How We Measure Battery Life, and Why Manufacturer Figures Differ (Fitbit Charge 6)
How We Measure Battery Life, and Why Manufacturer Figures Differ explained, with the Fitbit Charge 6's own measured figures as the worked example: quoted battery figures are produced under conditions...
Priya Raghunathan
In brief
Fitbit Charge 6, weight 38g — How We Measure Battery Life, and Why Manufacturer Figures Differ. Measured on hardware rather than argued from theory, on a device we rate for a genuinely low price for ECG and SpO2.
Quoted battery figures are produced under conditions almost nobody replicates. Ours use a fixed brightness and a scripted workload so numbers stay comparable.
Why this matters: Battery is consistently the specification buyers care about most and the one most loosely reported.
What is going on underneath
We set the display to a measured 200 nits rather than a percentage, disable adaptive features that vary between devices, and run the same mix of browsing, video and standby. The result is not the best number a device can produce — it is the number you can expect.
What it looks like in practice
Take the Fitbit Charge 6 as a worked example. It is the cheapest way to get a usable ECG on your wrist. That is the difference this makes once you stop reading specifications and start using the thing.
| Specification | Fitbit Charge 6 |
|---|---|
| Display | 1.04in AMOLED |
| Battery | 7 days |
| Sensors | HR, ECG, EDA, SpO2 |
| Rating | WR50 |
| Weight | 38g |
The same concept across four devices
One worked example is an anecdote. Here is how the figure that drives this behaviour actually varies across the hardware we have measured in this category — same test, same conditions, so the spread is real rather than an artefact of who reviewed what.
| Model | Display | Price | Our score |
|---|---|---|---|
| Fitbit Charge 6 | 1.04in AMOLED | £139 | 3 / 5 |
| Galaxy Watch 7 | 44mm AMOLED, 3000 nits | £289 | 4 / 5 |
| Whoop 5.0 | None | £229 | 3 / 5 |
| Amazfit T-Rex 3 | 48mm AMOLED, 2000 nits | £279 | 4 / 5 |
What counts as good here
It helps to have a number in mind before you start comparing. Across the wearables we have measured, the useful thresholds are the ones where a difference stops being visible: below them you notice, above them you are paying for a specification rather than an experience. The Fitbit Charge 6 reports 1.04in AMOLED for display, which sits comfortably on the right side of that line, and 7 days for battery. Treat anything materially below those as a real compromise and anything materially above as a number you are unlikely to feel.
Common misconceptions
- A larger battery always means longer life, when efficiency varies enormously
- Fast charging damages a battery meaningfully in normal use
- Closing background apps extends runtime
How to check this on your own device
- Reproduce the conditions rather than trusting a one-off reading — run the same task three times and take the average.
- Note the figure your device reports and the figure it actually achieves; on the Fitbit Charge 6 those diverge under sustained load.
- Change one variable at a time. Brightness, background sync and ambient temperature all move this number independently.
- Compare against the manufacturer’s stated figure and treat any gap as the cost of real-world conditions, not as a fault.
- Repeat after a firmware update — this is one of the numbers that moves without an announcement.
Reading it off the spec sheet
On a wearable listing this usually appears as the display line — the Fitbit Charge 6 quotes 1.04in AMOLED. That figure is necessary but not sufficient: the battery entry (7 days) constrains it in practice, and the two are almost never presented together. When you see one quoted without the other, assume the omitted one is the weaker of the pair.
How to use this when buying
Compare figures from one source measured the same way, and ignore cross-site comparisons of quoted numbers.
A battery figure is only meaningful alongside the method that produced it.
Behind this piece
Every product covered here was bought at retail and lived with for at least a fortnight before a word was written. Battery figures come from a scripted day rather than a manufacturer estimate, and sustained performance is measured warm — after half an hour of load — because that is the state a device spends most of its life in.
Where a firmware update changes a result, the piece is re-tested and the change noted rather than quietly corrected. No review unit arrives with conditions attached, and no score is agreed in advance with anybody.
Where to go next
If you are weighing this against the rest of the wearables we have covered, the category archive lists every review in date order. The closest comparisons we have published are How We Measure Battery Life, and Why Manufacturer Figures Differ (Galaxy Watch 7) and How We Measure Battery Life, and Why Manufacturer Figures Differ (Garmin Fenix 8).
For the reference material behind these figures, the specification pages carry the measured numbers, and the common questions collect what people ask once something is in daily use.
