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

1. Battery capacity

We will explain whether the capacity of the battery decreases on its own over time. Or with subsequent charging cycles? Isn't it the case that the 2600 mAh battery in question should maintain its full capacity throughout its lifetime, i.e. for 500 consecutive charge/discharge cycles?

Each battery slowly degrades from the moment it leaves the factory. These are irreversible processes, so even an unused battery may turn out to be useless after a sufficiently long time. How long this period is depends on the type/type and capacity of the battery. A certain rule here is that the more strained/capacious the battery in a given size, the faster it will degrade on its own. For example, in the case of international IEC technical standards, in the case of typical Ni-MH batteries (including the popular sizes AA R6, AAA R03), the permissible permanent degradation of capacity is specified at the level of up to 20% compared to the nominal value after the first year. After 2 years, according to the standard, there is not even a requirement that a given battery must meet when measuring capacity.

Of course, good quality rechargeable batteries should age much slower than the standards allow. The self-aging effect itself is also almost negligible in rechargeable batteries with the lowest capacities, but we must keep this in mind and avoid uncertain sources of purchase, where we can buy a low-quality product or an ordinary storage deckchair.

Similarly, it is a misconception that our battery will show the same high capacity all the time, e.g. 2600 mAh, for the next 500 charging cycles. Here, according to official standards, the end of the life of a Ni-MH rechargeable battery can only occur when it reaches about 60% of its original capacity. Thus, the full, nominal capacity of 2600 mAh will be obtained only at the beginning of the life of our battery, while at the end of its life, the permissible values will be much lower - even below 1600 mAh! In the case of Li-ion batteries, the value of 80% of the nominal capacity is more often assumed as the end of life.

2. Charging cycles

The manufacturer declares 500 cycles, and the batteries you have "do not survive" even 100. We explain why this is the case.

The durability calculated in the number of charging cycles also depends on the age of the batteries. In the case of a battery that is several years old, especially with a high capacity, the self-degradation associated with its age can be faster than the natural wear and tear resulting from charge/discharge cycles.
Another issue is the way manufacturers determine the value of their batteries' charging cycles. Over the years, this has even changed in technical standards. In the case of common Ni-MH batteries, it is still typical to give the number of half cycles (where the battery is discharged in about 50%), and such a value is always several times higher than the number of full cycles. Due to the arbitrariness in determining the lifespan / number of charging cycles, it is difficult to compare this value between different batteries.
In addition, the manufacturer measures the number of cycles in laboratory conditions - the batteries are neither discharged too deeply nor specially overcharged.
Under typical conditions of use, it is often impossible to avoid too deep discharge of the battery, and sometimes overcharging it - and this has a very negative impact on the target life of such a cell. Even Ni-MH batteries, commonly considered to be very resistant and forgiving, can irreversibly lose some of their capacity with each too deep discharge or significant overcharging. Even considered the best - the Japanese AA 2450 mAh battery, which is supposed to last 150 full charging cycles, will often not "live" longer than about 75 cycles in home conditions.

Also in this case, less strenuous batteries with lower capacities are best - they will show the highest service life (measured in charging cycles) in typical applications.

Summary

The basic conclusion from this article should be the principle of buying rechargeable batteries according to our needs, not buying them heavily in stock.

Aging issues of rechargeable batteries and conditions of use have a very large impact on their capacity and reliability over long life.
Once again, it turns out that a more expensive, more capacious battery will not always be better for us.

In the case of devices that operate on rechargeable batteries for many weeks, it is better to use rechargeable 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 last several times as many charging cycles and are subject to self-aging and degradation much more slowly. The most capacious and strenuous rechargeable batteries are better for professional use, with short periods between charges, where temporary performance, battery life is a priority.

Another important conclusion is the fact that the capacity declared by the manufacturer for a given battery is binding only for brand new cells. In the case of buying, for example, a 2-year-old rechargeable battery, we do not have any guarantee for maintaining any parameters. Therefore, it is necessary to choose a certain seller rationally.

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