Critically low auto-land with TWO batteries - drone or batter(ies) issue?

[JDJ]

Hi - hoping to get some advice from the brains trust! I have a Mavic 2 Prowith two batteries. Charged both up on sunday and in the afternoon did a short flight to scout a location. Came back the next morning to take the shots when this happened:
Flight 1 / Battery 1: this was the battery that was used the previous day but still had 50%+ charge, which should have been plenty. Fired it up and flew a very short distance (maybe 20m) before I got the critically low battery / auto land message; babied it back to the home point. Thought this might just have been less charge than I thought / battery collapsing, so changed it over to the full unused battery.
Flight 2 / Battery 2: Took off again with the second battery showing full charge and everything seemed fine. Flew over to the feature, maybe a minute and 500m away. Again suddenly got a critically low battery / auto land message; this time it went down and very narrowly avoided landing in the water. Managed to retrieve the drone with no small amount of toil.

Here are the logs one two

With one instance, I would just think it was a failed battery. But given it occurred with both batteries which have never had an issue before? How can I determine whether this is an issue with the batteries or the drone itself?

 

[Meta4]

With one instance, I would just think it was a failed battery. But given it occurred with both batteries which have never had an issue before? How can I determine whether this is an issue with the batteries or the drone itself?

Batteries deteriorate over time and yours must be several years old by now.
The % indication can be misleading but the cell voltage isn't.
Both of your batteries already have very low cell voltages before you launched, much lower than the % indication would have lead you to believe.
Within seconds of launching, when the battery was under load and the drone was climbing, the cell voltage went below the critical low level of 3.2 Volts.

One battery has 80 charge cycles on it, the other has 160.
When new they have a capacity of 3850 mAh.
One of yours are showing as 2803 mAh and there's no data for the other one.
These batteries can't hold as much charge as a new one, and they can't hold the charge like a new one would.

If you had topped them up before launching, you would have got more flight time but they probably wouldn't have lasted long.
With new batteries, your drone should be flying as good as new again.

 

[JDJ]

Batteries deteriorate over time and yours must be several years old by now.
The % indication can be misleading but the cell voltage isn't.
Both of your batteries already have very low cell voltages before you launched, much lower than the % indication would have lead you to believe.
Within seconds of launching, when the battery was under load and the drone was climbing, the cell voltage went below the critical low level of 3.2 Volts.

One battery has 80 charge cycles on it, the other has 160.
When new they have a capacity of 3850 mAh.
One of yours are showing as 2803 mAh and there's no data for the other one.
These batteries can't hold as much charge as a new one, and they can't hold the charge like a new one would.

If you had topped them up before launching, you would have got more flight time but they probably wouldn't have lasted long.
With new batteries, your drone should be flying as good as new again.

Yeah, that makes sense, thanks. Out of interest, how many charge cycles/years would you expect to get out of a battery? I've had to replace them a few times now and they're not cheap!

 

[Cafguy]

You should get around 300 cycles with new batteries depending on how you treat them. they start to lose their luster around the 220 mark tho.

 

[Not A Speck Of Cereal]

Just as an aside, are we no longer heeding the wisdom of never flying on a half-depleted battery (fly half of it one day, fly on the 2nd have the next day)?

Flying half-depleted batteries were once one strong possible reason for power failures in the sky (aircrafts falling to the ground and destroying themselves). I might touch down and then a few minutes later take off again, but once a battery has cooled off, I never use it again until it's be recharged.

We know this isn't the case here as Meta found the reasons for your issue in the logs (and your 2nd battery was still unused after charge), but when I read that you scouted with one the previous day, then flew on the 50% left the next day, I cringed.

But I ask because maybe that stopped being an issue with battery tech.

Chris

 

[JDJ]

@Not A Speck Of Cereal no idea, I've never heard about this before tbh

@Cafguy and @Meta4 I just bought a reconditioned battery (can't get new batteries for the mavic 2 in Australia at the moment) which has 75 charge cycles on it. Is there a reliable way for me to assess the overall health of this battery, given its the same number of charge cycles as my other one which has recently failed?

 

[Meta4]

@Cafguy and @Meta4 I just bought a reconditioned battery (can't get new batteries for the mavic 2 in Australia at the moment) which has 75 charge cycles on it. Is there a reliable way for me to assess the overall health of this battery, given its the same number of charge cycles as my other one which has recently failed?

If you do a short flight ... even just a few seconds and post the recorded data, I can dig deep in the data to find the actual capacity of the battery to compare with the 3850 mAh capacity of a new battery.

 

[JDJ]

@Meta4 that would be amazing, thanks! I charged all batteries up and flew test flights today. If you could let me know whether the new (reconditioned) battery is reasonable, that would be great, I've got a 30 day returns if Im not satisfied with it.

Apart from the new battery, I couldnt get the older batteries to recreate the critical battery issue again and they seemed to fly fine, although looking at the logs the 'cell deviation' was in the red multiple times so I dont think I trust them now.

New Battery 1
Old battery 2
Old battery 3

 

[Meta4]

@Meta4 that would be amazing, thanks! I charged all batteries up and flew test flights today. If you could let me know whether the new (reconditioned) battery is reasonable, that would be great, I've got a 30 day returns if Im not satisfied with it.

Apart from the new battery, I couldnt get the older batteries to recreate the critical battery issue again and they seemed to fly fine, although looking at the logs the 'cell deviation' was in the red multiple times so I dont think I trust them now.

The new battery is showing 75 charge cycles so it will have lost some of its original capacity.
Battery capacity data from the new battery is blank so I can't give any comparison numbers.
The cell voltage drop over time isn't alarming.
In DJI Go 4 app settings, you can configure the screen display to show the cell voltage like this (top right)

That way you can keep any eye on the cell voltage and be aware of how close it's getting to 3.2V (critical low voltage level).

The other two batteries are showing that they still have some life and can be used as long as they are freshly charged.
But if they are sitting around for a few days after charging, they won't be reliable.

I see a number of 3rd party batteries for the mavic 2 are available on Ebay.
That might be another option now that DJI have stopped selling the batteries for that model.