How to extend the life of Li-ion batteries？

We have an Apple device that has been in service for almost 10 years and the battery is 98% healthy!

This article is about batteries for mobile devices such as mobile phones, i.e. lithium-ion (mainly lithium cobaltate) batteries; by "life" in this article, we mean the amount of use before the battery capacity decreases to 80% - let's say the battery capacity drops to 80% after a cumulative charge of 1000Ah. is certainly better than a cumulative charge of 500Ah and then a drop to 80%.

 

The content of this article is essentially a qualitative deduction based on some existing findings and is not guaranteed to be accurate or completely correct, and is provided for information purposes only.

 

There is already a lot of science on the Internet about the protection of Li-ion batteries, and many things have been clichéd, but this article is still a brief description. If you already have some knowledge of lithium battery protection, you can skim the first half of the article. If you just want to draw conclusions, there is a very concise summary near the end of the article.

 

 

 1. Some already well known basics

 

Lithium-ion batteries, as rechargeable 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 you start charging without using up the electricity, then the next time you discharge and put it in this place, you cannot put it down again, leading to a reduction in battery capacity; if you start discharging the battery without filling it up, then the next time you charge it again to this place, you cannot charge it in, also leading to a reduction in capacity.

 

Therefore, for batteries with memory effects such as NiCd batteries, the best way to use them is to recharge them when they are finished and use them again when they are full. Lithium batteries do not have this effect. On the contrary, full charging and discharging of Li-ion batteries is extremely damaging to the capacity of Li-ion batteries. Therefore, lithium batteries do not need to be filled and discharged.

(2) Lithium batteries are extremely harmful whether they are overcharged or overdischarged. If overcharged, it will permanently damage the capacity of the lithium battery;

 

 

if overcharged further, there is a risk of explosion, etc. However, there is no danger of overcharging a LiPo device by leaving it plugged in and charging overnight, as these devices will of course stop charging when they are full or reduce the current to a very low level (only thereby compensating for the small amount of power consumed overnight).

 

In fact, the ten dollar 18650 lithium batteries I usually use for DIY have a built-in protection plate, so on a mobile phone etc, it's even more certain that they will. If it is over discharged, then the lithium battery will not be charged and can no longer be used directly, and will need to be removed for special activation; if it is over discharged again, it will be completely "starved" and will be completely unusable.

 

Therefore, due to the lithium battery itself has a certain slight self-discharge, long-term storage of lithium battery equipment before the device must ensure that there is a certain amount of power to avoid battery starvation; after a certain period of time, you should also check whether the need to replenish the power.

 

 

 (3) The life of a lithium battery does have a relationship with the charging current (or rather, the charging rate). Therefore, it can be said that the life span of a Li-ion battery is related to the power supply. Generally speaking, the life span of a lithium battery is lower if you use fast charging. However, as long as the current does not exceed the amount of 1 hour to fill, the charging rate has no significant effect on the life span. The only "problem" with using a rechargeable battery, computer USB port, etc., is that the supply current is low and charging is slow; this is not detrimental to battery life. With the exception of extremely poor quality chargers, devices are now largely unpredictable in terms of charging power, unlike the early days when each mobile phone battery had to be accompanied by a special charger. Slower charging currents take better care of the battery instead.

(4) Lithium batteries should ideally be operated at room temperature. Using, charging or storing LiPo batteries at high temperatures for long periods of time will permanently reduce their capacity. Charging LiPo batteries at low temperatures (<0°C) will cause permanent damage, but when used only at low temperatures, the reduction in capacity will basically be experienced only temporarily and will recover when the temperature returns to normal. There are no major problems with storing lithium batteries at lower temperatures, but they should not be stored at too low a temperature for too long.

 

2, more detailed knowledge of lithium battery capacity loss is an extremely complex process, involving a very large number of factors. There is no space here to explain exactly what mechanism each factor has an effect through, but a brief list of these factors.

 

The capacity loss of Li-ion batteries can be divided into two main components: calendar aging, which occurs over time (when a Li-ion battery is left unused, its capacity will decrease over time), and cycle aging, which occurs through use.

With regard to the former, the main factors involved are State of Charge. This refers to where the charge is at in terms of total capacity. For example, is it 40% or 60%.

 

Temperature (Temperature).

 

Storage time (time).

 

With regard to the latter, the main factors involved are：

 

Depth of Discharge (DDD) per charge. Whether you charge the battery from 0% to 100% and then put it back to 0%, or whether you start charging when it reaches 20% and unplug it when it reaches 80%, for example, is not the same.

 

The State of Charge (SoC), which is commonly referred to as the charge level. For the same DoD, the average SoC can be different. For example, keeping the charge cycled between 40% and 100% and keeping it between 20% and 80%, although the depth of charge is the same, will have a different effect on the battery due to the different state of charge.

 

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

 

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 other factors that are almost beyond our control. For example, a lithium battery at the start of its life will undergo a process of formation of a solid electrolyte phase interface film (SEI film). This process consumes a certain amount of lithium ions. This process cannot be bypassed as long as the battery is in use, so we do not need to overthink it.

 

Calendar aging is largely independent of cycle aging, so if it is possible to have the device use an external power supply directly without the Li-ion battery being involved in charging and discharging, then cycle aging can be dispensed with, which is beneficial to the life of the Li-ion battery. But what specific SoC should it stay in? This is what will be discussed below: the qualitative laws regarding the influence of each factor on the loss of Li-ion battery life.

 

 

(1) State of Charge studies have shown that both calendar aging and cycle aging are delayed when the SoC is lower. Therefore, if we want to minimise the life decay of a Li-ion battery, we should keep it low. For example, if one wants to keep the device running directly from an external power source without the Li-ion battery being involved in charging and discharging, then keeping the charge at 40% is better than keeping it at 60%. So, is the lower the charge, the better, as long as the use allows? Thanks to @TaoTaoMi, "A long period of low charge will promote faster growth of the negative SEI layer, resulting in an irreversible increase in internal resistance of the battery. Also, when the battery voltage falls below 3.6V (about 30% charge), the volume of the negative electrode expands significantly, and the physical expansion and contraction will destroy the negative electrode microstructure, which will also lead to an increase in internal resistance," so keeping the lower limit at about 30% charge is sufficient.

(2) Temperature (Temperature) What temperature is the most friendly for lithium batteries, different studies have come up with different data, but roughly similar to human comfort temperature. So, just keep the temperature at a room temperature that is comfortable for you. At higher temperatures (almost above the normal human oral temperature), the ageing process is much faster anyway. At lower temperatures (almost 0°C), storage is basically no problem, but charging can cause more damage than usual. At very low temperatures (almost <20°C), even storage is less suitable.

 

 (3) Depth of Discharge: The shallower the depth of charge, the better. It is better to have a few more short charging sessions per day than to use up almost all the electricity every day and then fill it up again at night.

You may have a question: wouldn't a shallow charge naturally result in more cycles? For example, if we can use 500 cycles at a 100% depth of charge and discharge, wouldn't we certainly expect to use 1000 cycles at a 50% depth? In fact, this is not the case, as the researchers refer to each cycle as having reached a cumulative charge/discharge of 100%. This definition still leads to the conclusion that the shallower the charge and discharge, the better, which means that, for example, at a depth of 50% we can expect to charge and discharge 2000 times.

(4) Rate of Charge: A lower rate of charge is better. If you are not in a hurry, it is advisable to reduce the use of fast charging. However, the rate of charge for digital products such as mobile phones and tablets is at best about 2C, far less than the 5C or even 15C that researchers have found to be more damaging in their studies, so the rate of charge for these devices is a relatively minor factor.

(5) Time and number of cycles

 

It is obvious that the newer the battery and the less it is used, the less it will lose capacity. However, if there is a new device, leaving it to be used less means that more of its life is taken away by the empty hours of time, rather than being turned into time spent with us. It seems to be more than just a technical issue. It's a matter of opinion and wisdom.

To sum up the above: try to use and charge LiPo batteries at room temperature.

Choose a relatively low charge level, keeping the actual charge above and below it, while avoiding always having a full charge.

Charge shallowly and discharge sparingly.

Use fast charging sparingly.