Choosing an 18650 Battery
Brand – The first thing to consider when choosing an 18650 battery for your device or project is the battery brand. Not all brands are created equal and often times you will find are not very truthful in their battery ratings. The goal of any brand is to sell as many batteries as possible but this is especially true for new Chinese battery manufacturers.
There are four major brands that manufacturer batteries who you can rely on to provide quality, consistent and truthfully rated batteries and those brands are; LG, Samsung, Sony and Panasonic/Sanyo. These brands are well established and have long standing tradition, values and standard to uphold. Any manufacturer specs put forward from these brands are likely exact if not underestimated and you will often see the battery outperform the manufacturer ratings.
Other brands such as Efest, Vapcell, Imren and MXJO have been established for a few years and their quality has certainly improved. These brands can tend to be slightly favorable in the battery ratings in order to sell more. They do often provide value with a thicker wrapped battery which provides an extra layer of protection and a free battery case.
There are various other unknown brands with all sorts of unrealistic ratings. As of 2018, there is not a single 18650 battery rated above 3500mAh capacity or a 30A CDR. If you see any 18650 battery marketed above these specs, you should be very weary of what you are actually getting as it is known to be false and exaggerated.
Capacity – Given the choice, everyone would choose a battery with the highest capacity. Capacity is measured in milliampere-hours (mAh) and this tells us how many milliamps we can draw from the battery for how many hours. However, capacity comes at the cost of current rating (CDR). Inside of each cell, they can only fit so much material so you often have to choose between a high capacity battery OR a high current battery. Take for example the LG HB6 which has a CDR of 30A but only a capacity of 1500mAh. On the other end of the spectrum is the Panasonic NCR18650B which has a CDR of only 4.9A and capacity of 3400mAh.
There are some batteries that manage to balance capacity and current rating such as the Samsung 25R, LG HG2, and Samsung 30Q. All three of these batteries provide a great balance of capacity and current.
Continuous Discharge Rating (CDR) – This is probably the most important thing to consider when selecting a battery. You need to know how much current the device you are trying to power requires before choosing your battery. If you choose a battery with a current rating less than what you need, you will notice the battery overheating as it is working beyond its ability to keep up. There are also two terms you should know that are discussed in battery current ratings. Those terms are continuous discharge rating (CDR) and the pulse discharge rating (Pulse).
CDR – The maximum current at which the battery can be discharged continuously without damaging the battery or reducing its capacity.
Pulse Rating - The maximum current at which the battery can be discharged for a short period of time without damaging the battery or reducing its capacity.
We avoid using any pulse ratings as there are far too many factors to consider when pulsing a battery such as pulse length, time at rest between pulses and battery temperature to accurately compare two batteries. For these reasons, we use the CDR rating which is the current rating at which the battery can be continuously discharged at safely without overheating or damaging the cell.
Voltage – You will notice that all 18650 batteries have a nominal voltage between 3.6V – 3.7V. 18650 batteries actually operate within a range from ~4.2V when fully charged down as low as ~2.5V when completely discharged. The nominal voltage is simply a midpoint in this range for classification purposes.
When someone refers to a battery “hitting harder”, what they are referring to is the voltage at which a battery can sustain midway through its cycle or even longer, commonly referred to as voltage sag. Some batteries can remain around 3.7V midway through the cycle while other batteries are operating at 3.2V.
Operating Temperature – If your battery is consistently reaching a high temperature and getting hot, this is a warning sign that the battery is being pushed too hard. A battery that is consistently rising above 45C will certainly age faster than a cool running battery. They can also be dangerous as the potential for venting and/or bursting is increased greatly with a high temperature lithium ion battery. You should consider choosing a battery with a higher CDR rating.
Flat top and Button Top – One final thing you want to consider is whether you need a flat top battery or a button top battery. This is referring to the positive end of the battery. A button top battery has a protruding surface which increases the battery’s length and may not fit in a device which requires a flat top battery. A flat top is as the name suggests a flat surface and may appear to be too short if your device is requiring a button top battery.
18650 Battery Chemistry
Battery chemistry is not an attribute most consumers need to be aware of when choosing an 18650 battery. A battery should be chosen based upon the factors listed above; Brand, Capacity, Discharge Rating, Voltage and Temperature. With that being said, some chemistry mixtures are more volatile than others and we have listed these in terms of least volatile to most volatile.
LFP (lithium-iron-phosphate) – Least Volatile
LCO (lithium-cobalt-oxide) LiPo – Most Volatile
Common prefixes that are used by manufacturers