Phavai Data Lab

Ultramarathon GPS Watch Battery Planner

See whether a specific watch mode fits your expected elapsed time and chosen reserve. Results are claim-based planning estimates—not promises about race-day battery life.

AI-assisted data organization · Formula and source check by Ben Yardley, outdoor-media professional, trail runner, ultrarunner, and climber. Phavai did not physically test these watches.

01 · Your scenario

Set the race assumptions

Factors that may reduce runtime

These create a warning only. Phavai does not invent a drain percentage.

02 · Claim-based result

Fits only without the full buffer

Garmin Forerunner 965 in GPS Only GNSS is listed for up to 31 hours. Your 24-hour scenario fits the published ceiling, but not the full contingency and reserve.

Manufacturer ceiling31.0 h
Buffered race time28.8 h
Claim-based finish charge22.6%
Buffered headroom-4.0 h
Minimum starting charge112.9%
Mode fidelitySingle-system GPS
Before race day

Rehearse this exact mode and feature setup on a long effort. Published maxima are not a safety guarantee.

Check the manufacturer claim
Same assumptions, every mode

Compare headroom without mixing GPS modes.

Download source CSV

A longer number can come from lower-fidelity or reduced-recording behavior. The table keeps each mode and its limitation separate; it does not automatically recommend the largest value.

Scroll sideways to compare every mode.

Manufacturer-published maxima checked September 8, 2026; calculated status uses the scenario above.
WatchModePublished maxMode fidelityScenario statusBuffered headroomSource
Garmin Forerunner 265 GPS Only GNSS 20 h Single-system GPS Recharge plan -12.8 h Official claim
Garmin Forerunner 265 All Systems + Auto Select 16 h SatIQ automatic mode selection Recharge plan -16.0 h Official claim
Garmin Forerunner 265 All Systems + Multi-Band 14 h Higher-accuracy multi-band Recharge plan -17.6 h Official claim
Garmin Forerunner 965 GPS Only GNSS 31 h Single-system GPS Tight -4.0 h Official claim
Garmin Forerunner 965 All Systems + Auto Select 22 h SatIQ automatic mode selection Recharge plan -11.2 h Official claim
Garmin Forerunner 965 All Systems + Multi-Band 19 h Higher-accuracy multi-band Recharge plan -13.6 h Official claim
COROS PACE 3 Standard Full GPS 38 h Standard GPS Fits assumptions +1.6 h Official claim
COROS PACE 3 All Systems On 25 h Multiple satellite systems Tight -8.8 h Official claim
COROS PACE 3 Dual Frequency 15 h Higher-accuracy dual-frequency Recharge plan -16.8 h Official claim
Polar Vantage V3 Performance Training 43 h Dual-frequency GPS, optical heart rate, 1-second recording Fits assumptions +5.6 h Official claim
Polar Vantage V3 Eco Training 140 h Reduced logging: single-frequency GPS, optical heart rate off, 2-minute recording Fits assumptions +83.2 h Official claim
Transparent math

What the planner calculates

Buffered race timeExpected elapsed time × (1 + contingency %)
Usable hours after reservePublished mode hours × (starting charge % − finish reserve %)
Buffered headroomUsable hours after reserve − buffered race time
Minimum starting chargeFinish reserve % + buffered race time ÷ published mode hours

Limits: Manufacturer figures are maxima, not guarantees. Firmware, satellites, temperature, display use, sensors, navigation, music, and battery age can shorten runtime. The toggles flag those risks but do not apply invented drain percentages. Fully charge the watch, rehearse the exact settings on a long training effort, and keep a verified charging contingency when navigation or safety depends on it.