Body Battery is one of the few wearable metrics people genuinely miss when they switch away from Garmin, and the reason is not accuracy. It is that a 0 to 100 reserve that drains and refills is immediately understandable in a way an HRV chart never will be.
What it is doing under the metaphor
Garmin computes it from heart rate variability, its stress metric, sleep quality and activity. The reserve drains as you spend energy and accumulate stress, and refills during rest and particularly during sleep.
The exact weighting is proprietary and unpublished, which has two consequences. Nobody can reproduce it exactly, and nobody outside Garmin can tell you precisely why yours dropped 20 points on a given afternoon.
The genuinely good design decision is the framing. A recovery score answers “how did you sleep?” and a battery answers “how much have you got left right now?”, which is a more useful question at 3pm.
What the Apple Watch gives you natively
Vitals. Five overnight metrics against your own typical range. A morning snapshot, not a running reserve, and with no interpretation attached. Covered in the Vitals app explained.
Training Load. Workout effort over 7 days against 28. The demand side only, and workouts only.
Move ring. Active energy in kilocalories. Counts up rather than down, and measures energy rather than stress.
None of these is a battery, and Apple ships no readiness score of any kind, which is the subject of does the Apple Watch have a readiness score.
What to look for in a replacement
It should read from Apple Health. Anything asking you to wear a second device has missed the point. The data is already there.
It should baseline against you. A score comparing your HRV to a population range is not doing the job. Personal baselines are the entire mechanism.
It should show its working. The most common complaint about Body Battery is that a drop is unexplained. An app that tells you which signal moved the number is strictly more useful.
It should count the whole day. A drain metric that only counts logged workouts will miss the stressful afternoon that actually emptied you.
The main iOS options are Helix, Athlytic and Bevel, compared in the best Apple Watch recovery apps.
The honest caveat about all of them
No wrist-worn energy metric, Garmin’s included, can tell a leg day from a back day. A whole-body reserve is a lossy summary, and it will happily read 80 while your quads are unusable. That limitation is structural rather than a flaw in any one implementation, and we took it apart in why your wearable has no idea which muscles are tired.
If you are still deciding whether to switch at all, the full comparison is Garmin vs Apple Watch for recovery.