Batteries in Cold Weather: Lithium, Alkaline and NiMH
A headlamp that dims within minutes on a frosty night is usually not broken. Cold slows the chemical reactions inside a battery, so it delivers less power until it warms up again. Choosing the right chemistry and keeping spares warm prevents most of these surprises.
How the main chemistries compare
Alkaline cells lose a large share of their usable capacity in freezing temperatures and struggle with high-drain devices. Primary lithium AA and AAA cells are lighter and keep working much better in the cold, which is why many winter campers carry them.
Rechargeable NiMH and lithium-ion cells are economical and convenient, but they also lose capacity in the cold, and lithium-ion cells should not be charged while below freezing.
Keeping batteries warm
Carry spare batteries in an inside pocket close to your body during the day. At night, keep the headlamp and spares in your sleeping bag or in a pocket of your sleeping clothes.
A cold battery that seems empty often recovers some capacity once it warms, so swap it for a warm spare rather than throwing it away.
Practical habits
Use the lowest light setting that does the job, since high output drains a cold battery fastest. Remove batteries from devices you will not use for a while, and lock lights so they cannot switch on inside a pack.
Do not mix old and new cells or different chemistries in one device, and pack used batteries out for proper recycling.