AA, AAA rechargeable batteries and other Ni-MH, Li-ion 18650 batteries and similar - how to interpret the capacity and the number of charge cycles specified in the specifications

1. Battery Capacity

We will explain whether the battery capacity decreases automatically over time. Or perhaps with each charging cycle? Isn't it true that the subject battery of 2600 mAh should maintain its full capacity throughout its entire lifespan, for example, for 500 consecutive charging/discharging cycles?
 
Every battery slowly degrades from the moment it leaves the factory. These are irreversible processes, so even an unused battery may become useless after a sufficiently long time. How long this period is depends on the type and capacity of the battery. A certain rule here is that the more stressed/more powerful the battery in a given size, the faster it will undergo automatic degradation. Thus, according to international technical standards IEC, for typical Ni-MH batteries (including popular sizes AA R6, AAA R03), an acceptable permanent capacity degradation of up to 20% relative to the nominal value is defined already after the first year. After 2 years, according to the standard, there is no longer even a requirement that a given battery must meet when measuring capacity.
 
Of course, good quality batteries should age much slower than the standards allow. The effect of self-aging is also almost negligible in batteries with the lowest capacities, however, we must keep this in mind and avoid uncertain sources of purchase, where we may buy a low-quality product or just an ordinary warehouse leftover.
 
Similarly, it is a mistaken belief that our battery will always show the same high capacity, e.g., 2600 mAh for the next 500 charging cycles. Here, according to official standards, the end of the lifespan of a Ni-MH battery may occur only when it reaches about 60% of its original capacity. Thus, the full nominal capacity of 2600 mAh will only be achieved at the beginning of the operation of our battery, while at the end of its operation, the acceptable values will be significantly lower - even below 1600 mAh! In the case of Li-ion batteries, an end-of-life value of 80% relative to the nominal capacity is more commonly accepted.

2. Charging Cycles

The manufacturer declares 500 cycles, while the batteries we have "do not survive" even 100. We explain why this is the case.
 
The durability counted in the number of charging cycles is also dependent on the age of the batteries. In the case of a several-year-old battery, especially one with high capacity, the self-degradation associated with its age may be faster than the natural wear resulting from charging/discharging cycles.
Another issue is how manufacturers determine the cycle values of their batteries. Over the years, this has even changed in technical standards. For common Ni-MH batteries, it is still typical to provide the number of half-cycles (where the battery is discharged to about 50%), and this value is always several times higher than the number of full cycles. Due to the accepted variability in determining lifespan/number of charging cycles, it is difficult to compare this value between different batteries.
Additionally, the manufacturer measures the number of cycles under laboratory conditions - the batteries are neither too deeply discharged nor specially overcharged.
In typical usage conditions, it is often impossible to avoid too deep discharging of the battery, and sometimes its overcharging - and this has a very negative impact on the intended lifespan of such a cell. Even batteries commonly considered very resistant and forgiving of mistakes, such as Ni-MH batteries, can irreversibly lose part of their capacity with each too deep discharge or significant overcharging. Even the best - Japanese AA 2450 mAh battery, which is supposed to withstand 150 full charging cycles, often does not "live" longer than about 75 cycles in home conditions.
 
In this case as well, batteries that are less stressed, with lower capacities, perform best - they will show the highest lifespan (measured in charging cycles) in typical applications.

Summary

The main conclusion from this article should be the principle of buying batteries according to our needs, not buying them in excess.

The issues of battery aging and usage conditions have a very large impact on their capacity and reliability during longer use.
Once again, it turns out that a more expensive, higher-capacity battery is not always better for us.

In the case of devices that operate on batteries for many weeks, it is better to use batteries with lower capacities, e.g., 700-800 mAh for AAA size, 1900-2000 mAh for AA size, or 2200-2600 mAh for Li-ion 18650 batteries. Such batteries usually withstand several times more charging cycles and age and degrade much slower. The most powerful and stressed batteries are better for professional applications, with short periods between subsequent charges, where momentary performance and battery operation time are a priority.

Another important conclusion is the fact that the capacity declared by the manufacturer for a given battery is only binding for brand new cells. In the case of purchasing, for example, a 2-year-old battery, we have no guarantee of maintaining any parameters. Therefore, a rational choice of a reliable seller is necessary.

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