How to extend the life of lithium batteries

Lithium-ion (mainly lithium cobalt-acid) batteries; the "life" in this article refers to the amount of use before the battery capacity decays to 80% - let's say the battery capacity drops to 80% after a cumulative charge and discharge of 1000Ah, which is certainly better than a cumulative charge and discharge of 500 Ah and then the capacity drops to 80%.

 

 

The Internet has been a lot of science on the protection of lithium batteries, many things have been cliché, but this article is still a brief description. If you already have a certain understanding of the protection of lithium batteries, the first half of the article can be skimmed. And if you only want to conclude, near the end of the article where there is a very concise summary. 
 

 

First, some already widely known basics of lithium-ion batteries as rechargeable batteries, with high energy density and many other advantages, are now widely used in digital products.

 

 

(1) Some early rechargeable batteries, such as nickel-cadmium batteries, have a memory effect. If the power is not used up to start charging, then the next discharge and then put into this place, it can not be put down again, resulting in a reduction in battery capacity; if the battery is not full and then start discharging, then the next charge and then charge into this place, it can not be charged in, also resulting in a reduction in capacity. Therefore, for nickel-cadmium batteries and other batteries with memory effect, the best way to use is to use up and then recharge, full and then use. Lithium batteries do not have this effect. On the contrary, the lithium battery is fully charged and discharged, the capacity of the lithium battery is greatly damaged. Therefore, lithium batteries do not need to be full of full discharge.

 

 

(2) lithium batteries are extremely harmful whether overcharged or overdischarged. If overcharged, it will permanently damage the capacity of the lithium battery; and then overcharged down, there is a risk of explosion and so on. However, there is no danger of overcharging lithium battery devices plugged in overnight, because these devices will of course stop charging when they are full or reduce the current to very little (only to make up for the small amount of power consumed overnight). In fact, I usually DIY with ten dollars of 18650 lithium batteries have built-in protection plate, then on the phone, etc., it is more sure to have. If you put too much, then the lithium battery will not be charged, can not be used directly, need to be removed for special activation; and then put too much, it will be completely "starved", completely unusable. Therefore, due to the lithium battery itself has a certain slight self-discharge, long-term storage of lithium battery equipment must ensure that the device has a certain amount of power, so as not to starve the battery; after a certain period of time, you should also check whether you need to replenish the power.

 

 

(3) The life of the lithium battery is indeed related to the charging current (or rather, the charging rate). Therefore, it can be said that the life of the lithium battery is related to the power supply. Generally speaking, the life span of Li-ion battery will be lower if you use fast charging. However, as long as the current does not exceed the amount of 1 hour full, the charging rate for the life of the impact is not significant. The use of rechargeable batteries, computer USB port charging, the only "problem" is that the supply current is relatively small, charging very slowly; for battery life, which is not detrimental. In addition to extremely poor quality chargers, now the device has basically not pick the charging power, unlike the early each cell phone battery must be paired with a special charger. A slower charging current instead takes better care of the battery.
 

 

(4) lithium battery is best to work at room temperature. Using lithium batteries at high temperatures, charging lithium batteries or storing them for a long time will permanently reduce the capacity. Charging lithium batteries at low temperatures (<0°C) will cause permanent damage, but only the use of low temperature, basically only temporarily experience a reduction in capacity, and will recover when it returns to normal temperature. Lithium batteries stored at lower temperatures are not too big a problem, but can not be stored at too low a temperature for too long.

 

 

Second, more detailed knowledge

 

 

The capacity loss of Li-ion battery is an extremely complex process involving a very large number of factors. There is no space here to explain each factor is specifically affected by what mechanism, but a brief list of these factors.

 

 

The capacity loss of lithium batteries can be divided into two main pieces: the capacity loss that occurs over time (calendar aging) (lithium batteries left unused for a long time will also reduce the capacity) and the capacity loss brought about by use (cycle aging).

 

Regarding the former, the main factors involved are

 

Charge state (State of Charge). This refers to where the power is in the total capacity of how much. For example, is 40% or 60%.
 

 

Temperature (Temperature).

 

Storage time (time)

 

Regarding the latter, the main factors involved are

 

Depth of Discharge (DDD). Let's say you are charging from 0% to 100% each time, and then put it to 0% and then recharge, or use 20% of the power prompt to start charging, and charge to 80% and unplug, it is not the same.

 

State of Charge (State of Charge), which is commonly referred to as power. For the same DoD, the average SoC can be different. For example, cycling between keeping the charge between 40% and 100%, and keeping it between 20% and 80%, although the depth of charge is the same, the impact on the battery is different due to the different state of charge.

 

Charging multiplier (rate of Charge). If the charging current can fill the battery in 1 hour, we say that the average rate of charge is 1C; if the charging current fills the battery in 2 hours, we say that the average rate of charge is 0.5C; and so on.

 

Temperature (Temperature).
 

The number of cycles (Number of cycles). Obviously, cycling two hundred cycles is a greater loss than one hundred cycles ......

 

In addition, there are some factors that we have little control over. For example, a lithium battery at the beginning of its life 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 can not be by passed, so we do not need to overthink it.
 

 

Calendar aging and cycle aging are basically independent, so if you can let the device directly use external power without the lithium battery involved in charging and discharging, then you can dispense with cycle aging, which is beneficial to the life of the lithium battery. However, what specific SoC should it stay in? This is what will be discussed below: the qualitative law about the impact of each factor on the life loss of Li-ion battery.

 

(1) State of Charge (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 Li-ion battery, we should keep its charge low. For example, if you want the device to use external power directly without the lithium battery involved in charging and discharging, then keeping the power at 40% would be better than keeping it at 60%.

 

So, as long as the use of needs allow, whether the lower the power is better?

 

"Long-term low power will promote the negative SEI layer growth accelerated, resulting in an irreversible increase in the internal resistance of the battery. There is also when the battery voltage is below 3.6V (about 30% power), the volume of the negative electrode expands significantly, and the physical expansion and contraction will destroy the negative electrode microstructure will also lead to an increase in internal resistance," so it is sufficient to maintain a lower limit of about 30% power.

 

(2) Temperature (Temperature)
 

 

What temperature is the most friendly to the lithium battery, different studies have not exactly the same data, but roughly similar to the human comfort temperature. So, just keep the temperature at a room temperature that is comfortable for you.

 

At higher temperatures (almost higher than the normal human oral temperature), the aging process is much faster anyway.
 

At lower temperatures (almost 0°C), storage is basically no problem, but if charging is done it will cause more damage than usual.

 

At very low temperatures (almost <20°C), even storage is not very suitable.

 

(3) Depth of Discharge

 

The shallower the depth of charge, the better. A few more times a day for a shorter period of charging is much better than almost using up the electricity every day and then filling it up again at night.

 

 

You may have a question: If the charge is shallow, won't the number of cycles be naturally higher? For example, if we can use 500 cycles according to the 100% depth of charge and discharge, then don't we certainly expect to use 1000 cycles according to the 50% depth? In fact, this is not the case. The researchers refer to each cycle as having reached a cumulative charge/discharge of 100%. Under this definition, we still get the conclusion that the shallower the charge and discharge, the better, which means that, for example, we can expect to charge and discharge 2000 times according to the 50% depth.
 

 

(4) rate of charge (rate of Charge) charging rate is lower is better. If not urgent, it is recommended to reduce the use of fast charging. However, cell phones, tablets, digital products such as fast charging rate top top is about 2C, far from the researcher found that the damage is greater 5C or even 15C. Therefore, the charging rate of these devices is a relatively small impact of factors.

 

(5) time (time) and the number of cycles (Number of cycles) This is obvious, the newer the battery, the less it is used, the less capacity depreciation of course. However, if there is a new device, putting it to less use means more life taken away by time empty, rather than into the use of time to accompany us. This seems to be more than a technical problem.