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AirData battery deviation- what causes the changes of it mid-flight?

AMann

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I was just glancing at my AirData flight files and was curious about my battery cell deviation values. They swing around during flight between 0.000 on up to O.031. What causes it to change midflight? Also what is a typical deviation value other people get?
 

old man mavic

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its relative to current draw cell condition,cell temperature and starting voltage of the cell they are basically 3 different batteries working in unison to deliver a specific voltage requirement to the motors at the request of the ESCs ,and power the electronics at the same time, its all those variables that cause the deviation
 

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Might wind not have something to do with that. Like the motors having to go faster or slower cause of a change
of wind direction.
 

old man mavic

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Might wind not have something to do with that. Like the motors having to go faster or slower cause of a change
of wind direction.
thanks Dc that flippin wind has a lot to answer for,it the bane of UAV flyers all around the world
 

AMann

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its relative to current draw cell condition,cell temperature and starting voltage of the cell they are basically 3 different batteries working in unison to deliver a specific voltage requirement to the motors at the request of the ESCs ,and power the electronics at the same time, its all those variables that cause the deviation
That explains it, thanks for the reply! So, since the condition of each cell has slight variations from the others, they experience deviations. OK, so what would be an acceptable maximum deviation one should accept before retiring a flight battery?
 

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That explains it, thanks for the reply! So, since the condition of each cell has slight variations from the others, they experience deviations. OK, so what would be an acceptable maximum deviation one should accept before retiring a flight battery?
You are looking at a difference of 3/100 of a volt!!
That's nothing.
Assuming that minor deviations aren't normal is probably incorrect.
 
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old man mavic

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it would have to be read when the battery was fully charged and i would think .05 of a volt would be significant anything more could have an effect on the cells as they discharge as the lowest cell would reach a critical voltage and could cause the drone to land or crash
 
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RadioFlyerMan

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I was just glancing at my AirData flight files and was curious about my battery cell deviation values. They swing around during flight between 0.000 on up to O.031. What causes it to change midflight? Also what is a typical deviation value other people get?
@old man mavic is correct that during any flight, cell deviations are a function of a lot of factors. I cannot put a hard limit on a deviation since it's magnitude and duration is a function of flying style. And I have found that deviations for a battery will change over it's life of use. The resulting effect of such deviations is to degrade the overall capacity of the battery, with one or more cells as the weakest link. It is this overall capacity that I focus on.

I process my flight records to track battery health as a function of total usage of the battery in watt-hours. I have attached example graphs of my five batteries for my MPP. You can see that there will be a natural decline in overall capacity. This is based on the nominal capacity of 3830 mAh or 43.6 Wh. With this data, I'm trying to judge or predict a catastrophic failure, which may be impossible. However, the higher the deviation at various power levels and the lower overall capacity makes me use the weaker batteries for ground work (calibrations, simulator, etc.). My Battery #2 is my weakest battery and is not used for flying.

The AirData analysis can help you in evaluating your batteries. However, and most important, inspect the batteries for ANY bulging. If there is, my advice is don't use it, not even for ground work. Dispose of it properly and don't burn your house down.

Screen Shot 2019-12-13 at 7.25.57 AM.pngScreen Shot 2019-12-13 at 7.03.47 AM.png
 
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kndll

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Here's something about deviation for you experts to weigh in on , I've always had deviations in cell number 3 , I finally got a second battery and it too has deviations only in cell number 3 -- thoughts .
 

old man mavic

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Here's something about deviation for you experts to weigh in on , I've always had deviations in cell number 3 , I finally got a second battery and it too has deviations only in cell number 3 -- thoughts .
its probably the plus output of the battery
 
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AMann

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You are looking at a difference of 3/100 of a volt!!
That's nothing.
Assuming that minor deviations aren't normal is probably incorrect.
Hey- The Hubble telescope had about a 1 mm error in the curvature of it’s 2.4 m mirror, that’s less than 1/2400ths off, don’t tell me my deviation isn’t significant! ;)

I know 3/100’s a pretty insignificant difference, and shows that my battery is healthy. I guess the difference could be caused by something as simple as variations of the Internal membranes inside the batteries, lead and circuit variations affecting resistance, or even with the battery chemistry...I was just curious about how those deviation figures work between each cell.
 
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DanMan32

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Since models wasn't mentioned in the topic and this topic was posted in a model agnostic forum, I'd like to point out the M2 has 4 cells.
 
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WithTheBirds

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The question you might ask could be phrased in the alternative- how can these cells be made to such tight tolerances that we only see 30mv variation in voltage when loaded in a series connection? To share an observation from using LiPO since they were commercially available in a multitude of applications anything less than 100mv variation in a LiPO battery in our drones is no cause for concern. Particularly if considered in isolation. Usable capacity, measured IR, operating temp (rise above ambient and observed excessive puffing are equally important.
 
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Why then does airdata flag anything above .07V as "major" deviation?

Also, this may be relevant here.
I used to think the deviation is the difference between the cell with the max voltage and the cell with the lowest; not so apparently. It is the difference between the cell with the lowest and the second highest.
 
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WithTheBirds

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Why then does airdata flag anything above .07V as "major" deviation?

Also, this may be relevant here.
I used to think the deviation is the difference between the cell with the max voltage and the cell with the lowest; not so apparently. It is the difference between the cell with the lowest and the second highest.
You should ask them.

They also recommend a deep discharge every certain number of cycles also.
 

jzilla80

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I've always had deviations in cell number 3 , I finally got a second battery and it too has deviations only in cell number 3 -- thoughts .
This is something Ive been wondering about myself.

I have 3x M2 packs, manufactered around 7/2018. The first 2x packs have about 70-80 flights and the 3rd pack has about 40 flights.

On all 3 packs, the 4th cell is the only deviator, sometimes pushing .08v, but on a sport mode climb or heavy wind, however airdata doesnt flag them as bad probably because its only sporadic events and not sustained deviations, but its always cell 4 shown as being close to problematic.

The attachments are from airdata and have been this way on all packs pretty much since my first flights... Just makes me wonder if my AC draws a bit more from cell 4 for some reason, or all my packs are from a lot that maybe has a slight offset, however the charge and discharge voltages, at rest, all come pretty close to each other, with the resting voltage of cell 4 starting to diverge as my packs get older.

My guess is Im looking at it a little too critically, but maybe some of you can comment on what your thoughts are!

Ive been very very kind to these packs, charging to 50-75% after flights, butbonly after well cooled, and storage alwaysbin 70deg room temp. Same with all my lithium batteries (on other types of gear, aka diving dpv, etc), and Ive never see cell divergence that starts from the beginning like this AND which is the cell showing issues on multiple separate packs.

But, I am glad theyre still flying, 70+ on a pack is getting to be where Im happy with their lifespan...
What sort of life cycles have you guys seen on your drone packs?
 

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kndll

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I just find it's interesting that it's all on cell # 3 where as on my Phantom batteries the deviations vary from cell to cell from battery to battery .
 

jzilla80

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I just find it's interesting that it's all on cell # 3 where as on my Phantom batteries the deviations vary from cell to cell from battery to battery .
Yeah, I agree, its strange...

Im wondering (all factors involving Batt chemistries aside) can an aircraft draw more current from one cell vs the whole pack?

I always thought that the batt chemistry, the pack resistance, and any imperfections during the individual cell manufacture, that were what factors into how the cells in each pack discharge...

If thats the case, then even though your packs are showing deviation in cell #3, and mine are showing deviation in cell #4, then it must be a coincidence?

You have 2x packs right?
Does your AC use 3 or 4 cell packs?

Check your pack manufacture date.
If they're all in the same month, I SUPPOSE that could explain both our situations somewhat, as all 3 of my M2 packs are from 7/2018.

Im glad mine are still flyin, but Ive never had a flight last longer than 23 mins, which is below what Ive heard other M2 users saying their max is, closer to 27min. (Mav2 states 31min flight time but thats DJI, not realworld)

Do your max flight times match what others have said theyre getting, with your AC model?
 

kndll

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My mavic pro batteries are 3 cell , manu . date july 2017 and aug 2018 , I fly for about 18 mins. on average with anywhere from 40% to 20 % battery left --- so max flight time ??????
 

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