In general, your choice boils down to three distinct flashlight batteries: rechargeable, alkaline and lithium. The energy characteristics of a battery are Amp Hours, Voltage, and Internal Resistance, and these factors affect how the battery works.
Larger batteries have a bigger milliamp hour rating (mAh), allowing them to hold more electricity than smaller batteries. Therefore they can do the same job for a longer period.
Higher voltages allow more electricity to be exerted and therefore using higher voltage batteries will allow the light emitting from the flashlight to be brighter.
Rechargeable flashlight batteries
Rechargeable batteries come in a variety of sizes and are often already built into rechargeable flashlights. They are ideal in lights that are used frequently for short periods of time.
Rechargeable Nickel-Metal-Hydride (NiMH) cells can be used in place of alkaline batteries. However, they often have reduced run times and cause the light to be less bright. They lose their charge if left in the flashlight unused.
Lithium Ion (Li-on) rechargeable cells can provide high outputs at low long-term operating costs. A disadvantage is that lights using these cells have a tendency to be more expensive.
All rechargeable batteries are capable of delivering high currents very rapidly. However, they lose some of their charge every day. Lithium Ion batteries are affected least by this characteristic, but if the batteries are overcharged, they will eventually hold less and less charge.
An important point to note is that not all flashlights will accept rechargeable batteries, even though you may be able to buy a size that fits. Check the instructions that come with your flashlight or check out the specs online. It should say clearly if rechargeables are a problem.
Non-rechargeable flashlight batteries
Alkaline batteries are the most popular for use in flashlights. They work best in lights that draw low to moderate currents and have a shelf life of up to seven years. They deplete during use thus reducing the flashlight’s light output and they are negatively affected by cold weather.
Most usefully, alkaline batteries are widely available, with the standard sizes usually easy to buy wherever you find yourself in the country – or, indeed, in the world. If your flashlight batteries and died at an inconvenient moment and you’re looking for replacements and the local mom and pop store, the chances are you’ll end up with alkaline.
Lithium batteries are made of a lithium metal formulation. They produce higher currents than alkaline batteries and are not affected by cold weather in the way that alkaline batteries are.
Lithium batteries have lower internal resistance, meaning they can give out their electricity a lot faster. They are very lightweight batteries. 123a lithium batteries are commonly used in high output, short run incandescent lights. They are a lot more expensive than alkaline, but are a good option for flashlight batteries.