Questions about buying batteries come up periodically on the /r/flashlight subreddit.  This is the guide I wish had existed when I had those questions.  The primary focus of this guide is on batteries that go into flashlights, though some of what’s here can certainly be applied to other battery-powered devices.

If you just want to know how to get 18650 batteries, skip down to the Lithium-Ion section.  Be careful when buying lithium-ion batteries from marketplaces like Amazon; unsafe batteries abound.  See the section for advice on making safe purchases.

§ Types of Batteries

Batteries can be separated into different types that largely have to do with their voltage.  A battery’s voltage is determined by the chemical reactions it uses to generate electricity (and occasionally with additional circuitry added to the battery).  The usual way we refer to batteries (AA, AAA, C, etc.) specifically references their size, not voltage.  Fortunately, for the most part, particular sizes only come in particular voltages.  I’ll note a few places you might have to take care.

Flashlight batteries generally fall into one of three categories (links go to the sections on each type of battery):

  • 1.5V - These include the most common battery types in use, including AAA, AA, C, and D.
  • 3V - The most common 3V flashlight battery is the CR123A.  Many button cells (watch batteries) are also 3V, like the common CR2032.
  • Lithium-Ion - This is a whole class of batteries that have higher outputs and last longer than many other flashlight batteries, but they require more care in handling.  Lithium-ion flashlight batteries usually have five-digit designations, like 18650 and 10440.

I’m omitting stuff like 9V batteries and 6V “lantern batteries”, since they’re not used in flashlights to the same degree that the above categories are.

§ 1.5V Batteries (AA, C, etc.)

Flashlights that use AAA, AA, C, and D cells are very common.  They’re useful because those cells are also very common.

People sometimes refer to these batteries as either “primaries” or, less often, “secondaries”.  “Primaries” are synonymous with non-rechargeable; you use them and then throw them away.  “Secondaries” are synonymous with rechargeable, though people will more often just call them “rechargeable”.

The main consideration when choosing 1.5V batteries is the chemistry used inside.  There are three common chemistries:

  • Alkaline - The cheapest and most common.  Not recommended unless they’re your only option.  They’re not rechargeable, so you have to replace them every time you use them up.  They lose their charge over time, so if you leave them alone for a while, they might not even be useful when you do pick them up.  They tend to leak, which becomes more likely the more they discharge (and remember, they lose charge even if you’re not using them).  When they leak, they can destroy whatever device they’re in.

  • Nickel-metal Hydride (NiMH) - Rechargeable.  People will often refer to “Eneloops”, a specific, well-regarded brand of NiMH batteries.  Good for frequently-used flashlights because you can reuse them rather than buying new ones all the time.  They also don’t leak, so you don’t run the risk of damaging your devices.  Standard NiMH batteries lose charge much faster than alkaline batteries, but you can get “low self discharge” NiMH batteries that only lose their charge slightly faster than alkalines do.  (Rough comparison: after a year without use or charging, an alkaline battery will have 80-90% of its original charge, an LSD NiMH will have 70-80%, and a regular NiMH will have 15-20%.)  Although alkalines usually claim more energy storage than NiMH on paper, NiMH batteries tend to give longer runtimes in flashlights in practice because of the way modern flashlights use electricity.

  • Lithium - Expensive, but long-lasting.  Not rechargeable.  These typically cost three times or more what alkalines do.  (So do NiMH batteries, but those are rechargeable, so the cost is amortized over many reuses.)  They lose their charge more slowly than alkalines, they can store more energy than alkalines or NiMH, and they don’t leak.  Good for devices you want to leave alone for months or years at a time and still work as soon as you pick them up again.

There are rechargeable alkaline and rechargeable lithium batteries, but rechargeable NiMH are the most common at the moment.  Nickel-cadmium (NiCd) used to be the most common rechargeable chemistry, but it’s been replaced by the NiMH, which is better than NiCd in practically every way.

In most cases, you should get NiMH rechargeable batteries for flashlights that get used frequently.  For flashlights that sit and wait to be used (emergency flashlights, bug out bags, etc.), use lithium primaries.

Lithium batteries handle temperature extremes better than NiMH and alkaline batteries, so lithium is also the best choice for things like flashlights that live in cars.

The Wirecutter has recommendations for NiMH AA and AAA batteries and NiMH AA and AAA chargers.

§ 3V Batteries (CR123A, etc.)

3V batteries are common in a number of more niche devices, like cameras.  There are a lot of flashlights that use 3V CR123A batteries.  Pretty much every 3V battery uses lithium, so everything about lithium in the 1.5V section applies to 3V batteries, too.

The higher voltage lets some CR123A flashlights put out more light than similarly-sized AA flashlights.  Aside from that, there’s not much to consider about buying CR123A batteries.

The Parametrek battery database lists several CR123A batteries ranging from $1.50 to $5 apiece.  On Amazon, Amazon Basics, Streamlight, and Energizer CR123A batteries range from $1.50 to $2 apiece.

Some places sell “RCR123A” batteries, which are basically CR123A-sized lithium-ion batteries.  (Specifically, they’re 16340 cells; lithium-ion naming conventions are covered below.)  Some RCR123A batteries have integrated voltage-regulating circuitry to deliver a constant 3V so they behave just like a regular CR123A.  Others do not; like other lithium-ion batteries, they’ll be 4.2V when fully charged.  If you’re going to buy RCR123A batteries, either make sure your device can handle voltage up to 4.2V or check the specs on the RCR123A to see whether it has a 3V output.  (Lithium-ion batteries will often be listed as having a 3.6V output or so.)

All of the usage considerations in the lithium-ion section apply to RCR123A batteries, too.

§ Lithium-Ion Batteries

Lithium-ion batteries brought a revolution in compact energy storage.  They can hold more energy and discharge it faster than any of the common handheld battery technologies that came before them.  Lithium-ion batteries are used, in some form, in devices ranging from smartphones to laptops to electric cars.

Lithium-ion batteries supply 4.2V when fully charged.  As their energy is drained, their voltage drops.  When they reach 2.5V or so, they’re considered empty.  Lithium-ion batteries are usually referenced by the average voltage across their entire discharge range, which is usually 3.6V or 3.7V.

Although a lithium-ion battery can continue to supply power below 2.5V, doing so runs ths risk of permanently damaging the battery’s chemistry.  That might reduce the energy the battery can hold when full, render the battery useless, or cause an internal short circuit that could lead to a fire.

Lithium-ion batteries are also potentially more dangerous than the other batteries described above.  If they get too hot, they can catch fire or explode.  Charging and discharging lithium-ion batteries both generate heat, so doing either one too fast can cause a fire or explosion.  A short circuit—connecting the positive and negative ends without enough resistance in between—will almost certainly discharge the battery too rapidly.  (For people who remember the Samsung Galaxy Note 7 fires, those were caused by unsafe lithium-ion batteries.)

The above doesn’t need to put you completely off lithium-ion batteries.  They’re incredibly useful; you just need to take a little more care with them than other common batteries.  Some lithium-ion batteries are more safe than others; that’ll be covered below.

You do need to be careful about where you buy your lithium-ion batteries.  Many large marketplaces, like Amazon and AliExpress, have unsafe or mislabeled lithium-ion batteries for sale.  Because of the dangers of unsafe usage of such batteries, you need to make sure you’re getting them from a reputable seller.  That will be covered in the buying lithium-ion batteries section.

Some flashlights have built-in charging circuits.  If yours doesn’t, you’ll also need a charger, covered in the chargers section.

# Lithium-Ion Names and Shapes

The Lithium-ion batteries that flashlights use—at least, flashlights with removable batteries—are generally cylindrical and are described by a five-digit identifier, like “18650”.  The first two digits give the diameter of the cylinder in millimeters (mm).  The last three digits give the length of the cylinder in tenths of a millimeter.  Thus, an 18650 cell is nominally 18mm by 65mm.  There’s some variation in those values, particularly in the length, but they give a rough approximation.

Some common sizes are:

  • 18650 - The most ubiquitous size for lithium-ion flashlights, as well as for a lot of other things (laptop batteries, smartphone power banks, and so on).  Because this is currently one of the most popular sizes in industrial use, it’s gotten the most research into making it efficient.  As of January 2018, no other shape matches the energy density of the 18650.  (e.g. a 26650 has twice the volume of a 18650, but the best 26650 only has 1.5 times the energy of the best 18650.)
  • 26650 - The 18650’s larger sibling.  Used by some flashlights to give more runtime per battery.
  • 21700 - A relatively newer size that some companies are starting to use.  It seems possible that 21700 might someday replace 18650 as the most popular (and, thus, best-engineered) battery size.  For now, there are a few flashlights that make use of the larger capacities and discharge currents that 21700 cells have in comparison to 18650 cells.
  • 18350 - Almost half the size of an 18650.  A number of flashlights have options for swappable longer and shorter battery compartments, so you can decide on a daily basis whether to have a shorter light that uses 18350s or a longer light (with longer runtimes) that uses 18650s.
  • 16340 - More or less the same size as a CR123A.  There are used in “RCR123A” batteries as described in the 3V section above.
  • 14500 - More or less the same size as a AA battery.  Some flashlights can use either AA or 14500 cells.  Don’t use a 14500 battery in a AA light unless the flashlight manual says you can. If the flashlight only expects 1.5V batteries, using a 4.2V 14500 can destroy the light and possibly start a fire.
  • 10440 - More or less the same size as a AAA battery.  Some flashlights can use either AAA or 10440 cells.  Don’t use a 10440 battery in a AAA light unless the flashlight manual says you can. If the flashlight only expects 1.5V batteries, using a 4.2V 10440 can destroy the light and possibly start a fire.

A number of flashlights allow you to use either an 18650 battery or two CR123A batteries.  As with 14500/AA and 10440/AAA, don’t do this unless the flashlight manual says you can, since two CR123A batteries in series will give the flashlight 6V.

When speaking, most people break up the five digits of a lithium-ion battery into three groups: xx-y-zz.  Thus, “18650” is pronounced “eighteen-six-fifty”.  (“14500” is usually pronounced “fourteen-five-hundred”.)

# What You Need to Know About Lithium-Ion Options

With 1.5V batteries, you have just one thing to decide about: the battery chemistry.  With lithium-ion batteries, there are four options you need to consider: protection, top shape, capacity, and discharge rate.

If in doubt, you’ll probably be okay with protected, button-top batteries of the highest capacity you can afford (ignoring discharge rate).

– Protection

Dimensions of plain, button-top, and protected 18650s.

Dimensions of plain, button-top, and protected 18650s.

As noted above, lithium-ion batteries should not be discharged below 2.5V or so and should not be discharged too quickly.  Many manufacturers take plain lithium-ion cells and add small protection circuits to them.  These circuits stop providing power if the battery voltage drops too low or if the current draw gets too high, protecting the cell from things that could damage it.  This makes the protected batteries a bit safer, since it’s more difficult to accidentally push them too hard.

A protection circuit makes the battery a little longer, and sometimes a little wider.  There are flashlights that have so little extra space inside that they must be used with unprotected batteries.  Usually such flashlights will have their own low-voltage protection (LVP) and will stop trying to use the battery if the voltage gets too low.  If you use an unprotected battery in a flashlight without LVP, you’ll have to be careful not to drain the battery too far or you risk permanently damaging the battery.

Protected batteries usually cost a little bit more than their unprotected counterparts, typically in the realm of an extra $1.50 or so.

Some high-powered flashlights need to draw so much current that they can’t use protected batteries because they’d trip the protection with their power usage.  For those flashlights, make sure you get unprotected batteries with a high enough discharge rate (covered later).

Flashlights that need unprotected batteries should say so on their website and in their manual.  If there’s nothing about protection, you should be able to use protected batteries (and you ought to do so).

– Top Shape

Tops and bottoms of flat top, button top, and protected 18650s.

Tops and bottoms of flat top, button top, and protected 18650s.

Lithium-ion batteries, like all other batteries, have a positive end and a negative end.  Putting a lithium-ion battery in backwards can damage the flashlight, the battery, or both.  In some cases, it can start a fire.

On a plain cylindrical lithium-ion cell, the disk on the positive end is a little smaller than the disk on the negative end.  Some manufacturers take bare cells and put buttons on top of them, like the buttons on top of 1.5V batteries.  This makes the battery a little longer, but not as much as a protection circuit does.  Most unprotected-batteries-only flashlights will still work with button top batteries.

Button top batteries usually cost slightly more than flat top batteries.  The extra cost is generally somewhere around ten to twenty cents.

Many flashlights will work with either button top or flat top batteries.  Some are shaped so that only a correctly-inserted button top battery will work.  This serves as mechanical enforcement of correct battery polarity.  If your flashlight takes more than one battery in series, you’ll need to use button-top batteries.

Protected batteries pretty much always come with button tops.

In general, any flashlight that works with flat tops will also work with button tops, except for rare cases where the battery compartment spacing is incredibly tight.  Consequently, I’d recommend getting button top batteries unless you specifically know you need flat tops.

– Capacity

A battery’s capacity, most commonly measured in milliamp-hours (mAh), governs how long it can continue providing power.  More mAh generally equals more flashlight runtime.  Even if you don’t expect to run a battery all the way down, keep in mind that as a lithium-ion battery discharges its voltage drops.  In many flashlights, that means that a partially-discharged battery can’t support the brightest modes on the light.  A higher-capacity battery will continue to provide higher voltages for longer periods of time.

If all else is equal, you should get the highest-capacity battery you want to spend money on.

Many disreputable battery vendors claim impossibly high capacities for their batteries.  As of January 2018, here are the highest manufacturered capacities for some common lithium-ion sizes; if a battery claims significantly higher numbers, it’s probably lying (and if it’s lying about capacity, it’s a lot more likely to be lying about other things, like safety):

  • 16340 - 700mAh (see the note below about Efest)
  • 18350 - 1200mAh
  • 18650 - 3600mAh (but see the note below)
  • 26650 - 5500mAh

(Note: Efest, a reasonably reputable brand, sells “850mAh” 16340s, but testing indicates that they’re more than a little optimistic about that claimed capacity.  In practice, 700mAh is the most you’ll get out of a 16340.)

(Note: Only one 18650 cell claims a 3600mAh capacity, and it’s arguably cheating a little to get that number.  For most practical purposes, you can regard 3500mAh as the highest available 18650 capacity, and consider any “3600mAh” battery to really be 3500mAh.)

– Discharge Rate

Depending on their particular chemistry, lithium-ion batteries can have a maximum discharge rate anywhere from 3 amps (A) to 40A.  Most flashlights stay under 3A-4A, so pretty much any battery will be fine for them.  Some of the higher-output flashlights need or can benefit from 10A, 15A, or even 20A batteries.

There’s a tradeoff between battery capacity and discharge.  The chemistries that do very well on one metric are not as good on the other.  As of January 2018, the best high-capacity batteries store 3500mAh with a maximum discharge of 10A, while the highest-discharge batteries can sustain 40A but only store 2000mAh.

The most-demanding flashlights I’ve seen top out at about 20A, so you probably don’t need to go out looking for batteries with higher discharge rates than that.  (Unless you’re also using the batteries in your vape or something.)  Many people with high-drain flashlights like to use Sony VTC6 or Samsung 30Q batteries; both are 3000mAh/15A.

Some people refer to high-discharge batteries as “IMR” batteries, after a commonly-used chemistry for such batteries.

In general, you should see if your flashlight has a maximum current drain listed.  If it doesn’t, ignore discharge rate and get the highest capacity batteries you want.  Otherwise, get the highest-capacity batteries with a high enough maximum discharge rate.

– Other Considerations

There are all sorts of other characteristics that people care about with their batteries, but those are less relevant than the above four things, especially if all you care about is getting your flashlight to work.

There’s actually a really complex relationship between batteries' capacity, voltage, and current.  Batteries are a little less efficient at higher amperages, so a flashlight that’s constantly used on its turbo setting will generally drain its battery even faster than the numerical difference between the light’s brightness levels would indicate.  Similarly, batteries providing higher amperages will have their voltage drop a bit relative to the same battery with the same charge but at a lower current draw.  Different batteries will have different balances among those relationships (e.g. Samsung 30Qs exhibit slightly more voltage sag than Sony VTC6s, even though their top-line ratings are the same).

These sorts of things only tend to matter to people who want to squeeze every last lumen out of their lights, and those are just a small subset of the people who use lithium-ion flashlights on a regular basis.  If you’re interested in this level of detail, though, you will want to look at HKJ’s battery and charger reviews.  The website is a little confusing in its layout, but there’s a wealth of information about all of the batteries HKJ has tested, and HKJ has tested a lot of batteries.

# Where to Buy Lithium-Ion Flashlight Batteries

Don’t just go to Amazon, search for “18650”, and buy the first search result.  There are a lot of cheaply-made and more-unsafe-than-necessary batteries in large marketplaces like Amazon.  You should buy from a vendor who will only sell properly-labeled stock from trusted manufacturers.

One of the easiest ways to do that, as well as to search for batteries that match all of the options you need, is to use the Parametrek Battery Database.  The person who maintains the database has links to purchase batteries from reputable sellers.  For a search example, here’s all of the protected 18650 batteries, with the highest-capacity ones first:

Note that to search for capacity, the mAh numbers I’ve talked about are on the “mAh” category.  The “capacity” section sorts by watt-hours (Wh) instead.  (The basic difference is that milliamp-hours are only directly comparable for batteries at the same voltage, while watt-hours give meaningful comparisons even between batteries with differing voltages.  Lithium-ion batteries are generally marketed with their mAh rating—since the voltage is known—so that’s what this guide uses, too.)

If you have questions about a particular battery seller, you can always come ask about it on the /r/flashlight subreddit.

– Notes on Particular Lithium-Ion Battery Brands

Unprotected batteries are pretty much all made by LG, Panasonic, Samsung, Sanyo, or Sony.

Some of the more popular brands for protected batteries include AW, EVVA, and Keeppower.  (As mentioned previously, these companies buy unprotected batteries from the above vendors, add their own protection circuits, and sell the resulting batteries.)

Many flashlight manufacturers have their own branded batteries.  Those are generally of good quality, but they’re often more expensive than equally-good batteries from other reputable sellers.  Some people prefer to pay the extra amount just to avoid trying to figure out whether a particular other seller is reputable or not.

Batteries from Olight are a little unusual.  They’re a reputable manufacturer (and seller, if you buy directly from them), but they do some extra things to their batteries.  The tops of their batteries have a positive button, like any button top battery, but also a negative ring around the button.  This is required for the batteries to work in their proprietary flashlight charging cradles, but it increases the chances of short-circuiting the batteries.  (The protection circuit should prevent a short-circuit from starting a fire, but it’s still not something you want to do to a battery.)  Unless you’re using an Olight flashlight with an Olight charger, you probably don’t want an Olight battery.

Ultrafire batteries should be avoided.  They’re known to cut corners on their batteries in order to make them cheaper.  If you buy one of their batteries, you might get something that works, but you also might get a battery with a defective protection circuit, or a battery that contains a smaller, cheaper battery, and a lot of sand to fill the extra space.  Given the care that needs to be taken with lithium-ion batteries, the risk isn’t worth the lower prices.

§ Chargers

If you go with rechargable batteries, you’ll need a charger.  (Some lithium-ion flashlights have built-in charging, but even with those an external charger can be useful sometimes.)

The best option is to look at the list of chargers reviewed by HJK, pick one with the features you need (number of bays, NiMH, lithium-ion, etc.) and a good rating (two or more smiling faces), and buy it from one of the reputable battery vendors discussed above.