Before assuming a lamp has failed, check three things in order: whether it was switched on in the pack, how cold the cells were, and how old the pack is. All three produce the same complaint, and only the third means the lamp is worn out.
First: was it on in the bag?
A headlamp pressed against something in a rucksack lid runs until the cell is empty, and it is silent about it. This is by a long way the commonest cause of a lamp that seems to have lost half its life.
Most lamps have a lock mode — a long press, or a mechanical switch — that ignores the button until it is unlocked. Where a lamp has one, use it every time it goes in the pack. Where it does not, a lamp with a removable battery can travel with one cell reversed or with the door a click open.
The symptom that confirms it: the lamp is warm, or has been running, when you take it out.
Second: how cold was it?
Cell chemistry slows as temperature falls, and the same battery delivers a smaller share of its capacity in the cold than in a warm room. A lamp that gives a full evening in autumn can look badly diminished on a frosty morning without anything having changed.
The remedy is placement rather than purchase. Carry the lamp inside a jacket until you need it, and keep the spare cell or pack in an inside pocket rather than in the lid of the rucksack. A cold pack that has warmed up usually recovers most of what it appeared to have lost, which is itself a useful diagnostic.
Bear in mind that an IP rating describes water and dust and says nothing about cold behaviour. Cold is the failure mode that no figure on the box covers.
Third: how old is the pack?
Rechargeable cells lose capacity with age and with the number of charge cycles they have been through. On a lamp with a sealed pack, that decline is the lamp’s own lifespan, which is the trade you accept for the weight saving and the USB-C socket.
The tells are consistent: the lamp charges quickly and empties quickly, or drops from apparently full to nothing without the intermediate stage. Where the maker sells a replacement pack, that is a repair; where the pack is sealed in, it is not.
On a lamp taking AAA cells the equivalent question is what is in it. A set of cells that has been sitting in the lamp for a season, or a mismatched set with one older cell among new ones, behaves like a lamp with a fault.
Fourth: what were you comparing it against?
Sometimes the runtime has not dropped; the expectation was wrong from the start.
Runtime published under the ANSI/PLATO FL1 convention is measured from switch-on to the point at which output has fallen to a tenth of its initial value. It is not the time the lamp stays useful. On an unregulated lamp, most of those stated hours are spent well below the opening brightness, so a lamp that seems to fade fast may be doing exactly what its specification says.
Check also which row of the table you remembered. The runtime quoted for a middle setting is a different number from the one quoted at maximum, and the maximum is normally held only briefly before thermal management steps the lamp down.
What to do about it
If the lamp is fine and the nights are simply longer, buy on the honest column: the output the maker publishes for the sustained setting, the runtime at that level, and whether the word regulated appears anywhere in the specification.
If the pack is spent, check whether the maker sells a replacement before replacing the lamp. And follow the maker’s instructions on charging and storage — most state a charge level for long storage, and a lamp left flat for months is one that ages faster than it needed to.
What people ask before buying
Why does my lamp die so fast in winter?
Cell chemistry slows as the temperature falls, so the same battery delivers less of its capacity in the cold. Carrying the lamp or a spare cell inside a jacket rather than in a pack lid is the usual answer, and it costs nothing.
My lamp was on when I opened the pack. Is that why it is flat?
Almost certainly. A lamp switched on inside a rucksack runs at whatever level it was left on until it is empty. Most modern lamps have a lock mode for exactly this, and using it is the single easiest way to stop losing a night's charge to a pack lid.
Last reviewed 6 September 2026