Body battery

Yes, it's designed that way for a reason - it's because the watch does not collect HRV information during an activity recording, because it cannot accurately do so during exercise. After the activity is finished, the watch estimates the body battery drop that likely would have happened during the activity (probably using an algorithm factoring in time and exercise intensity), and applies that estimate to your body battery chart all at once at the timestamp where your activity began, resulting in what looks like a sudden sharp drop.

It is indeed weird to see a drop at the beginning of an activity. It doesn't seem to be happening to me

Looking at your graph, it is also weird that the body battery didn't decrease in the morning at all (very weird flat line). So I assume the BB caught up when you started the activity (although something else interrupted the flat line just before you started the activity).

One possible explanation is a window of time where the heart rate was not read properly. Maybe the watch was not snug enough or thought that you were sleeping (very low stress level).

I was lying in bed this morning so it stayed on 100%, but the sudden battery drop only occurs very seldomly so saying it’s designed that way isn’t correct and as you can see it declined steadily during the walk

Karlax said:

I was lying in bed this morning so it stayed on 100%

It wouldn't be a strict flat line. It could have been trying to update to a value higher than 100% and that created the flat line. So my best guess is that the BB was over estimated, and then it caught up first when you started moving, then when you started the activity.

I have noticed that BB can be optimistic the day after a firmware upgrade. Could that be the case here?

Well I do get 8 hrs sleep a night and usually recover 65-70 points, so I was already at 100 when I woke and then did nothing all morning, heart rate at 50 and stress around 15, so I would expect it to stay steady 

I was just about to ask the same question. It started happening one week ago (early April 2023).

So far the sudden drop happened twice for me. Once with a walk (instant -5% and then continued normally dropping), and once with a HIIT activity (instant 12% drop and then continued dropping quite steeply but as I expected).
So far it did not happen with activities recorded with non-garmin apps (ie. Works+) or cycling recorded with Garming Edge 1030 Plus.

And, this never happened before early April 2023. I have the watch since early January and I also had a Garmin Venu before this. I never ever seen such strange sudden drops happen instantaneously after starting an activity. And to be honest it makes zero sense to me. It looks like a bug that may happen only occasionally after the latest update.

For now I restarted my watch and I will keep an eye on this issue. If it keeps repeating I will try the "magic" of a Garmin Express update. Sometimes just running Garmin Express with the watch connected can fix all kinds of issues.

If it keeps repeating I will have to contact Garmin Support as this is something that seems to be incorrect and certainly wasn't happening in the past.

My fênix 7 is facing a issue with body battery too. the body battery dropped from 80 to 8, when I was doing physical activity. This has happened to me 3 times. I suspect it has something to do with the fact that I'm listening to Spotify on the watch during these workouts.