tested the exterior temp with my hand maybe 20 seconds after launch and it was barely warm. The ESC was not warm to the touch.

The motor is rated at 43.71A on a 6x4E Prop (http://innov8tivedesigns.com/images/specs/Cobra_2217-8_Specs.htm)
The battery is 60C so 1.8 * 60 = 108A. More then double the rated draw of the motor on that prop.

Why am I seeing battery puffiness? I should be well within the AMP draw of the battery, especially for the second flight which I really toned it back on the throttle to see if the other battery was just bad. I would hate to have this plane be $30 bucks per-flight if it kills batteries.

As it is, I won't fly it again until I can figure it out. I don't want to kill all my packs :(
 

PsyBorg

Fly Angry
Mentor
#2
Several factors could be going on here.

Difference in weight, drag as you stated, ventilation of the gear in the fuselage and holding more heat in. The daily temps are more then likely higher now then when you flew the Strix. Over / under propping. Prop on the wrong way changing from pusher to tractor config maybe?

That 20 -30 second full throttle run on a hot day would surely brutalize a battery only rated at 60c specially if inadequate ventilation for heat is going on. The second run coming down at 65c (149 F) makes me think the air flow is a huge part of it too as you state you were much more gentile with it. If the esc is overheating it will have a much higher current draw.

Batteries for my quad when I beat on them in hot weather only come down around 120 degrees F (48c) on hot days.
 

lrussi750

Rogue Pilot
Mentor
#3
I would categorize this as using a motorcycle engine to power a big old sports car. While this setup is quick when used on a slippery flying wing (with cooling designed into the canopy) it is not optimal for a draggy old world airplane design like the MIG3 or Mustang, FT Racer. Speed setups for these aircraft usually involve bigger, low KV motors spinning larger 9" to 10" props.

20180422_200721.jpg
 
#4
Several factors could be going on here.

Difference in weight, drag as you stated, ventilation of the gear in the fuselage and holding more heat in. The daily temps are more then likely higher now then when you flew the Strix. Over / under propping. Prop on the wrong way changing from pusher to tractor config maybe?

That 20 -30 second full throttle run on a hot day would surely brutalize a battery only rated at 60c specially if inadequate ventilation for heat is going on. The second run coming down at 65c (149 F) makes me think the air flow is a huge part of it too as you state you were much more gentile with it. If the esc is overheating it will have a much higher current draw.

Batteries for my quad when I beat on them in hot weather only come down around 120 degrees F (48c) on hot days.
Thanks for the response. The battery isn't vented really at all in the Mig3 but it was 65F outside not 65C at the battery. Both batteries were barely warm to the touch on landing. The ESC is vented and sits right behind the air inlet that vents out the rear of the plane.
 

PsyBorg

Fly Angry
Mentor
#5
Well no wonder.. How did you stand to even be outside to fly in that temperature. If ambient temps were 150 degrees and the battery was slightly warm then you were well over the normal operating temps for the battery I am sure. You started the flight already overheated.
 
#6
I would categorize this as using a motorcycle engine to power a big old sports car. While this setup is quick when used on a slippery flying wing (with cooling designed into the canopy) it is not optimal for a draggy old world airplane design like the MIG3 or Mustang, FT Racer. Speed setups for these aircraft usually involve bigger, low KV motors spinning larger 9" to 10" props.

View attachment 108738
Would this setup be better on a lighter/lower drag model like the bloody wonder? I honestly was just looking for a setup that could give me a higher top speed. If the Mig3 is the wrong model for that, it just means I have an excuse for another build.
 
#7
Well no wonder.. How did you stand to even be outside to fly in that temperature. If ambient temps were 150 degrees and the battery was slightly warm then you were well over the normal operating temps for the battery I am sure. You started the flight already overheated.
PsyBorg,

No, really I mean the weather was 65 degrees F, not C. It was cool and overcast. I live near Seattle which isn't some crazy thermal plane oven :) The weather was cool and all the components came down no more then slightly warm to the touch. Other then the battery puffing, nothing would indicate there was a problem.

Could just being model with lots of drag and a burst to 100% throttle or two really be accounting for 50 additional amperes of draw?

How much does drag impact amp draw? Shouldn't it impact it just like trying to fly a heavier model? The thrust accelerates the plane less with either high drag or high weight? The prop would be spinning faster then the airspeed basically the same.

I am just trying to figure out how to reason about the results I got. Like I said, I would be just as happy to put this power pod into another plane but I want to rule out something about the power setup itself as the cause before I do more damage to my batteries.
 
#8
Would this setup be better on a lighter/lower drag model like the bloody wonder? I honestly was just looking for a setup that could give me a higher top speed. If the Mig3 is the wrong model for that, it just means I have an excuse for another build.
Would an 1100kv motor on 4S with a 9x6 or 9x4.5 APC prop be more appropriate? The increased pitch makes up the RPM difference for top speed. I bought a few of these Finwing 2220 1100kv motors in the same order just because they were on sale:
https://www.readymaderc.com/products/details/penguin-m2220-motor

I also have a Cobra 2217/12 1550kv laying around but that also recommends a 6" prop @ 4S:
http://innov8tivedesigns.com/images/specs/Cobra_2217-12_Specs.htm
 

Namactual

Well-known member
#9
One thing to consider is not all batteries are created equal. You could have just got a lemon so to speak. Or the manufacturer is being really generous with the specs. Probably a little of both.

All of the drag in the world would not draw any more amps than a static thrust test. I would invest in a watt meter and test your setup while on the ground. That will tell you more than any of this speculation.

As far as the speed you want, I would look at just changing the airframe rather than changing your power setup. If you want to use a mig, cut the wings down. Anything with a smaller wing profile will be faster as a result of less drag.
 
#10
One thing to consider is not all batteries are created equal. You could have just got a lemon so to speak. Or the manufacturer is being really generous with the specs. Probably a little of both.

All of the drag in the world would not draw any more amps than a static thrust test. I would invest in a watt meter and test your setup while on the ground. That will tell you more than any of this speculation.

As far as the speed you want, I would look at just changing the airframe rather than changing your power setup. If you want to use a mig, cut the wings down. Anything with a smaller wing profile will be faster as a result of less drag.
Thanks, I think a watt meter is my next purchase. That should help me figure out if there is something causing the excess draw or the batteries are just advertised 'aggressively' or even just old.
 

lrussi750

Rogue Pilot
Mentor
#11
One thing to consider is not all batteries are created equal. You could have just got a lemon so to speak. Or the manufacturer is being really generous with the specs. Probably a little of both.
Namactual makes a good point about the battery, have you checked the voltage of the RMRC pack, are the cells even? I've had batteries drain unevenly and that caused some swelling and a good amount of heat.

I also purchased those same Finwing 2220 motors and will be trying them out because they are crazy cheap and I love a deal!!

As far as the Alatus power system goes, I have a hard time wrapping my head around what to use it on. I've seen the Alatus fly at FF and I know its crazy fast. But I wouldn't use it in anything other than a decent size wing. It's a heavy motor that should be using a 5" prop on 4s or a different brand 6x4 prop, because the APC is a little to aggressive on 4s per the i innov8tivedesigns chart.

I like NerdNic's recommendations for the MIG3, I've seen him fly them and they are fantastic, and scary fast.

BASIC
MOTOR - Turnigy D3530 1100kv
PROP - 9x6 APC
ESC - 30a Dynam
BATTERY - 2200mah 3S
SERVOS - 9g

ADVANCED (50mph)
MOTOR - NTM 35-36 1400kv
PROP - 9x6 APC
ESC - 60a Dynam
BATTERY - 2200mah 40c 3S
SERVOS - 9g

SPEED (85-120mph)
MOTOR
PROP - 10x10 APC
ESC - 80a Dynam / 100a Plush
BATTERY
SERVOS - 12g Emax ES08MA II metal gear
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#12
There are a few things you should always ensure if you want to fly without damaging your planes electrics.
NEVER overvoltage
Never overprop
Always ventilate

Overvoltage is simply using a battery which has a higher cell count than the ESC is rated for
Over prop is just a prop that is too big for the motor power and rotational speed rating
Lack of ventilation can cause battery destructton or ESC to overheat and cut out totally if it does not suffer from a total failure.

The idea that a minor overprop is just going to cause a minor increase in current is wrong. It is the motors rotational speed that provides the "Back EMF" that controls the motor current draw for a given setting of throttle and rotational speed. Without back EMF a motor is just a number of wire links. When overpropped a motor is given more current than it is designed to handle for a given speed and is often way above the capacity of the ESC to supply continuously. The number of times I have seen motors streaming smoke because the owner forgot to fit the prop for use with a 4S setup, (retaining the 3S setup prop) when using the 4S battery is astounding.

As for the puffing of the battery all of the extra current has to come from somewhere! When it comes to batteries the centre cells or should I say the cells not on the top and bottom of the stack do not get the same cooling as the end cells, (he end cells are the ones you feel the temp of when you handle the pack post flight). It is this lack of internal cooling that causes the individual cells to overheat and to GAS. It is the gas pressure that causes the "Puffing".

The internal cell temperature rise is caused by the current drawn from or through the cell multiplied by the cells internal resistance. This heat needs to be dissipated quickly to prevent gas generation and cell degradation. Too much current or insufficient cooling will ALWAYS damage the battery. The amount of damage is related to the current and the internal cell resistances. Some packs will have low internal resistance and some high, (all within specification), and so the amount of puffing of a battery pack is almost a lottery! As a side issue the overheating damage of a battery pack is cumulative, (it adds up over time), because the gassing also increases the individual cells internal resistance making it more likely to suffer greater damage next time.

Just my thoughts on the issue!

Have fun!
 
#15
There are a few things you should always ensure if you want to fly without damaging your planes electrics.
NEVER overvoltage
Never overprop
Always ventilate

Overvoltage is simply using a battery which has a higher cell count than the ESC is rated for
Over prop is just a prop that is too big for the motor power and rotational speed rating
Lack of ventilation can cause battery destructton or ESC to overheat and cut out totally if it does not suffer from a total failure.

The idea that a minor overprop is just going to cause a minor increase in current is wrong. It is the motors rotational speed that provides the "Back EMF" that controls the motor current draw for a given setting of throttle and rotational speed. Without back EMF a motor is just a number of wire links. When overpropped a motor is given more current than it is designed to handle for a given speed and is often way above the capacity of the ESC to supply continuously. The number of times I have seen motors streaming smoke because the owner forgot to fit the prop for use with a 4S setup, (retaining the 3S setup prop) when using the 4S battery is astounding.

As for the puffing of the battery all of the extra current has to come from somewhere! When it comes to batteries the centre cells or should I say the cells not on the top and bottom of the stack do not get the same cooling as the end cells, (he end cells are the ones you feel the temp of when you handle the pack post flight). It is this lack of internal cooling that causes the individual cells to overheat and to GAS. It is the gas pressure that causes the "Puffing".

The internal cell temperature rise is caused by the current drawn from or through the cell multiplied by the cells internal resistance. This heat needs to be dissipated quickly to prevent gas generation and cell degradation. Too much current or insufficient cooling will ALWAYS damage the battery. The amount of damage is related to the current and the internal cell resistances. Some packs will have low internal resistance and some high, (all within specification), and so the amount of puffing of a battery pack is almost a lottery! As a side issue the overheating damage of a battery pack is cumulative, (it adds up over time), because the gassing also increases the individual cells internal resistance making it more likely to suffer greater damage next time.

Just my thoughts on the issue!

Have fun!
Hai-Lee,

Thanks. I did see a slight voltage difference in the cells at the end of the flight. I didn't write it down but it was ~0.1v for one cell on both flights. It was the fourth cell both times but no telling if that was really wired in order. Based on the above and everything from this thread it sounds like I might be drawing more current then I expect and lack of cooling on the pack in the Mig3 is causing the damage. It looks like for future flights I should step down to at least a 5" prop which means I have to pick another airframe as it just won't have enough clearance from the body on the Mig.

I'm looking at a watt meter that can also tell me the battery internal resistance to see what state they are in now.

Thanks everyone for the input here! I knew going outside the FT power packs was going to have some bumps so I appreciate all the time everyone took to respond.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#16
In my case I actually refrigerate my batteries before I charge them or use them. This actually lowers the internal temperature of the "Buried" cells and even when taken out of the refrigerator the buried cells do not reach ambient temperature very quickly due to the insulating effect of the end cells.

Since doing this I have not lost a battery. They are getting a little tired lately but then I fly around 100 flights each month and the batteries are around 18 months now!

Managing the cell temperature is the key to long battery life!

Have fun!
 
#17
So, I have data! I got a 'turnigy meta meter' and was able to ackwardly do a bench test with this inline for my Mig.

50% sustained, 75% peak throttle test
Peak: 20.49A, 285.39W
Post test resting voltage was at 15.11V

100% peak throttle test
Peak: 18.92A, 261.29W
Post test resting voltage was at 15.00V

So, there are two options:
1) Trust these results
2) Don't trust these results and refine my method

Lets start with 2
Issue #1: I can't photograph the meter while doing the test without killing myself so I can't get 'resting' amp rate. I think the fact that the 50% sustained test has a higher peak is a fluke, likely I ramped the throttle faster or something.
Issue #2: These were short tests. I need to repeat with longer sustained test to ensure heat buildup doesn't sway the results.
Issue #3: The battery was outside the plane because of the watt meter harness. Anything due to the fuselage insulating the battery won't be picked up.
Issue #4: The possibility that this meter is a POS.
Issue #5: The battery was at storage charge, not full charge. Lower V means lower A & W.
Issue #6: I can't figure out the terrible directions for measuring internal resistance on the two batteries I used.

Ok, I'll repeat the test. But it raised a question for me. If you are far below the AMP rating on a battery, and the battery isn't trash, will it generate enough heat to puff when well insulated? The mig3 has almost zero airflow on the battery. I can start cutting holes if needed. Can I rule out (good battery + low amps + no airflow) = puffy anyways?
 
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Hai-Lee

Old and Bold RC PILOT
Mentor
#18
Firstly the test results you have recorded might be accurate when you are doing static tests and the propeller pitch is designed to give its best thrust and hence current drain at high forward speed. The drop in current draw could be due to blade stall, (similar in nature to cavitation in a marine propeller. When stalled the drag figure becomes lower due to the paddling of the air out of the blades path and any drag/load figure due to the thrust/lift of the blade is diminished.

As for a puffy battery it is the heat that causes the issue. Even well ventilated batteries can "Puff" up as the heat generated internally, current multiplied by battery internal resistance is heat that must be dissipated. When a battery is in a closed and sealed place the heat generated has seriously reduced dissipation and therefore the battery pack temperature will rise significantly.

Often in planes with poor ventilation the ESC is located close to the Battery, often in the same space. Here the Puffing will be greater as the ESC heat, (including the BEC generated heat), can heat the environment reducing the battery's ability to dissipate its internally generated heat.

Sometimes shifting the ESC to the outside of the plane under the "CHIN" where the prop airflow can force cool it can save a battery from some of the heating problem. As for what I do it is a combination of everything suggested so far, (varies upon the model) PLUS I store my batteries in a refrigerator and ensure that they are quite cold/cool before I use them in my aircraft.

Just my thoughts and what works for me!

Have fun!
 
#19
In my case I actually refrigerate my batteries before I charge them or use them. This actually lowers the internal temperature of the "Buried" cells and even when taken out of the refrigerator the buried cells do not reach ambient temperature very quickly due to the insulating effect of the end cells.

Since doing this I have not lost a battery. They are getting a little tired lately but then I fly around 100 flights each month and the batteries are around 18 months now!

Managing the cell temperature is the key to long battery life!

Have fun!
There is absolutely no way I am getting away with LiPos in the fridge. I would have to do a garage mini-fridge for that and I am not quite there yet :). Don't cold cells have higher internal resistance? That should lead to self-heating themselves back to normal with a shorter then normal flight time (due to the energy to heat conversion). It makes sense that it reduces the incidence of self-heating to puffiness though.
 
#20
Firstly the test results you have recorded might be accurate when you are doing static tests and the propeller pitch is designed to give its best thrust and hence current drain at high forward speed. The drop in current draw could be due to blade stall, (similar in nature to cavitation in a marine propeller. When stalled the drag figure becomes lower due to the paddling of the air out of the blades path and any drag/load figure due to the thrust/lift of the blade is diminished.

As for a puffy battery it is the heat that causes the issue. Even well ventilated batteries can "Puff" up as the heat generated internally, current multiplied by battery internal resistance is heat that must be dissipated. When a battery is in a closed and sealed place the heat generated has seriously reduced dissipation and therefore the battery pack temperature will rise significantly.

Often in planes with poor ventilation the ESC is located close to the Battery, often in the same space. Here the Puffing will be greater as the ESC heat, (including the BEC generated heat), can heat the environment reducing the battery's ability to dissipate its internally generated heat.

Sometimes shifting the ESC to the outside of the plane under the "CHIN" where the prop airflow can force cool it can save a battery from some of the heating problem. As for what I do it is a combination of everything suggested so far, (varies upon the model) PLUS I store my batteries in a refrigerator and ensure that they are quite cold/cool before I use them in my aircraft.

Just my thoughts and what works for me!

Have fun!
Hai-Lee,

Assuming I had any confidence in my method on the first attempt it throws me that the results don't match the motor manufacture's static thrust results on the same prop. I agree that physics doesn't play fair and can ruin my results just because I am not really moving 90 mph. My watt meter doesn't fit in the Mig3 but we might be able to swing that with enough double sided tape :). Being 100% off the mgr results for the 'same test' makes me assume something else is off though.