Charge Cycles or “Cycle Life”
Most 18650 batteries are rated for between 200 and 500 cycles but what does this mean? A “cycle”, is defined as a full charge to 4.2V, and a full discharge to the lower voltage limit (normally between 2.5V and 2.8v) at the maximum discharge rating. The Cycle life of a battery is considered to be reached when it will only deliver 80% or less of its rated/nominal capacity. For example, the Samsung 25R has a standard charging current rating of 1.25A and a maximum discharge rating of 20A down to 2.5V. Samsung (in their factory datasheet) specify that the 25R will retain a minimum of 60% (1500mAh+) of its rated capacity of 2500mAh after 250 cycles. This is the lower end of the scale in the world of batteries but the Samsung 25R was designed with the ability to be run quite hard.
Can you extend cycle life? Yes, you can. While the manufacturers provide maximum ratings for their cells, this isn’t a “target” and you don’t have to run them that hard. In fact, over specifying the rating for a battery or pack will help it run/live for longer. Nothing likes to be run on its ragged edge for too long and having some head room in use will help to prolong the aging process in Lithium-ion cells. Only ever running them at 80% of their maximum discharge capacity and limiting the depth of discharge (DoD) to 3.0V for example can help to extend the number of cycles you can get from them and increase the retained capacity for a longer time. You can also be kinder in charging. The Samsung 25R actually has a factory fast charge rating of 4A. Charging at 1A is much kinder on the cell and will help to improve or retain cycle life.
Can you worsen the cycle life? Absolutely. For those using the 18650 batteries for vaping (or any application where you place over-specified load on them) – It’s unlikely you will ever reach the factory stated minimum cycle life and you’ll notice a reduction in capacity far sooner than if used in a battery pack with sufficient operating headroom. This is because of the way the batteries are used. I don’t like to use the term “pulse discharge” in the vaping community because there is currently no standard/internationally recognised method of testing however this does refer well to the way in which they are used. Often vapers are pulling far more current than the cell was ever intended for because its only for a few seconds at a time and this has become widely recognised as “generally safe”. For example “mechanical device” users who favour the Sony VTC5A (rated at 2600mAh and 25A max constant discharge) for its low internal resistance and voltage drop (sag) often place resistances of around 0.10ohms (or less) on them. From a fully charge (4.2V) cell, this will be drawing 42amps/176w. Drop the resistance just 0.03 to 0.07ohms and you’re now at 60amps! This is a huge strain on a single battery and while a good/new battery will take this kind of abuse (to a point), it does harm the internal chemistry over time and eventually you’ll start to notice large drops in capacity/more time on charge and they’ll start to lose their ability to hold a charge too. This is accelerated with age as the cell starts to struggle to expend this amount of energy as fast s the device is trying to draw it, leading to sudden rise in cell temperature which then further damages its capacity.