There isn't really a straight answer; its very subjective and will depend on what you want it to do...

However, let's start with the basics.

Why do we use 18650 and close relatives? 

The answer is quite simple. Energy density. Gram for gram, lithium based cells are far more powerful that anything else readily available that can be easily put in an out of a device by anyone familiar with nothing more complicated than changing the batteries in a TV remote. The other option is an integral battery but often when they fail or reach their "end of life" (normally several hundred charge cycles) the device is redundant with replacements either not available or not economically viable.

What should you look for in choosing a battery for vaping?

This is where things get a little hazy, because contrary to popular belief or what people on Facebook might tell you, there are many right answers, not just one. There are also many wrong answers.

First, put price at the bottom of your priorities and consider that you're going to put this in front of your face/in your mouth. Is not paying an extra £1 to a reputable vendor rather than a hooky eBay shop really worth the risk?

Next, we need to look at what your needs are. You can have high discharge, or high capacity. You can't really have both. You can have a reasonable mix of the two, but any increase in discharge output (current, measured in Amps) will incur a capacity output penalty (measured in mAh). Do you want something that will hit hard all the way to empty (technically known as "low sag") or do you need something that will get you through the day?

We'll look at these separately below;

Discharge rate (constant current or "CDR" - Constant Discharge Rating, measured in Amps)

We stock cells that range between 1.5A and 40A output. The KeepPower 8A 18650 is a fantastic cell in its field, arguably one of the best and even though it will fit in some vape devices, it wont last more than a few seconds before something terrible happens. This is because the discharge rating is far too low to power a vape device. It's a torch/laser cell. This is where subjectivity comes in.

For regulated or mechanical devices, you need to select the right cell for the job and again, this is where it's subjective and a little bit of Ohm's Law knowledge is required (or you can cheat, and Google an online calculator). For the sake of simplicity, we're not going to use pulse discharge ratings here. Just the CDR.

For example - you're running a single 18650 mechanical device and you place a RDA with a 0.20Ohm coil(s) on top. With this "load" and a fully charged 18650 (4.2V), it will be drawing 21A from the cell. Consider a Samsung 30Q for instance; one of the top selling batteries in the industry. It's Constant Discharge Rating is 15A. This puts you at almost double the rated discharge. It doesn't mean its not a top performing battery, its just not right for this application. The Sony VTC5A and Vapcell VTC5D and VTC6A re-wrap however would be perfect, with ratings of 25A+

Capacity (how long the cell will last between charges, measured in mAh - Milliamp Hours)

Again, we have cells ranging from 1500mAh, to over 4000mAh. Everyone wants the highest mAh possible. Less time charging and the ability to carry less batteries is convenient, but it's a trade off. You cant have a 4000mAh 30A 18650 cell (no matter how many times China put figures like that on a wrap!)

Broken down in the simplest possible way, an 8A 4000mAh cell will provide 1A of power for 4 hours. 2A for 2 hours. 4A for 1 hour. 8A for half an hour, and so on (so long as the current doesn't exceed the maximum constant discharge rating)

The above has very little relevance in vaping, because it's used in very short bursts, but you get the idea. More mAh, longer run time.

Internal Resistance

This is a mostly overlooked but actually fairly important factor. The lower the internal impedance or "resistance" of a cell, the easier it is to deliver its energy. High resistance cells will struggle to expend their energy efficiently, instead turning the pressure of not being able to discharge at the rate being requested by the device into heat. Low resistance cells are much more efficient, offer less voltage sag and this is where the "hard hit" bit comes from. Lower resistance = More instantaneous power.

Understanding Battery Codes

Our compliance markings aside, there are lots of markings on batteries. We'll break them down to make them a little easier to understand;

• IMR - Lithium Manganese - IMR is one of the most stable and one of the highest current producing chemistries. It has the lowest running temperature in comparable tests making it far safer than older ICR technology. Interestingly, a lot of re-wrap companies mark their batteries IMR, when they're actually INR.

• INR - Lithium Manganese Nickel - INR is probably the most common in vaping. It blends nickel and manganese to form the positive cathode, providing low resistance and the ability for high current output. A lot of effort is put into this chemistry by manufacturers, shown in the Samsung 25R and the LG HE2.

• NCA - Lithium Aluminium - NCA is a much lesser used but still comparable chemistry to INR. It does away with the manganese element of the cathode in favour of aluminium. You wont get the high level discharge ability of an INR cell, but you do get a much longer run time and increase shock resistance. They're currently being used by lots of e-bike manufacturers, and Tesla use them in their vehicles!

• ICR - Lithium Cobalt - ICR chemistry is used for one purpose. Energy density. Unfortunately this comes at a cost and that cost is stability. It bugs me that cells with this internal chemistry are available individually to end users on eBay and Amazon because they're of almost no use to the general public bar DIY pack repairers. The Samsung 26F for example (most often found in laptop batteries) has a wrap the same colour as the Samsung 30Q; they can be very easy to confuse. The 26F is a 5.2A cell (factory data sheet rating) which is of almost no use in vaping. Put one of these in a vape device and run it at above 20w and you're almost guaranteed to have a bad time.

• IFR - Lithium Phosphate - IFR (more commonly known as LiFePo4) has very specific uses and is rarely seen in vaping because of its super low energy density. There are some that can be discharged at very high rates however, often upto 30C (30x its capacity) The average capacity is about 1200 mAh, some are much lower. So for example a 30C rated IFR cell could be discharged at 30 x 1100mAh, so 33Amps! But, it won't do it for very long at all. They also have a voltage cut off of only 3.2v. Much higher than the normal 2.8v or even 2.5v of other lithium based cells.

So. Which is the best 18650 battery? The answer is easy. All of them, in their own little way. Take time to consider what you want from the cell and pick the cell most appropriate to your needs, not just what everyone on social media is shouting about!

I hope this serves to answer the question but if its left you with more than you started with, feel free to drop us a message.