A brief history
The invention of the zinc-carbon battery, the first battery to combine long shelf life, reasonable durability and a paste, rather than a leakage-prone liquid for an electrolyte quickly lead to the invention of the flashlight. The "flash" in the name "flashlight" comes from significant voltage sag of these batteries, which resulted in a brief flash of light followed by a considerable reduction in output. It is still manufactured due to its low cost, but it has been superseded in every other way by the alkaline battery.
The nickel-cadmium rechargeable battery was the next evolution of batteries that could reasonably be used in flashlights. Though invented in 1899, it wouldn't become popular in sealed, consumer-friendly cells for many decades. Rarely seen today, it has been largely replaced by the nickel metal hydride battery.
A refinement of the zinc-carbon battery, the modern alkaline was patented in 1960. Alkaline batteries are inexpensive and offer fairly high energy density, but only under light loads. They're well-suited to low-cost, low-power flashlights that are used infrequently, but their non-rechargeable nature makes them wasteful for frequent use. For high-powered lights, they suffer from the same voltage sag that gave the flashlight its name. They're also prone to leaking electrolyte during long-term storage, which can damage devices; any device intended for potential emergency use should not be stored with an alkaline battery inside.
Commercially viable lithium metal batteries appeared in the 1970s. They offer high-performance direct replacements for common sizes of alkaline battery as well as coming in dedicated sizes with higher voltage. Lithium batteries offered a substantial increase in capacity over existing battery types along with excellent shelf stability, good performance in a wide range of temperatures and the ability to handle high loads. Surefire introduced a handgun-mounted light powered by the CR123 battery in 1985 and later expanded to handheld flashlights. The lithium battery soon became the standard for high-end flashlights, but the cost of disposable lithium batteries made these lights unappealing to most consumers. Lithium metal batteries remain the best choice for infrequently-used devices that need to work in an emergency, such as weapon-mounted lights and laser sights.
The nickel metal hydride battery was commercialized in consumer-friendly cells in 1989. It has slightly less capacity than the alkaline under very light loads, but much more under heavy loads. NiMH is a direct, rechargeable replacement for the alkaline battery, providing similar capacity for low-powered devices and much better performance for high-powered devices. A flashlight that produces, say, 100 lumens from a single AAA battery should typically be run on NiMH batteries, as alkalines can't keep up. NiMH batteries are the power source most flashlight manufacturers use to test lights powered by AA and AAA batteries; advertised output and runtime usually won't be achieved with alkalines.
Early attempts to design a rechargeable version of the lithium battery were problematic. Lithium is a fairly reactive material and the batteries were prone to burning or exploding. A solution was found in the form of lithium salts with no metallic lithium present, and Sony introduced the first commercial lithium-ion battery in 1991 and it quickly became the technology of choice for battery packs powering laptop computers, power tools and much more. It would be some time before it found its way in to flashlights, however.
In the early 2000s, hobbyists began putting the most common Li-ion cell, known as the 18650, based on its 18x65mm dimensions in to flashlights designed for two CR123 lithium primaries in series. The 18650 is slightly wider, so this remained the realm of those willing to take machine tools to expensive lights for several years. Streamlight used a modified 18650 for its Strion series in the mid 2000s, but Chinese manufacturers looking for a way in to the premium market really popularized the standard, unmodified 18650 in flashlights. Now, the 18650 is the standard battery for high-powered flashlights and hundreds of models are available. A recent trend that lowers the barrier to entry is to build a charger right in to the light using a MicroUSB port that's compatible with most phone chargers.
Batteries in Flashlights
The flashlight you have determines what kind of batteries you can use. Some lights can run on several battery types. The Nitecore SRT3, for example can run on a lithium metal CR123, lithium ion 16340, alkaline, NiMH or lithium metal AA or lithium ion 14500. Most lights are more limited, accepting only one or two battery types. Casual buyers and newcomers to the world of higher-end flashlights often focus on retail battery availability when selecting lights, while those more experienced usually maintain a stock of quality rechargeable batteries. The initial investment is higher with rechargeable batteries, but it's almost always beneficial in the long run. Furthermore, not having to consider the cost or hassle of replacing batteries makes regular use of a light more attractive.
For most people, a combination of size requirements and performance characteristics should determine the type of rechargeable battery selected. Larger batteries can, of course provide better performance, but sometimes the light needs to fit in a specific pocket more than it needs to be bright or long-lasting. Li-ion and NiMH batteries in most shared sizes actually have similar capacity despite being sold by milliamp-hours, which makes NiMH appear to have more capacity. It's watt-hours that determine how long a battery can produce a given amount of light, which can be computed by multiplying the amp-hours by voltage: 1.2 for NiMh and 3.7 for Li-ion. In most cases, using Li-ion in a light that can take both will result in higher brightness for a shorter time due to its higher voltage.
The best performance per size, by a large margin is given by the 18650 Li-ion battery. There's no technical reason other Li-ion sizes couldn't have similar performance, but market demand has resulted in battery manufacturers pouring their best technology in to the 18650. It's used in power tools, laptops, USB powerbanks and numerous consumer devices, usually in a non-removable form. A Tesla Model S contains 7104 of them. If a light powered by one or more 18650 batteries fits a users's size requirements, the performance advantages should be given serious consideration. A single 18650 has double the volume of a AA battery, but has the performance of four or five NiMH AAs, and in higher-powered applications, matches 9-12 alkaline AAs.
NiMH batteries slowly self-discharge while not in use. Some do so quickly and will be significantly depleted after a month or two of non-use. Low-self-discharge models are available that retain 80% of their charge after a year. Flashlight hobbyists largely prefer the Sanyo/Panasonic Eneloop brand for shelf life and the Eneloop Pro for capacity, though many others are available. In retail packaging, low-self-discharge cells are sometimes advertised as pre-charged. There's some loss of capacity involved in reducing self-discharge, so a light that's frequently used is better off with the highest capacity cell available.
A quality charger is necessary to maintain NiMH batteries. Detecting that the battery is fully charged can be difficult for charger manufacturers using inexpensive components, and overcharge can damage the battery or even cause it to vent hydrogen gas. It's also possible for NiMH batteries to lose capacity after repeated partial discharges. A full discharge followed by a full charge can restore the lost capacity, and some chargers have a refresh mode to automate the process.
Li-ion batteries have a reputation for safety issues. Several models of laptop computer batteries have been recalled for a risk of fire, usually due to faulty charging circuits overcharging the batteries. In reality, lithium-ion batteries are more stable than lithium metal batteries, however, their rechargeable nature tends to subject them to more stressful conditions. Two additional conditions are hazardous with Li-ion batteries. One is overcharge, caused by a charger with an incorrect design or faulty manufacturing, or by user error when using chargers with multiple voltage settings, as a few Li-ion batteries are designed for higher voltage than most. Mild overcharge usually just wears the battery out faster, reducing performance. Severe overcharge can cause dangerous heating and gas production. As much as Li-ion batteries don't want to be overcharged, they also react poorly to being discharged too far.
Quality Li-ion cells contain two safety devices to prevent explosions. The first is a positive-thermal-coefficient resistor, often referred to as a PTC or thermistor. This increases the battery's resistance as temperature increases, reducing its ability to supply current. The PTC can regulate some unsafe conditions without damage to the battery. In more extreme conditions, the battery gets hot and produces gas. Upon exceeding a certain threshold, a pressure relief valve releases the gas and also acts as a current-interrupt-device or CID. This permanently disables the battery. Lower-quality lithium batteries may have unreliable safety devices or lack them entirely.
Many flashlight manufacturers as well as third-parties sell 18650 batteries with an added protection circuit to prevent over-charge, over-discharge and short circuits. Over-discharge protection also prevents reverse-charging when multiple batteries are used in series. Protection circuits cannot prevent harm under all conditions and are not a substitute for safely storing and transporting batteries such that they are physically isolated from conductive objects that can short them, using a charger that charges and terminates correctly and recharging batteries promptly when depleted. Unprotected batteries cost less, are slightly more efficient and work perfectly well in most lights.
The major Japanese and Korean electronics firms make the best Li-ion batteries in terms of both performance and safety. Any battery made by Sanyo, Panasonic, Sony, LG or Samsung is of good quality, should meet its advertised performance specs and incorporates a quality PTC and CID. Chinese Li-ions range from acceptable to fire-hazard, but are the only option in some less popular sizes.
Most lights meant for the 18650 can use any 18650, so it makes sense to get the largest one available: a 3500mAh cell such as the LG MJ1 or Sanyo NCR18650GA, or a rewrapped protected versions thereof. A few lights require a button top for correct fit, while others require a flat top. Some, mostly recently-introduced models from Zebralight striving for the smallest possible size require unprotected 18650s, which are slightly shorter.
Custom hot-rod lights may even more stringent battery requirements due to their unusually-high power consumption. These usually specify "IMR" cells or a specific current rating. "INR" is also usually suitable for this application. IMR and INR refer to inclusion of manganese and nickel, respectively in the chemistry and can deliver more current safely than most Li-ion batteries, which use cobalt ("ICR"). While the latest, highest capacity cells do use an INR chemistry, lights of this sort will be at their best with a higher current rating like the 20A LG HG2, as batteries with a higher current rating tend to have more stable voltage under load.
A recent trend is Li-ion chargers that also function as USB powerbanks when a battery is installed. The Nitecore F1 is a budget-friendly, compact and versatile example.
The following sizes of Li-ion batteries are commonly used in flashlights:
|Li-ion size||Equivalent size||Capacity||Notes|
|10440||AAA||300-350 mAh||works in many 1xAAA lights, but will damage some; will damage nearly all 2xAAA lights|
|14500||AA||600-840 mAh||works in many 1xAA lights, but will damage some; will damage nearly all 2xAA lights|
|16340||CR123||550-800 mAh||usually a direct replacement for CR123, but make sure 4.2 volts per battery is acceptable in your light|
|18350||700-900 mAh||some 1xCR123 lights can also use these|
|16650||2000-2500 mAh||one of these can usually replace two CR123s|
|17670||~1800 mAh||one of these can sometimes replace two CR123s|
|18500||1000-2000 mAh||two of these can usually replace three CR123s|
|18650||1500-3500 mAh||the most popular Li-ion size by far|
|26500||C||~3000 mAh||used in custom builds based on C-battery lights|
|26650||2500-5200 mAh||used in a few flashlights like the Fenix PD40 where one 18650 isn't enough but two doesn't fit the form-factor|
|32650||D||4000-6000 mAh||most often seen in high-powered Maglite conversions|
Advertised capacities significantly higher than those listed in the time of this writing have consistently been found to be false. There is simply no such thing as a 600mAh 10440 or a 5000mAh 18650.
Alkaline batteries are inexpensive, readily available and have a fairly long shelf life, but their advantages end there. Their capacity under light loads such as TV remotes and wall clocks compares favorably with NiMH, but the higher loads of a flashlight can reduce their capacity to a third or less. Used alkalines left inside a device often leak corrosive electrolyte, which damages the device, especially when the battery has previously been subjected to high loads, as in a flashlight.
Lithium metal batteries are the best option for long-term storage. They're available in AAA and AA sizes in 1.5 volt chemistries that work as direct replacements for alkaline and NiMH. These are safe in all lights meant for AAA or AA and deliver similar output, but even longer runtime than NiMH. Also popular for flashlights is the 3 volt CR123A. Many lights are designed specifically for the CR123A, and most (but not all; check your manual) 18650 lights can use two CR123As in place of one 18650 at a cost of about a third of the runtime. They will still deliver good performance after being stored for 10 years and offer the best performance of any common flashlight battery in extreme cold.
Lithium metal is flammable in air and attempting to extinguish burning lithium with water will cause it to explode. Abuse of a lithium metal battery causes it to get hot, produce gas and eventually rupture, which can produce a short circuit and ignite the lithium metal as well as flammable components in the electrolyte. Excessive load, such as a short circuit, attempting to charge the battery and combining a dead battery with a fresh battery, which charges the dead battery in reverse can all produce this kind of reaction. Quality lithium metal batteries use the same PTC and CID protections Li-ion batteries do.
You probably want an 18650-powered light, even if you previously thought you didn't. Other battery sizes are indicated primarily when size is a more important consideration than performance.
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