A few things you need to know when purchasing a battery for your flashlight.

[DuaneWessels]

Li-Ion or IMR
Li-ion is a general term referring to any battery that uses lithium ions to create an electrical current.
IMR batteries are a type of li-ion battery that uses manganese as the primary cathode material. Usually seen in flat-top cells, this chemistry is preferred and sometimes required for high drain devices.

In plain old English, most batteries on the market are Li-Ion cells but IMR batteries are also popping up these days with top end, high output flashlights. These batteries can handle a higher drain current than a Li-Ion battery.

18650, 14500, 21700, 16340 meaning.
Li-ion batteries are named with a specific convention that tells us the exact dimensions of the battery so you will know if it will fit into your flashlight or not.

Li-ion battery names break down into three parts.

The first two numbers represent the diameter of the battery in mm. So an 18650 battery is 18 mm in diameter and a 16340 battery is 16 mm in diameter etc.

The next two numbers represent the battery length in mm. Going back to the 18650 example, the middle two numbers tell us the battery is 65 mm long and a 16340 battery will be 34 mm long. – this part is important, so remember it we will come back to it.

You'll notice the last number on all cylindrical batteries is always a zero. This is because the final digit represents the shape of the battery...round like a zero.

CR123, RCR123 both refer to a 16340 size battery with a few key differences.

A CR123 is the non-rechargeable lithium-manganese version of a 16340 size battery while an RCR123 is the rechargeable lithium-ion counterpart.

You may have also noticed that two 16340 batteries is roughly the length of an 18650 battery. Many times you'll see a flashlight support either 1x 18650 battery or 2x CR123/RCR123 batteries. Even though the 16340 batteries are slightly narrower in diameter, the length allows for a proper contact and can be substituted.

What does mAh mean?
Li-ion/IMR cell available power is measured in milliamp hours (mAh). This is the battery capacity and tells you how much charge the battery can hold.

What this means for you is the larger the capacity of the battery, the longer runtime you'll get on a single charge.

Battery Voltage
Measured in (V) for Voltage .This is the electronic potential of the battery, or in simpler terms, how strong the electric current is when discharged from the battery. Most li-ion batteries have a low voltage between 3 and 3.7V.

When two batteries are placed in a series, the voltage of each battery is combined to calculate the new total voltage. This means that although many flashlights can swap between 1x 18650 or 2x 16340 batteries, some devices will not be able to handle the extra voltage. Always follow the manufacturer's recommendations to avoid damage to your flashlight.

Battery Output Current
Measured in Amps (A), the measurement of the electrical current and discharge rate. A standard, button-top 18650 battery will usually have a discharge rate around 5A. As advances in LED technology have demanded higher and higher outputs, nowadays you'll see some flashlights that require a high discharge rate to enable the brightest modes. If this is the case for your flashlight, you'll want to seek out batteries that meet the minimum discharge rate specified by the manufacturer.

Now the important bit...
Button-top, or protected cells vs Flat-top, or unprotected cells.

Protected or button top batteries have a small electronic circuit integrated into the cell packaging. This circuit protects the battery against common dangers, such as overcharge, over discharge, short circuit/over current, and temperature.

Most protected batteries have these components:

PTC: Protect against overheating and indirectly over current.  Once cooled down or the current has  reverted to normal will automatically reset.

CID or Pressure Valve: Will disable the cell permanently if the pressure is too high in the cell (Can be due to overcharge).

PCB: Will protect against over discharge, over charge, and over current, depending on design. The PCB will reset automatically or when placed in a charger.

Unprotected flat top batteries do not have this electronic circuit in the cell packaging.  Because of this, they can have more capacity and current capability than a protected cell. However, there is always a danger of overheating, short circuiting, or over discharging.  Unprotected batteries are also much cheaper.

Protected button top batteries are safer to use in your flashlight.  Protected batteries are less likely to ignite and cause personal or property damage.

Unprotected flat top batteries might be cheaper and offer you a bit more power. They however do not offer you any protection and can cause damage to you and your flashlight.

While some flashlights do use unprotected flat top batteries...most likely IMR, these flashlights have their own protection built in to prevent the above dangers unprotected batteries pose.
Protected batteries are safer to use in your flashlight.

Now let's look at the popular 18650 batteries. How do you know if it is protected or not?

Well, top manufacturers will state it on the battery and packaging either by saying protected or what type of protection is built in.

Next clue will be the button top...no button top, no protection.

Next will be the length of the battery...remember 18 is the diameter in mm, 65 is the length in mm. If the battery is shorter than 65mm long it most probably does not have protection.

Price...cheap batteries usually do not have protection.

Most top manufacturers design their flashlights to only work with the correct length of 65mm...shorter batteries will either not make contact or keep cutting out.

Some manufacturers even add to the length to force you to just use their propriety batteries...I am not a fan of this.

So in closing.

Make sure that the batteries you buy and use in your flashlight is the correct type, protected, size, voltage and amps.

And always follow the manufacturer's battery recommendations.

Using a cheap battery might lead to damage to yourself and your flashlight and most manufacturers will not cover you for damages.

And again, there are only a handful of battery cell manufactures out there. Most flashlight companies use the same cells, the difference is in the protection they add to these cells to make their batteries safe for use and extend the life cycle.

Flashlights are not cheap. Do not go ruin your flashlight with a cheap battery.

In Afrikaans there is a saying...
Goedkoop koop is duur koop.