Batteries all work by a chemical reaction that causes electrons to flow from one end to the other. When you plug a battery into a device such a headlamp, you connect the battery’s two terminal ends, causing the electrons to flow, powering the device. Not all batteries, however, are alike. Here are the most common.
Lithium: These batteries last up to five times longer than standard alkaline batteries, and lose less power in cold temperatures. A fresh set of these will get you through most any roadtrip or alpine climb. Two AAs cost around $8.
Alkaline: Duracel and Energizer batteries use zinc and manganese-oxide with an alkaline electrolyte for power. These batteries cost half that of lithium, but go dead faster.
Nickel-cadium: These are the rechargeable types. Expensive, but can be worth it if you charge them properly. Never recharge a battery unless it is dead—NiCad batteries “remember” how low the were: If, for example, you charge a battery that is only 50 percent dead, it will die next time it reaches 50 percent drained. Always discharge a battery completely, then charge it fully. Expect to pay $10 for 4 AAs, not including charger.
Zinc-carbon: Least expensive, shortest-lasting battery on the market. Don’t bother.
Other tips: Dead batteries can come back to life, briefly, if you let them rest by switching off your headlamp for five to 30 minutes. You can also stretch the life of dying batteries by warming them in your hands or inside jacket pocket, a useful tactic in cold weather.