Batteries require frequent calculations regarding capacity fade and remaining service life. Generally, a battery pack must be replaced once its capacity drops to 80% of its original rating. However, the ultimate end-of-life (EOL) threshold for a battery pack varies depending on the specific application, user preference, and company maintenance policies.
The energy storage of a battery can be visualized as three distinct zones: the empty zone (available for charging), the available zone (energy ready for use), and the inactive/unusable zone (caused by usage and aging), often referred to as the "rock zone," as illustrated in the figure below.
Battery degradation begins as soon as the battery is manufactured. A new battery is expected to deliver 100% of its rated capacity, but most battery packs in actual use fail to achieve this level.
As the usable region of the battery shrinks, the amount of energy that can be stored decreases, and the charging time gradually becomes shorter. In most cases, battery capacity declines linearly due to cycle usage and aging. In addition, deep discharging places more stress on the battery than partial discharging. Therefore, it is advisable not to fully deplete the battery, but instead to recharge it frequently. However, for nickel-based batteries and smart batteries used for calibration purposes, periodic deep discharge is recommended, as it helps eliminate the "memory effect" in nickel-based batteries. Nickel-based lithium batteries can typically undergo 300 to 500 full charge-discharge cycles before their capacity declines to 80%.
Charge-discharge cycling is not the only cause of capacity degradation. Storing lithium batteries at high temperatures can also lead to capacity loss. A fully charged lithium battery stored at 40°C (104°F) for one year without use may lose up to 35% of its capacity. Ultra-fast charging and discharging are also harmful, potentially reducing battery life by half-this effect is particularly evident in individual lithium cells. Battery packs have higher energy density, but they are more sensitive due to variations among individual cells.
Device specifications are often based on new batteries; however, this represents only a brief initial stage and cannot be maintained for long. Like an athlete, performance gradually declines over time, and if left unchecked, this will eventually lead to battery-related failures.
Battery capacity degradation and overall lifespan need to be regularly evaluated. When the capacity drops to 80%, the battery pack should be replaced. The end-of-life threshold of a battery pack may vary depending on the application, user preferences, and the company's warranty policies. Since mechanical failures are relatively rare, capacity degradation becomes the most reliable indicator for determining replacement timing. This can be assessed by verifying the capacity of batteries in service every three months. In addition, technologies that characterize the charging status of chargers are also under development.
In addition to aging-related degradation, sulfation and grid corrosion are the main factors affecting the degradation of lead-acid batteries. Sulfation refers to the formation of a thin film layer on the negative plate when the battery remains at a low state of charge. If detected early, this condition can be eliminated through equalization charging. Grid corrosion can be mitigated by improving the charging regime or adopting optimized float charging methods.
For nickel-based batteries, the so-called inactive "rock zone" is usually caused by the formation of crystalline structures in the active materials, also known as the "memory effect." Deep charge-discharge cycles can often restore the battery capacity to its full level. Periodic discharging can also help control the crystallization process and prevent damage to the separator.
The aging of lithium-ion batteries is due to the oxidation of internal materials. This is part of the usage and aging process, occurring naturally and irreversibly.
https://www.ryderelectronics.com/electric-bicycle-battery/low-speed-vehicle-battery.html
