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.