In the industry, cycle life is generally calculated based on the number of full charge-discharge cycles of a lithium-ion battery. During use, irreversible electrochemical reactions occur inside the lithium battery, leading to a decrease in capacity. These reactions include electrolyte decomposition, deactivation of active materials, and the collapse of the positive and negative electrode structures, resulting in a reduction in the number of lithium ions inserted and extracted.
Based on different materials, the existing power batteries on the market are mainly divided into lithium iron phosphate batteries, ternary lithium batteries (including NCA and NCM), lithium manganese oxide batteries, lithium cobalt oxide batteries, nickel-metal hydride batteries, and lithium titanate batteries. Among them, lithium iron phosphate batteries are currently the mainstream battery type in the Chinese electric vehicle market, but in recent years, their market growth rate has been significantly lower than that of ternary lithium batteries.
The theoretical lifespan of ternary lithium batteries is approximately 800 cycles, which is average among commercially available rechargeable lithium batteries. Lithium iron phosphate batteries have a lifespan of about 2,000 cycles, while lithium titanate batteries are said to reach 10,000 cycles.
Currently, mainstream battery manufacturers promise more than 500 cycles (charge and discharge under standard conditions) in their ternary lithium battery cell specifications. However, after the cells are assembled into a battery pack, due to consistency issues, mainly the voltage and internal resistance cannot be exactly the same, and the cycle life is about 400 cycles.
The manufacturer recommends a SOC (State of Charge) window of 10% to 90%, and discourages deep charging and discharging, as this can cause irreversible damage to the battery's positive and negative electrode structures. If calculated using shallow charging and discharging, the cycle life is at least 1000 cycles. Furthermore, if lithium batteries are frequently discharged at high rates and high temperatures, their lifespan will drastically decrease to less than 200 cycles.
Three points to note when using lithium-ion batteries:
1. How to charge a new battery
It's important to note that lithium batteries enter a dormant state after being stored for a period of time, at which point their capacity is lower than normal, and their usage time is shortened. However, lithium batteries are easily activated; just 3-5 normal charge-discharge cycles are sufficient to activate them and restore their normal capacity. Due to the inherent characteristics of lithium batteries, they have virtually no memory effect. Therefore, users do not need special methods or equipment to activate new lithium batteries. Not only is this true in theory, but from my own experience, using the standard charging method from the beginning—this "natural activation" method—is the best approach.
2. When should charging begin during normal use?
Because the number of charge-discharge cycles is limited, lithium batteries should be fully discharged before recharging. However, I found an experimental table regarding lithium-ion battery charge-discharge cycles, and the data on cycle life is listed below:
Cycle life (10% DOD): >1000 cycles
Cycle life (100% DOD): >200 cycles
The principle of fully depleting the battery before recharging doesn't mean going to extremes. A widely circulated saying, similar to prolonged charging, is to "use up the device's battery as much as possible, ideally until it automatically shuts down." This practice is actually only used with nickel-cadmium batteries to avoid the memory effect, but unfortunately, it's still prevalent with lithium-ion batteries. There have been cases where people continued using their devices after receiving low battery warnings, continuing until they automatically shut down. In these cases, the devices became unresponsive to charging and powering on, requiring repair. This was because the battery voltage was too low due to over-discharge, rendering it incapable of normal charging and powering on.
3. Correct handling of lithium batteries
In summary, the most important tips regarding the charging and discharging of lithium batteries during use are:
1. Charge according to the standard time and procedure, even for the first three times;
2. When the machine displays a low battery warning, you should start charging it as soon as possible;
3. Lithium batteries do not require special activation methods; they will activate naturally during normal use of the device. Insisting on using the widely circulated method of "activating with three 12-hour long charges for the first three times" will not actually be effective.
Therefore, all practices that aim for 12-hour ultra-long charging times or draining lithium batteries until they automatically shut down are incorrect. If you have been following these incorrect practices, please correct them as soon as possible; it may not be too late.
