What Do We Actually Mean by Battery Longevity?


Right now, battery power is at something of an apotheosis, but it is only in recent years that battery development has been truly kicked up a gear. And this phenomenon has many manifestations, from the massive multi-celled li-ion batteries which are powering the electric vehicle revolution to the small USB-C rechargeable AA batteries that are bringing li-ion technology and massively expanded battery longevity into the realm of the common household batteries.

There are of course challenges. One of the major ones is producing enough batteries to power the increased number of EVs that are expected to take over our roads completely by some time in the 2030s. Another is answering the myriad ecological issues which surround the manufacture of said batteries. Perhaps less urgent (although no less vigorous) has been making the new household batteries that are constantly being developed last longer.

What is Battery Longevity?

All this new research though has led to an important question being raised. What actually do we mean by battery longevity? Well, for those actually involved in the production of newer more long-lasting batteries, “longevity” is not a term which is used. Batteries in fact have three distinct measures of longevity – run-time, shelf-life, and cycle life.

What is Run Time?

A battery’s run time is commonly confused with its shelf-life and cycle-life, but these are all in fact distinct things. Run time refers specifically to how long a battery will last on a single use. For non-rechargeable batteries this naturally refers to how long they can be used (not how long they actually last) before having to be thrown away. For rechargeable batteries, this refers to how long they can be used before having to be recharged. Naturally, newer and more advanced batteries have a longer run time.

Run time, however, is impossible to accurately state. This is because the run time varies with what device the battery is being used to power. Each device will draw power from the battery at a different rate (measured in watt-hours) and therefore the battery’s run time will be different for each device.

What is Shelf Life?

Another metric of battery longevity is shelf life. As you might have guessed, this refers to how long a battery can be stored, unused, before “going bad”. The same distinction as with run time again applies between rechargeable and non-rechargeable batteries. And just like run time, shelf-life is difficult to accurately calculate. This time, this is because shelf life is affected by factors intrinsic to the battery, but also by environmental factors such a temperature and humidity. Nevertheless, quality batteries do have, across the board, longer shelf lives.

What is Cycle Life?

Cycle life refers to how many complete charges and discharges a battery can undergo until it dies for good. Naturally, cycle life only applies to rechargeable batteries (or you could say that a non-rechargeable battery has a cycle life of one). Although still affected by environmental factors, cycle life is something which can be much more accurately stated for each battery product. The USB-C rechargeable AA batteries soon to be released by Pale Blue Earth, for example, boast over a thousand cycles.

Last Word

So, these three metrics are actually what is meant by battery longevity. Of the three, it is only cycle life which can be accurately stated for a given battery product and it is usually this metric of longevity that is used by researchers attempting to make batteries last longer. Therefore, if you are shopping for the latest cutting-edge battery product, keep an eye on the cycle life – it’s the best metric of quality.

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