Race-day battery
Will it last the day?
The number on the box is the best case. Pick the watch, the day and the settings you will actually run with, and this works through the same arithmetic as the battery guide — with the margin the guide says to add.
Your day
Garmin Forerunner 265
Comfortable
14 h multiband
At least 30% spare after every setting you chose, on the maker’s own figure. That is the margin the guide asks for on a first attempt.
Your first 30 days with the Forerunner 265 · What could you run? Race predictor
Every watch, same day, same settings
The whole catalogue under the settings above, longest first. Where a watch lacks a setting you chose — no multiband, no music, a transflective screen — it simply is not charged for it.
| Watch | Hours | Margin | Verdict |
|---|---|---|---|
| 76 h | 744% | Comfortable | |
| 65 h | 622% | Comfortable | |
| 60 h | 567% | Comfortable | |
| 60 h | 567% | Comfortable | |
| 55 h | 511% | Comfortable | |
| 42 h | 367% | Comfortable | |
| 41 h | 356% | Comfortable | |
| 40 h | 344% | Comfortable | |
| 38 h | 322% | Comfortable | |
| 37 h | 311% | Comfortable | |
| 35 h | 289% | Comfortable | |
| 35 h | 289% | Comfortable | |
| 34 h | 278% | Comfortable | |
| 31 h | 244% | Comfortable | |
| 31 h | 244% | Comfortable | |
| 30 h | 233% | Comfortable | |
| 30 h | 233% | Comfortable | |
| 26 h | 189% | Comfortable | |
| 26 h | 189% | Comfortable | |
| 26 h | 189% | Comfortable | |
| 25 h | 178% | Comfortable | |
| 25 h | 178% | Comfortable | |
| 24 h | 167% | Comfortable | |
| 23 h | 156% | Comfortable | |
| 23 h | 156% | Comfortable | |
| 21 h | 133% | Comfortable | |
| 21 h | 133% | Comfortable | |
| 20 h | 122% | Comfortable | |
| 20 h | 122% | Comfortable | |
| 20 h | 122% | Comfortable | |
| 20 h | 122% | Comfortable | |
| 20 h | 122% | Comfortable | |
| 20 h | 122% | Comfortable | |
| 19 h | 111% | Comfortable | |
| 19 h | 111% | Comfortable | |
| 18 h | 100% | Comfortable | |
| 14 h | 56% | Comfortable | |
| 14 h | 56% | Comfortable | |
| 10 h | 11% | Tight | |
| 8 h | -11% | Will not make it |
How the sum works
- Start from the mode you will actually use. The maker’s GPS figure, or the multiband figure if you turn multiband on — the catalogue carries both, verified against the spec sheets.
- Music: about a third. Only Garmin publishes a figure — the Forerunner 965 at 31 h GPS and 10.5 h with music. Nobody else quotes one, so the same ratio (× 0.4) is applied to every watch that plays music, as an estimate.
- Always-on screen: a fifth off. On an AMOLED. A transflective screen is lit by the sky and the quoted figure already assumes it is on.
- Cold: another fifth off. Lithium cells lose capacity below freezing.
- Then the margin. Comfortable means at least 30% spare — finish time plus a bad patch, a wrong turn or weather. Tight means it should finish with less than that. The failure mode is asymmetric: spare battery costs nothing, a dead watch costs the day.
Solar is not counted: hours of direct sun add a modest slice of a day, and it is safer as margin than as a plan. Battery age is not counted either; after two years, take a further tenth off yourself.