How to extend the service life of lithium batteries
1. Some well-known basic knowledge
As a rechargeable battery, lithium-ion batteries have many advantages such as high energy density, and are now widely used in digital products.
(1) Some early rechargeable batteries, such as nickel-cadmium batteries, have a memory effect. If the battery is not used up and start charging, then the next time it is discharged and placed in this place, it can no longer be placed, resulting in a decrease in battery capacity; if the battery is not fully charged, it will start to discharge, then the next time it will be charged and then recharged to this place , It can’t be charged, which also leads to a decrease in capacity. Therefore, for nickel-cadmium batteries and other batteries that have a memory effect, the best way to use them is to recharge them when they are used up and use them when they are full.
Lithium batteries do not have this effect. On the contrary, fully charging and discharging the lithium battery will greatly damage the capacity of the lithium battery. Therefore, the lithium battery does not need to be fully charged.
(2) Lithium batteries are extremely harmful whether they are overcharged or overdischarged. If it is overcharged, it will permanently damage the capacity of the lithium battery; if it is overcharged, there will be risks such as explosion. However, there is no danger of overcharging if the lithium battery device is plugged in overnight, because of course these devices will stop charging or reduce the current to a very low level when they are fully charged (only to make up for a small amount of power consumption overnight). In fact, the ten-dollar 18650 lithium battery that I usually use in DIY has a built-in protection board, so it is sure to have it on mobile phones and so on.
If it is over-discharged, the lithium battery will no longer be charged and cannot be used directly. It needs to be disassembled for special activation; if it is over-discharged, it will be completely "starved to death" and completely unusable. Therefore, because the lithium battery itself has a certain degree of self-discharge, it is necessary to ensure that the device has a certain amount of power before storing the lithium battery device for a long time to prevent the battery from starvation; after a certain period of time, check whether it needs to be recharged.
(3) The life of a lithium battery is indeed related to the charging current (or charging rate). Therefore, it can be said that the life of a lithium battery is related to the power supply. Generally speaking, the service life of lithium batteries will be lower if fast charging is used. However, as long as the current does not exceed the amount of full charge in 1 hour, the impact of the charging rate on the life is not significant.
The only "problem" when using power banks, computer USB ports, etc., is that the power supply current is relatively small and the charging is very slow; for the battery life, this is not harmful. Except for extremely inferior chargers, today's devices basically don't pick up the charging power source, unlike in the early days, each mobile phone battery must be equipped with a special charger. On the contrary, the slower charging current is more protective of the battery.
(4) Lithium batteries are best operated at room temperature. Using lithium batteries at high temperatures, charging them, or storing them for long periods of time will permanently reduce the capacity. Charging the lithium battery at low temperature (<0°C) will cause permanent damage, but only using it at low temperature will basically only temporarily experience a decrease in capacity, and it will recover when it returns to normal temperature. It is not a big problem to store lithium batteries at lower temperatures, but they cannot be stored at too low temperatures for too long.
2. More detailed knowledge
The capacity loss of lithium batteries is an extremely complex process involving many factors. There is no space here to explain the specific mechanism through which each factor has an impact, but a brief list of these factors.
The capacity loss of lithium batteries can be divided into two parts: the capacity loss over time (calendar aging) (the lithium battery is left unused, the capacity will be reduced for a long time) and the capacity loss caused by use (cycle aging).
Regarding the former, the main factors involved are:
State of Charge. This refers to the place where the power is in the total capacity. For example, it is 40% or 60%;
Temperature (Temperature);
Storage time (time).
Regarding the latter, the main factors involved are:
Depth of Discharge each time. For example, each time you charge from 0% to 100%, and then charge to 0%, or start charging when the battery is 20%, and unplug it when it reaches 80%. This is not the same;
State of Charge (State of Charge), which is commonly referred to as electricity. For the same DoD, the average SoC can be different. For example, when the battery is maintained between 40% and 100%, and the battery is maintained between 20% and 80%, although the charge and discharge depth is the same, the impact on the battery is different due to the different charging state;
Charge rate (rate of Charge). If the charging current can fully charge the battery in 1 hour, we say that the charging rate is 1C on average; if the charging current can fully charge the battery in 2 hours, we say that the charging rate is 0.5C on average; and so on;
Temperature (Temperature);
Number of cycles. Obviously, cycling for two hundred cycles is more lossy than one hundred cycles...
In addition, there are some factors that are almost beyond our control. For example, a lithium battery in the initial stage of use will undergo a process of forming a solid electrolyte phase interface film (SEI film). This process consumes a certain amount of lithium ions. As long as the battery needs to be used, this process cannot be bypassed, so we don't have to think about it too much.
Calendar aging and cycle aging are basically independent. Therefore, if the device can directly use an external power source without charging and discharging the lithium battery, then cycle aging can be avoided, which is beneficial to the life of the lithium battery. However, what SoC should we stay in? This is what will be discussed below: the qualitative law about the influence of various factors on the life loss of lithium batteries.
(1) State of Charge
Studies have shown that when the SoC is lower, both calendar aging and cycle aging will be delayed. Therefore, if we want to minimize the life decay of lithium batteries, we should keep its power low. For example, if you want the device to directly use an external power source without charging and discharging the lithium battery, it is better to keep the power at 40% than at 60%.
So, as long as the use requires permission, is the lower the battery, the better? It depends on whether you let the lithium battery not participate in charging and discharging, such as storing it on hold or using only external power (only calendar aging at this time), or let it participate in charging and discharging (cycle aging is dominant at this time).
In the former case, the power is indeed as low as possible. However, if the battery is too low, the risk of starvation caused by forgetting to recharge the battery is even greater. Under the circumstance of ensuring that the battery will not starve to death, the protection effect is better if the battery is discharged to 5% or even close to 0% and then stored.
In the latter case, the situation is slightly more complicated. When the battery is too low, the internal resistance of the battery will increase. Let's make an extreme hypothesis to illustrate this hidden problem. The capacity of a certain battery is 10Wh. Its internal resistance is very high when its power is 0~10%, so that when you store 1Wh of power to charge the power from 0% to 10%, you can only use 0.1Wh when you take it out, and the other 0.9Wh is working in the device. When turned into heat on the battery. At this time, we only got the effective use of 0.1Wh, but caused a cycle aging of 1Wh to the battery. The internal resistance is low when its power is between 98% and 100%. Although the cycle aging caused by the battery from 98% to 100% is only 0.2Wh, it generates less heat during use and can also obtain an effective value of 0.1Wh. use. Although the aging of the same 0.2Wh battery under high power will be greater than the equivalent 0.2Wh aging under low power, it is also likely to be less than the actual 1Wh of low power.
Among the limited data I can find, I can’t tell if the battery level is less than 20%, whether the high internal resistance brings greater harm, or the low battery charge has a greater benefit, so I can’t answer here. Should the battery be so low? But what is certain is that at least at a power level above 40%, the internal resistance of the battery can be ignored. Therefore, for example, keeping the power at 40%±20% is more beneficial than keeping it at 60%±20%.
(2) Temperature
What temperature is the most friendly to lithium batteries? The data obtained from different studies are not exactly the same, but it is roughly similar to the comfortable temperature of the human body. So, keep the temperature at a room temperature that makes you comfortable.
At higher temperatures (almost higher than a person's normal oral temperature), the aging process is much faster anyway.
At a lower temperature (almost 0°C), storage is basically no problem, but if it is charged, it will cause more damage than usual.
At extremely low temperatures (almost <20°C), even storage is not suitable.
(Insert: The research on humidity seems to be rarely seen, but according to common sense, don’t think it’s too damp...)
(3) Depth of Discharge each time
The shallower the charge and discharge depth, the better. It is much better to charge for a short period of time a few more times a day than to charge the battery almost completely every day and then charge it again at night.
You may have a question: If the charge is shallow, wouldn't the number of cycles naturally increase? For example, if we can use 500 cycles according to 100% of the charge and discharge depth, don't we of course expect to use 1000 cycles according to the 50% depth? In fact, this is not the case. The researchers say that each cycle is based on the cumulative amount of charge and discharge reached 100%. Under this definition, we still get the conclusion that the shallower the charge and discharge, the better, which means that, for example, at a depth of 50%, you can count on 2000 charge and discharge times.
(4) Rate of Charge
A lower charging rate is better. If you are not in a hurry, it is recommended to reduce the use of fast charging. However, the fast charge rate of digital products such as mobile phones and tablets is at most about 2C, which is far less than the 5C or even 15C that researchers found to be more harmful during the study. Therefore, the charging rate of these devices is a relatively small factor.
(5) Time and Number of cycles
This is obvious. The newer the battery and the less used it is, the less capacity loss will of course be. However, if there is a new device, putting it for less use means that more lifespan will be taken away by time, instead of turning it into use time to accompany us. This seems to be more than just a technical problem. The benevolent see benevolence, and the wise see wisdom.
To summarize the above:
Try to use and charge lithium batteries at room temperature;
Choose a relatively low power level, keep the actual power level above and below it, and avoid always being fully charged;
Lightly charge and discharge, eat less and more meals;
