Critical Low Power. Aircraft Landing.

[Fletcher500]

Interesting discussion regarding battery life. I have AGM batteries on my boat. They are very $ and don't like to be discharged too low or they lose lifespan over time. I.E., a boat operator will not allow large V drops on purpose if they know what they are doing. I would assume the same principle applies to a lead acid car battery...sulphation.

Sounds like these quad batteries are a different animal.

 

[wayy510]

There is your problem. You have effectively damaged one of the cells in your battery packs. I will toss that pack if I was you..... Or be VERY carefull when u fly. The next part will start to unravel as the battery becomes older... You will find that it will show 100% charged .. then within 5 Min of takeoff it will drop to 60% or lower and then minutes later it will hit levels below 30%. Storing lipos at anything above 30% is never advisable.. storing them at 100% is a definate NO NO.... It permanantly damages the battery

I've always stored all my batteries at 100% since day one and I've have zero issues.

I fly, charge all my batteries to 100% and put back in case until next flight.

I fly 17-20 minutes average in constant sport mode. I never fly "regular" mode.....always sports mode and as fast as the Mavic will go.

 

[dwallersv]

Storing lipos at anything above 30% is never advisable..

This is flat out wrong, and is bad advice. Storing Lipos at 30% will shorten the cycle life as well as promote voltage sag before the battery is discharged. Bad idea.

Lipos should be stored at 50%, and this is why DJI batteries auto-discharge to that level. DJI engineers are not idiots on this, nor are the thousands and thousands of other engineers (like myself) that know the specs on these cells.

storing them at 100% is a definate NO NO.... It permanantly damages the battery

THAT is absolutely correct.

 

[dwallersv]

I've always stored all my batteries at 100% since day one and I've have zero issues.

I fly, charge all my batteries to 100% and put back in case until next flight.

I fly 17-20 minutes average in constant sport mode. I never fly "regular" mode.....always sports mode and as fast as the Mavic will go.

You must fly often? If not, you must be flying on partially discharged batteries.

You shouldn't. Capacity estimation doesn't work as well starting with a partially discharged battery, and you could find yourself in trouble when you thought you'd have plenty of gas. Worse, low-battery RTH might not calculate correctly either.

 

[ZeroG]

Went for a fly a couple of days ago with battery at 53% and in ideal conditions with no wind and about 15C. Took off and spent a little time in hover while I checked and adjusted settings.

After 96secs climbed to 24m, selected Sport and flew for approximately 17secs in a straight line at constant height when I received Critical Low Power. RC showed battery at 43%. MP started to land.

Bit panicky at this stage but used Full Throttle up which stopped descent at 8m, which surprised me.

Then still in Sport I flew back to the take off site taking approximately 19secs and landed without further issues.

I have uploaded the flight log to phantomhelp.com. My file name is DJIFlightRecord_2017_06_16[14-04-26].txt and the LogViewer reference is B7Y3806P7IRT7PQGSFF7.

I note that the log shows battery voltage at start of flight at 11.354, 9.842 at the warning and 10.867 upon return landing.

It would appear that the battery was under stress but the short amount of time it took to descend was sufficient to allow it to recover.

I have read on the forum that extended use ie multiple minutes of Sport Mode especially whilst flying both forward and up can place severe drain on the battery however I was flying level and only for a very short period.

Any advice would be appreciated.

Yes, a Lipo at 53% will be susceptible to sudden drops in voltage if you spike the throttle, especially in sport mode where you can access more power from your battery. My suggestion is to tame down your throttle inputs, and fly smoothly, so the battery doesn't dip down below the critical level Happy flying, ZeroG

 

[ZeroG]

Yes, a Lipo at 53% will be susceptible to sudden drops in voltage if you spike the throttle, especially in sport mode where you can access more power from your battery. My suggestion is to tame down your throttle inputs, and fly smoothly, so the battery doesn't dip down below the critical level Happy flying, ZeroG

 

[Cymruflyer]

CyberNate wrote:
"When you fly into the wind with sport mode the motors are pushed to maximum RPM which has considerable current draw on the battery."

This statement is totally wrong, but understandable coming from anyone who is not a real pilot, due to their misunderstanding of real flying and head, tail or crosswinds.

To clear up this misconception for all who misunderstand winds aloft, an aircraft flying through the air, has absolutely no idea where the wind comes from, what speed it is or its direction. And if you were really flying the aircraft, any aircraft and you were in cloud or had your eyes closed

 

[HFMan]

This is flat out wrong, and is bad advice. Storing Lipos at 30% will shorten the cycle life as well as promote voltage sag before the battery is discharged. Bad idea.

This is also flat out wrong. There is no harm in storing lipos at 30% (in fact it's good practice) AS LONG as you don't store them so long that they self-discharge below 10%.

 

[Cymruflyer]

... Continuation to above post (for some reason it posted itself before I was finished) ...and you were in cloud with no instruments to watch or had your eyes closed, you would not have a clue what the wind was doing either. You fly in a volume of air in the same way that a fly inside a car is flying in a volume of air. The fly will fly the same speed for example, in that volume, regardless of whether the car is stationary, moving forward at 5mph or 80mph or moving in reverse at any speed or sideways at any speed.

Now in relation to the ground, there is a difference in speed of the fly but not airspeed. So the airspeed of 36mph mention was actually ground speed but the airspeed, assuming the max possible was 46mph, would have been 46mph.

 

[HFMan]

CyberNate wrote:
"When you fly into the wind with sport mode the motors are pushed to maximum RPM which has considerable current draw on the battery."

This statement is totally wrong, but understandable coming from anyone who is not a real pilot, due to their misunderstanding of real flying and head, tail or crosswinds.

To clear up this misconception for all who misunderstand winds aloft, an aircraft flying through the air, has absolutely no idea where the wind comes from, what speed it is or its direction. And if you were really flying the aircraft, any aircraft and you were in cloud or had your eyes closed

You clearly don't understand his statement, nor do you understand the Mavic behavior. The flight controller in the Mavic deals with GROUND speed, not airspeed. Hence, if there is a heavy headwind, the mavic will use more power to maintain the same ground speed. Get it?

 

[dwallersv]

This statement is totally wrong, but understandable coming from anyone who is not a real pilot, due to their misunderstanding of real flying and head, tail or crosswinds.

To clear up this misconception for all who misunderstand winds aloft, an aircraft flying through the air, has absolutely no idea where the wind comes from, what speed it is or its direction. And if you were really flying the aircraft, any aircraft and you were in cloud or had your eyes closed

This statement is totally wrong, but understandable coming from anyone who is not a real sUAV pilot, due to their misunderstanding of modern sUAV flight control systems and how they manage ground speed vs. airspeed.

As a real pilot with a ticket, I know how to do real flying just like maybe you. I also understand the details of the rather sophisticated flight control system for the Mavic, 'cause I'm a real sUAV pilot!

Nate is right, and you are wrong. The reason is, you don't understand that the Mavic has ground speed information available via GPS, and the on-board controller tries to maintain a ground speed relative to stick input. So, when flying into the wind, the aircraft will fly at a faster airspeed (that's one of those fancy terms real pilots that do real flying know about) than with a tailwind.

Real aircraft generally don't have systems like this because that would be far too much interference in control for real flying, and real pilots wouldn't put up with it.

 

[dwallersv]

This is also flat out wrong. There is no harm in storing lipos at 30% (in fact it's good practice) AS LONG as you don't store them so long that they self-discharge below 10%.

Fair enough. You've made your case, I've made mine. Those reading can decide for themselves.

 

[Cymruflyer]

... Did it again, bugger
Continuation to above post...
... Would have been 46mph of the drone. The motors would be running just the same and the battery drain would also be just the same for the drone to fly at 46mph, whether it flew through calm air, a 10mph headwind or tail wind or cross wind, or any speed wind, because it is flying through a volume of air. Now the ground speed would of course be different to its airspeed for everything mentioned above except the calm wind condition because that scenario would make ground and airspeed the same.

Therefore, there is no additional strain put on the motors or battery when flying at max speed, other than that strain associated with flying at that speed. If you were trying to make the drone hold a stationary position of land and there was wind, then yes there would be additional strain put on the motors and battery as it fought the wind speed component to hold its position, but if it were a calm day, then there would be less strain on motors and battery because there would be no external effects to fight as there would be, if fighting wind.

This headwind thing is a common misunderstand also held by real pilots in training, until they grasp what is really happening to their aircraft as their experience increases. So remember, a headwind places no additional stress or strain on the drone motors or battery when it is flying, but it would take longer in flight time to cover a set distance over the ground when going against the wind and less time to cover the same distance if flying with a tail wind.

If this is still difficult to understand just think of yourself walking on a tread mill fitted to the back of a truck, at a set speed of say 5mph. You would expend the same amount of energy to maintain that 5mph over the treadmill regardless of whether the truck was standing still, driving at 5 or 40mph either forward or backward or sideways (if it could do that). So the same is true for the drone flying through the air, the drones airspeed is the same as your speed on the treadmill. The ground speed of the drone or you on the treadmill would of course be different to your walking speed over the treadmill, depending on the speed and direction of the truck, which effectively would be the wind for the drone. Hope that makes sense to all who read, feel free to ask questions if you still don't understand it.

 

[HFMan]

Have you bothered to read any responses to your long rambling attempt to learn us? The mavic controls speed using GROUND SPEED. Is this difficult to understand?

 

[Cymruflyer]

Dwalletsv, no disrespect but you need to go back back to ground school for retraining. Any stated max speed for a flying craft is going to mean airspeed, it will always mean airspeed. So a stated max speed of 46mph for example will always mean airspeed and never have anything to do with ground speed. Only a GPS will be measuring ground speed and ground speed has no effect on what the motors and battery are going through, only throttle setting will affect that.

So Nate is correct in stating that flying in sport mode at max throttle is going to put additional stress on the battery and motors. However, he is incorrect and so are you if you think there will be an added stress when flying into a headwind, because the aircraft is flying through a volume of air only. The same stress mentioned by Nate will be incurred by the battery and motors when in sport mode and flying at max throttle, whether flying into the wind or with the wind or across the wind direction. Don't confuse airspeed with ground speed. I was a flight instructor for many years and if you are a real pilot then you should fully understand what is being said.

If you are watching your GPS speed, which is ground speed only, and asking the aircraft to go 30mph ground speed, then yes there is a difference to the stress on the battery and motors when flying into a 10mph headwind and less when flying with a 10mph tailwind because you are asking the aircraft to maintain a ground speed. That is a completely different thing and nothing to do what was being discussed.

What Nate stated is that there is greater stress on battery when flown in sport mode at max throttle/speed when flying into a headwind, and that is simply an incorrect statement. The same stress or strain is put on the battery when flown in any direction in sport mode at max throttle/speed (which is always and only airspeed), regardless of wind speed or direction.

 

[Cymruflyer]

HDman, sorry but the Mavic is a flying machine and throttle setting only controls airspeed, your are only seeing ground speed because only a GPS is showing you a speed reading. Again you are being confused by ground speed vs. airspeed.

That is why the stated max speed of say 46mph in sport mode at max throttle means the max speed showing via the GPS in still air, or no wind. If you were to fly at max throttle of 46mph into a 20mph headwind, your GPS would show you were going at no more than 26mph but that would be ground speed, the Mavic would still be doing 46mph airspeed and if you en turned it around and few back with the now tailwind, you would see the GPS, which only shows ground speed, at a reading of 66mph, but again, the Mavic would still only be doing its max airspeed of 46mph.

You might like to ask DJI about that stated max speed, you will find that is airspeed, not ground speed. Your GPS is the only thing you have to show you a speed indication, so don't confuse it with airspeed.

 

[Cymruflyer]

HFMan... Sorry not HDman.

 

[HFMan]

You are dead wrong, where are you getting your information??? The Mavic has no airspeed sensor... it does have pre-programmed expected flight envelopes (for given tilt and power, aircraft should go 'x' mph). It cannot measure this as air speed, so its behavior is to measure ground speed using GPS. If it is not making expected ground speed in sport mode, it will actually increase motor power to make that ground speed (to a limit). What don't you get about this? The Mavic does not calculate nor utilize air speed whatsoever!!! It is impossible!!!

I fully understand the difference between airspeed and groundspeed- I am also a pilot. What you don't seem to understand is how the Mavic behaves. It actually DOES increase power if there are headwinds- especially in sport mode. I have observed this many many times. It does so because it knows how fast it SHOULD be going but it measures (via GPS) that it is not reaching expected ground speed. Hence it applies more power.

Do you even own a Mavic? Do you understand how it behaves in Sport mode? The Mavic power control system is NOTHING like a full scale aircraft... quit thinking of it this way.

 

[HFMan]

Put it another way.. put your Mavic in Sport mode. You get a Power indicator graphic where your Radar normally displays. Push the stick full forward into a head wind, and watch the average Power reading (it fluctuates as the ground speed is not constant). Turn around and fly downwind- and again push full forward on the stick. The average Power will now be quite a deal less. This is completely counter to your argument....

 

[dwallersv]

@HFMan, walk away from this trainwreck. Guy's full of himself, clearly didn't bother to read anything in response to his initial ignorant, laced-with-arrogance posting.

I didn't bother to read any of his blather after my first response. He REFUSES to understand the facts.