Reading Battery Run Times on Camp Lights

A run time on a box isn't "how long the light works". In the US flashlight standard, it's how long the light takes to fall to 10% of its starting brightness.¹ A light can be almost useless well before then. Dim, yes. Dead, no.

What the number measures

Under that standard, initial output is read 30 seconds after switch-on, with fresh or newly charged batteries. Run time is then counted until the beam has dropped to 10% of that first figure.¹

Three things follow from this.

• The figure is for the batteries named on the box, so a different type can give a different result.¹
• Ten percent of the original brightness is dim. You'd probably have swapped batteries long before.
• The standard does not evaluate how the output behaves during the run.¹ Two lights with the same run time can feel very different in use.

Why lights dim before they die

Some lights have a regulator, which holds output steady. A regulated flashlight may run at only a slowly declining output and then abruptly cut off. Unregulated types may have steeply declining output after only a short time.¹

Batteries add to this. The voltage of an alkaline battery declines steadily during use.² In a light with no regulator, a falling voltage can mean a fading beam. How much capacity you get is also strongly dependent on the load.² Here's one example. An AA alkaline might have an effective capacity of 3000 mAh at low drain. At a heavy 1 ampere load it could be as little as 700 mAh.²

Put simply, how hard you drive the light changes how much you get out of the same batteries.

Headlamp figures are different

Makers of headlamps may use another standard. It rates run time until output falls to 1 lux at 2 metres (about 6.5 feet). That number isn't comparable to the flashlight standard's run time.¹

Don't compare them. A headlamp's run time and a lantern's run time may be measured two different ways, and that makes any side-by-side look unfair from the start.

Reading a spec sheet

• Find which setting each run time belongs to. A figure for low is not a figure for high.
• Look for a standard name or a stated cut-off, such as 10% of starting brightness.¹ Without one, the number may mean something looser.
• Note which batteries the figure assumes.
• Plan for a lower number than the box says. If you need light all night, carry spares or a way to recharge.

A dimming light is a warning, not a reserve. Change the batteries or recharge while you can still see what you're doing.

Each battery type has its own trade-offs, set out in rechargeable vs disposable batteries. The brightness side of the box is decoded in the lumens page.