Voltage sag at beginning of flight - what happened?

[Yaros]

Today I checked the logs from yesterday's flight, and found that the voltage was very low at the beginning of the flight.
At around 1 minute in the flight, the voltage went down to 3.4V, but then in 30 seconds it went up and stayed normal for the rest of the flight.

Here is the log on Airdata:

And on PhantomHelp Log Viewer

 

[Kilrah]

Battery warmed up.

 

[Yaros]

So, should I be worried about such behavior, or is it normal?

 

[Kilrah]

It's kinda normal for them to do that, it's how much it does that could indicate a problem but for that you'd need to look at how it evolves over time in similar conditions for each given battery.

 

[slup]

This isn't normal at all ... yes, the battery warmed up which helped the battery recover but voltage drops to these low levels in ambient 19C degrees with this moderate flying clearly indicates that this battery is coming close to it's end of safe predictable airborne use.

The battery should have a capacity of 3500mAh new, but it is indicated to only have 2984mAh left ... which is a substantial loss since new. Furthermore it's been charged 167 times. With this it's fair to assume that the internal resistance also have increased to levels where some moderate amp draw will make the voltage drop fast & in turn generate excessive heat which in turn will increase the internal resistance.

3,4V/cell isn't far from 3.0V ... which results in a forced auto landing. So if you don't want to experience that surprise you better take this seriously & only use the battery where a force landing can be handled + desk use.

 

[Don Testme]

Considering what I read in post #5. May I ask, how old is the battery you are using and or how many cycles does it have?

 

[slup]

...how old is the battery you are using and or how many cycles does it have?

The battery is manufactured 17:th of December 2020 & have 167 charge cycles registered on his Airdata account ... so it can be more if not all flight logs with that battery is on Airdata.

@Yaros ... I might add that I myself have a battery in the same state for my Air 1 ... With slow flying in normal mode, limiting applying both full ascend with full forward the battery behave reasonably ok, but it's always hotter to the touch than my other batteries which clearly indicate a higher internal resistance. Never use that battery when high amp draw is required, never Sport mode, only slower ascends & only close & near over ground where I can let it low battery auto land if that happens. Usually this battery serve as a desk battery, but charge it full & bring it with me for flights anyway to keep it up & running... but with full knowledge of what can happen.

 

[Yaros]

Oh, wow! I guess I'm lucky I checked this specific log in Airdata, because I almost never check my logs on Airdata.
I don't stress my batteries much, but I do use Sport Mode sometimes.

3,4V/cell isn't far from 3.0V ... which results in a forced auto landing.

Is this "forced auto landing" the same type of landing that happens when the battery reaches 10% of charge, that you can still fly and ascend a bit? Or does it just go down, and you can't do anything about it?

I need to mention that the battery was used for about 1 week in very cold weather (-10°C), but I always let it hover and warm before flying in that conditions, and never used sport mode.

This battery has 163 charge cycles to be precise, that's not much for LiPos, I expect DJI batteries to reach at least 200 cycles before starting to degrade. And I don't really notice any performance drop for now.

One thing that might cause that, is that the battery was charged about 6 days prior to this flight, and it was sitting charged for 5 days, it discharged to 96%, and I didn't charge it before flying. Can that be the reason?

This weekend I will probably go to the field and fly around in Sport Mode, while closely monitoring the voltages, and see if it drops below 3.5V while 50% or more charged.

 

[Yorkshire_Pud]

@slup, there is a possible point regarding the charge count.
This stems from my own flights using Mavic Mini batteries in a Mini 2 AND from checking the airdata charge count against the count shown in the APP and unencrypted Mavic Mini logs.

For my mavic mini batteries it seems that the charge count shown in airdata reflects ONLY the number of times airdata has 'noticed' each Mavic Mini battery having been charged. I.e. when the MM battery has been used in a Mini 2.
It is not the actual count shown in the app NOR the charge count shown in the unencrypted logs when the same battery is used in a mavic mini.(My mavic minis are normally flown with a pre 1.2 App so that I can process the logs via TXTlogToCSVtoolMM).....
So, in some cases, airdata thinks the charge count is low, say 1 - 4 whilst the actual count is in the upper teens or low 20s.
Also, going by my log book, which records the charge % at the start and end of EVERY log (where that info is available), some partial charges do not increment the charge count...anywhere. That is something I noticed only the other day and I will need to look at that in more detail at some point

I have not yet checked my Mini 2 batteries and the counts shown in airdata against the last set of app screen captures that I took.

 

[slup]

...this "forced auto landing" it just go down, and you can't do anything about it?

When it comes to "critical" low voltage (below 3.0V/cell) auto landing the craft will usually just descend no matter what you do ...

I need to mention that the battery was used for about 1 week in very cold weather (-10°C)...

In general both colder & hotter ambient environments will increase the internal resistance momentarily during the flight ... but as that increase the internal heat, it will also speed up the electrolyte decomposition which is permanent ... adding high amp draw in environments like this will just make it worse.

...And I don't really notice any performance drop for now.

That's not true ... you've just noticed a severe performance drop in the log & as you rarely look at this it can have happened before this flight ... in next flight you might fall below 3.0V/cell. You can't fully trust this battery anymore.

One thing that might cause that, is that the battery was charged about 6 days prior to this flight, and it was sitting charged for 5 days, it discharged to 96%, and I didn't charge it before flying. Can that be the reason?

This discharge & the danger with it is mostly if the cells have discharged differently ... the battery is unbalanced when you start the flight, but that wasn't the case in your flight. All batteries have a slight voltage drop in the beginning of the flight... if fully healthy & pretty new it's usually about 0.2V/cell which recovers in a similar pattern as in your flight. Your battery started out with approx 4.2V/cell & all 3 fell down to approx 3.4V ... that's 0,8V & ended up close to a critical low voltage auto landing.

 

[slup]

@slup, there is a possible point regarding the charge count.
This stems from my own flights using Mavic Mini batteries in a Mini 2 AND from checking the airdata charge count against the count shown in the APP and unencrypted Mavic Mini logs.

For my mavic mini batteries it seems that the charge count shown in airdata reflects ONLY the number of times airdata has 'noticed' each Mavic Mini battery having been charged. I.e. when the MM battery has been used in a Mini 2.
It is not the actual count shown in the app NOR the charge count shown in the unencrypted logs when the same battery is used in a mavic mini.(My mavic minis are normally flown with a pre 1.2 App so that I can process the logs via TXTlogToCSVtoolMM).....
So, in some cases, airdata thinks the charge count is low, say 1 - 4 whilst the actual count is in the upper teens or low 20s.
Also, going by my log book, which records the charge % at the start and end of EVERY log (where that info is available), some partial charges do not increment the charge count...anywhere. That is something I noticed only the other day and I will need to look at that in more detail at some point

I have not yet checked my Mini 2 batteries and the counts shown in airdata against the last set of app screen captures that I took.

I know ... but the difference in the long term usually is minor. He said 163 & Airdata 167 ...

 

[Yaros]

battery started out with approx 4.2V/cell & all 3 fell down to approx 3.4V ... that's 0,8V & ended up close to a critical low voltage auto landing.

So you want to say that if I would have started flying in Sport Mode during these 3 minutes of that flight the voltage might have dropped to 3.0V causing a landing over those houses? That's scary!

By the way, I was about to switch into sport mode, but then changed my mind at the last moment because it was night and there was low visibility.

 

[brett8883]

Without reading all the other posts I don’t see anything to worry about here. AirData has voltage drop alerts and this flight didn’t trigger any. If you watch your flight at the time the voltage drop happened you were pushing full pitch forward and throttle up for a sustained period so you were pulling more than 16 amps for a good little while. All seems normal to me.

 

[slup]

Without reading all the other posts I don’t see anything to worry about here. ...the voltage drop happened you were pushing full pitch forward and throttle up for a sustained period...

To put the OP's flight in perspective & compare the voltage drop logged in total from motor start over a period with near max throttle together with full elevator (between approx 00m50s & 01m05s) to where one cell reached 3,42V at 01m25s ...

With a total initial voltage drop of 0,8V/cell in average it's clear that this behavior isn't normal & nothing you just can ignore if you aren't fond of risking your drone in a low voltage forced auto landing & in worst case damage something or someone.

A near brand new battery:
This is the voltage drop from a very new battery ... only 6 charge cycles. Battery was fully charged & max throttle + elevator was applied between 00m28s & 01m40s. The total voltage drop from motor start until the sticks was released was from approx 4,25V down to approx 4,1V per cell.



A older well used battery showing increased internal resistance but still safe:
Below is from a older battery ... in total 67 charge cycles. Sustained full throttle & elevator between 00m39s & 01m39s. The total voltage drop from motor start until the sticks was released was from approx 4,25V down to approx 3,95V per cell.



From an even older battery (with fewer charge cycles though...) showing obvious signs of high internal resistance:
And here from a flight with a much older battery ... which after this flight was taken out of flight duty, at the time the battery had 51 charge cycles. Sustained full throttle & elevator between 00m40s & 0m50s. The total voltage drop from motor start until the sticks was released was from approx 4,25V down to approx 3,2V per cell in average with the lowest cell down to 3,16V.

A DJI drone will initiate a low voltage forced auto landing when going below 3,0V/cell ... this flight was performed above water & neither Airdata or the app showed any voltage drop alerts besides what is shown in the chart below.

 

[Yaros]

@slup Thanks for this analysis. A couple of days ago I did another flight with the same battery, here are the results:
As you can see, there are two voltage drops, but not as low. Do you think the battery is still good to fly?

 

[slup]

... there are two voltage drops, but not as low. Do you think the battery is still good to fly?

Both drops occurred in P-mode in about 14C degree ambient when you had mainly full elevator & started to sneak in some throttle up to half max during a shorter period... This isn't normal.

Imagine what could happen if it's a bit colder, you put in some more throttle together with full elevator... & applying the input during a longer period... or if you forget about this battery & use it in Sport mode.

You can't trust it anymore... use it at your own risk & only where you can handle a forced low battery voltage landing.

 

[Yaros]

Okay... the battery has only 165 charge cycles, I'm sad that it's this weak...
I did fly this battery in pretty extreme conditions though. In heat of +40°C and cold of -10°C.

Here is a screenshot of my 2nd battery:


And here is my 3rd battery:


Both seem pretty normal, right?

 

[Yaros]

I will really look into this issue, I can't understand when this really went wrong, because I found a flight log from earlier this year, and looking at it, this battery was outperforming my other batteries!

Take a look at this older log using the same battery:

 

[Yaros]

This is strange.
@slup take a look at this log, please:

People on the DJI Forum are telling me that it is more or less normal behavior for a battery, but I don't think so.
Then I got contacted by DJI directly and they have requested the logs from that flight, which I provided on their custom file sharing website.

 

[slup]

It's perfectly normal if using a degraded battery with a high internal resistance.

The voltage drop down to approx 3,3V/cell occurred 59s after takeoff with a calculated battery percentage of 91%. There you gave high levels of both Throttle & Elevator commands... flying into a near headwind of approx 6-8m/s.

This manoeuvre resulted in a very high current draw... which in turn caused a rapid voltage drop & a heat build up (15C degrees here during 39s). Both the voltage drop & the heat build up will be worse if the battery have a high internal resistance.

The battery you used there isn't up for this kind of flying, I guess it have a substantial number of charge cycles & is close to a couple years old. In this flight you were 0,3V away from a forced low battery voltage landing (imagine if you had kept the stick commands for a little longer)... and you were above water the whole incident.

Stick commands in dashed graphs... they correlate very well with the voltage drop, especially the throttle.