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Types of Batteries Used in Flashlights and Spotlights

Batteries are rated by their "energy density," the amount of electricity they can store for a standard size and weight. Right now, the most energy dense batteries available to the general public are lithium-based. The capacity of batteries can be expressed in watt-hours, joules per kg, or, for most small batteries, milliwatt-hours (mAh). Lead-acid batteries are the least expensive ($0.17/watt-hr) but also store the least energy by weight (146,000 joules/kg) and are not practical for flashlights or similar devices. Lithium-ion batteries are the most expensive ($4.27/watt-hr) but store the most energy by weight (460,000 joules/kg).

Lead-Acid Batteries

Over 100 years ago lead-acid batteries became the most common way for storing electrical energy. Even today they are the cheapest way to store energy per watt-hour. They are, however, the heaviest way to store energy among the most commonly available batteries. They are used in vehicles, but they are rarely used in small portable devices because of their relative weight.

Alkaline Rechargeable Batteries

Alkaline rechargeable batteries are only 10% more expensive than lead-acid batteries, based on watt-hours, and have over twice the energy density of lead-acid batteries. They are often the batteries of choice for small portable devices, such as flashlights.

NiCad

Although NiCad rechargeable batteries were very common several years ago for small home devices, such as cordless phones and some larger flashlights, they are rarely used now because of high cost and low energy density. Alkaline batteries are 1/8 the cost of NiCad batteries in terms of watt-hours and hold more than twice the energy. NiMH batteries cost 1/3 less than NiCad batteries and also hold more than twice the energy. NiCad batteries also have "memory" problems; if you charge them for a short time when they need a long charge, it can change the battery's capacity. NiCad batteries are likely to disappear from the market place because they are not competitive with other battery types.

NiMH

Nickel-metal hydride (NiMH) batteries are replacing NiCad for almost all applications that NiCad batteries used to serve, such as for larger flashlights and spotlights. Although they cost more than alkaline rechargeable batteries, they hold more energy by weight. NiMH batteries are the type currently used by hybrid cars.

Lithium-ion (Li-Ion)

The most expensive batteries are lithium-ion; they also hold the most energy by weight. They are used in cell phones and laptop computers, among other applications, and are available for flashlights. They store over three times the energy of lead-acid batteries by weight. They can be charged quickly and have no "memory" problems. Some early versions had overheating problems, especially with laptop computers, but these problems seem to have been solved with better electronics managing battery usage.

Lithium-polymer (Li-Po4)

Lithium-polymer batteries are now available that store more energy than Li-Ion batteries, are more stable, and recharge more quickly. They are being designed and tested for hybrid cars but are already available to the public in some standard battery sizes, such as for flashlights.