DEEPCAVE

Headlamp Runtime Calculator

Enter your battery's capacity and voltage and your lamp's draw at the brightness you plan to use, and get a real-world runtime estimate — plus the total across every spare set you're carrying.

Per battery or pack, from the label or datasheet.
3.7V for a single Li-ion cell; 4.8-6V for 4xAA/AAA.
Include spares to see total runtime for the trip.
At the brightness mode you plan to use most.
Real-world loss factor; 85% is a reasonable default.
Extra capacity loss in a cold cave; 0-20% is typical.

Runtime estimate

Energy per set
12.58 Wh
Effective energy (all sets)
10.69 Wh
Runtime per set
10.69 h
Total runtime
10.69 h

Energy in, light out

A battery stores a fixed amount of energy — its capacity in amp-hours times its voltage, in watt-hours. A headlamp on a given brightness mode draws that energy at a roughly constant rate, in watts. Dividing the two gives hours of runtime, the same arithmetic that underlies any battery-powered device's spec sheet.

Real lamps and cells fall short of the theoretical figure, which is why this tool applies a driver efficiency factor and lets you add a further cold-weather derate — caves are often cooler than the surface, and cold reliably shortens usable battery life.

Frequently Asked Questions

How is headlamp runtime actually calculated?

Runtime is usable energy divided by power draw. A battery's energy in watt-hours is its capacity in amp-hours (mAh ÷ 1000) multiplied by its voltage. Dividing that by the lamp's power draw in watts at your chosen brightness gives the hours it can sustain that mode — before accounting for real-world losses.

Why apply a driver-efficiency percentage?

A headlamp's LED driver isn't perfectly efficient, and a battery rarely delivers 100% of its rated capacity at a useful voltage all the way to empty. An 85% factor is a reasonable planning default; lamps and cells vary, so treat the result as an estimate rather than a guaranteed runtime.

Does cave temperature really affect battery life?

Yes — cold measurably reduces the usable capacity of most battery chemistries, lithium included, and caves are often noticeably cooler than the surface. The cold-derate percentage lets you knock a further chunk off the estimate for a cold system; 10-20% is a common planning range, but check your specific cell chemistry's cold-weather data if you have it.

How many spare batteries should I carry?

Enough that your total runtime across all sets comfortably covers your planned trip length plus a margin for delays — getting stuck, taking a wrong turn, or waiting out a group ahead of you are all normal reasons a trip runs long. Carrying a fully independent backup light, not just spare batteries for one lamp, is standard caving practice.

Planning estimate only. Always carry an independent backup light source underground — don't rely on a single calculated runtime for your only light.