How long does an 18650 battery last?

How long does an 18650 battery last?

There are several aspects to look at when asking how long an 18650 battery will last both from the perspective of charge cycles, time in use (capacity), and age.

Let’s break them down;

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.

Capacity

Capacity, measured in Ah (Amp hours), mAh (milliamp hours) or Wh (Watt hours) is the measure of energy contained within a cell, battery or battery pack. This is a quick way of identifying how long a battery will last when comparing to others. You could argue that on face value, a Samsung 30Q with 3000mAh would outlast a Sony VTC5A with 2600mAh for example. This only applies however, if comparing the same “load”. The Sony VTC5A has a factory rating of 25A constant discharge. The Samsung 30Q has a rating of 15A. If you were to run both of them at 25A, its highly likely that the VTC5A would actually last longer, because the 30Q would be struggling over its limit, wasting energy in converting it to heat instead. Selecting the right cell for the job is imperative if you want to get the most from them. As mentioned earlier, running cells at or beyond their limit significantly reduces capacity by harming internal chemistry.

Cycle life mentioned previously is determined by retained capacity and you can extend/hold on to good figures by not abusing them but if you must, ensure you are aware of the rate at which its happening. Capacity will naturally decrease over time and in use and its worth keeping an eye on or record of the capacity delivered (if your device has this function) or capacity put back in during charging (many of our XTAR chargers have this feature). It’s a good way to keep an eye on cell health and determine if they’re still going strong, or its time to replace them. More so if you are using them outside of the specified use/limits such as vaping

Age

It’s a known fact that old batteries are not as good as new ones. Months and years even just in storage take their toll on cells, especially if not stored correctly. The international/industry wide standard for shipping cells is with a 30-50% SOC (State of Charge). While 18650 Batteries have a relatively low rate of self-discharge, over time this can worsen, especially if abused with large load and then stored, even if correctly (the right humidity, temperature, etc). Samsung (for new cells) recommend storage at 50% of charge (circa 3.6V) and expect that they’ll retain 90% or more of their nominal capacity when recharged and used up to 1.5 years later. After this point, retained/useable capacity starts to tail off.

As you can see, there are a lot of factors which contribute to how long an 18650 battery lasts and the vast majority of them are not so much down to the battery/cell itself, but how they’re used. Treat them nicely and you’ll get close or sometimes even exceed the manufacturers rated life span. Abuse them, and you’ll likely get far less, at best.

I hope this has helped to clear up any questions about 18650 battery lifespan but as always, if you have any questions, we’re always happy to answer them!

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