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Lithium ion cells and batteries are found in a huge number of electronic circuit designs, electronic gadgets, mobile phones as well as many power tools, vacuum cleaners, and now in automobiles.
One of the key issues with the lithium ion batteries and cells, in common with other forms of rechargeables, is that they have a limited lifetime or lifespan. With use their charge capacity falls, and eventually it will reach a point where the useful life for a particular application is over.
When the useful lifetime for the battery is over, this will need to be replaced. Unfortunately the cost of repair, even if an organisation is able to undertake this means that the item has to be scrapped. This can be both costly and inconvenient apart from creating more electronic waste which is environmentally unfriendly.
In particular, many mobile phones are replaced, not only to keep up with the latest trends, but also when the battery performance has fallen. Often mobile phones are replaced at intervals of between two and up to four or five years. Normally by the five year timespan, the battery will have significantly degraded.
Also for electric cars, the cost of replacement batteries can be enormous, often rendering the car beyond economic repair. In view of this, manufacturers are looking at battery lifetimes for automobiles of between ten and fifteen years, and this should be extended as the technology develops.
Lithium ion battery lifetime
Improvements in lithium ion battery technology have enabled the lifetime of new batteries to be much longer than they were, even a few years ago. However the battery lifetime is still finite and can cause issues.
Typically the lifetime of a cell and in this case a lithium ion cell or battery is taken as the point where it an only store 80% of the charge it could at manufacture.
The lifetime is often measured in terms of the number of charge discharge cycles that battery can undergo before the maximum charge it can store is only 80% of the charge capacity when new. It is the continual repetition of charge and then discharge that causes the wear and degradation in the batteries.
As the lifetime is measured in terms of the number of charge / discharge cycles, it may appear that there is little that can be done to preserve the battery, apart from not using it. This is not the case and there are several steps which can be put in place to get the longest lifetime possible. Some may not be practicable, but at least having an understanding of them enables them to be implemented in the situations where it might be possible.
Battery lifetime and charge capacity degradation
With the degradation in the charge capacity of a battery being the limiting factor for the useable lifetime of a battery, it is useful to understand a little more about this.
There appear to be several factors that reduce the charge capacity of a lithium ion battery and hence its useful lifespan.
Loss of Active Material: The loss of active material, LAM, in the cell occurs on both the positive and negative electrodes. It is essentially a mechanism whereby the material in the electrodes which is available for electrochemical activity is reduced.
Loss of lithium inventory: The loss of lithium inventory or LLI occurs when the number of lithium ions available for transporting charge reduces.
Electrode imbalance: Over time it is found that the state of the electrodes becomes imbalanced to that one electrode can either accept more lithium ion charge carriers than the other one can receive or vice versa.
Impedance change: There are several mechanisms that occur to cause the impedance to rise, but over time it will be seen that a cell with a very low impedance will start to have its impedance rise. This occurs with usage and time.
Ways of preserving battery lifetime
There are several ways in which the battery lifetime in terms of the number of charge / discharge cycles can be preserved. In other words the lifetime before the battery has reached its end of life and not the lifetime in terms of the charge for that cycle.
These measures can help ensure that the lithium ion battery does not need replacing any sooner than is necessary.
By employing these measures, costs, and waste can be reduced along with the inconvenience of having batteries replaced can be reduced.
Only fast charge when you really need to: Fast charging of a lithium ion battery is very convenient because it enables the device, vehicle or other items to be up and running again as son as possible. However it does cause more damage to the battery and continually fast charging can mean the lifetime will not be as long as it might otherwise be.
Don't fully discharge lithium ion batteries: Fully discharging lithium ion batteries puts them under strain and this reduces their lifetime. Fully discharging them can strip the lithium ions out of the electrodes and this can lead to their early demise.
Also, Li-ion batteries should never be depleted to below their minimum voltage, 2.4 V to 3.0 V per cell.
Don't overcharge: Lithium ion batteries do not like being fully discharged, neither do the like being fully charged. Again all the lithium ions will have migrated from one electrode to the other and this too can lead to the disintegration of the electrodes.
Ideal charge band: Possibly the sweet spot for charging lithium ion batteries is to charge them when they reach about 20% charge and then stop at around 80%. This is not always easy to achieve but it is useful have this as an aiming point which can help in general management. As the battery management systems will prevent them reaching full charge and even low charge levels. They tend to give a reasonable margin. However it is still a good idea to prevent them getting too close to either end of the charge limits than is absolutely necessary.
- Keep cool: Li-ion batteries should be kept cool. By keeping them cool but not frozen the ageing process becomes slower. As a result, Li-ion batteries should not be kept in cars on sunny days as the temperatures rise significantly. It is even recommended that electric cars should not be parked in the sun for the same reason.
Non-use care: If a Li-ion battery is not to be used for an extended period of time it should ideally be brought to a charge level of between about 40% and 60% of full charge. Ideally it should be periodically charged to overcome the effects of self discharge. By keeping it at around 50% charge level, this ensures it is in the best condition with neither electrode depleted of lithium ions - a key factor in lifespan reduction.
Also, storing the batteries between about 5° and 20° is best for them as it provides a temperature that is neither too hot or cold.
It is also worth removing them from the equipment if this is possible and also periodically charging them to replace and charge lost by self discharge.
- Don’t freeze: Li-ion batteries should not be exposed to very low temperatures - most lithium-ion battery electrolytes freeze at approximately -40°C. This may preclude them from some applications where equipment needs powering in extremes of temperature.
- Buy new batteries only when needed: Li-ion batteries should be bought only when needed, because the ageing process begins as soon as the battery is manufactured. Leaving the purchase until they are needed means that they will not have deteriorated too much, although they will have been stored by the manufacturer and distributor before they were delivered.
How long with a lithium ion battery last
This is the main question that everyone wants to know. Unfortunately it is not easy to give a definitive answer. There are many variables involved.
Parameters such as the type of battery (whether it is one used in a mobile phone, photographic camera, electric tool, electric vehicle and so forth). Different types of equipment use different types of lithium ion battery and this will have an impact on the lifetime achievable.
Also items such as the temperature under which they are used, whether they have been stored, how quickly they have been charged and discharged, whether they have been left discharged for any period of time, and a whole number of other factors.
Another big variable is the question of what counts as a charge / discharge cycle. Sometimes the battery will have undergone a deeper charge cycle than others, sometimes it may be a 20% to 80%, other times it may only be a top up, say 30% to 60% and whether this counts as a cycle.
But taking a sensible approach to all of these questions, most lithium ion batteries will last for at least 3 years and around 1000 charging cycles.
If these are taken as a good minimum, then anything greater is a bonus. By adopting the guidelines for preserving the battery lifespan as much as possible, it should be possible to improve on the basic expected lifetime and reduce costs by having to replace batteries less frequently.
In terms of real world data, it has been found that the batteries on some early electric cars have only degraded slightly over time, and accordingly, they should enable the cars to operate for very significant lifetimes without the need for replacements. This will obviously depend to soem extent on the car manufacturer and the standards of battery manufacture, and the electronic circuit design of the battery management system.
Another major user of lithium ion batteries is the mobile phone industry. Even though battery standards are generally high, the need for rapid charging and the fact that many people run their phones right down and charge them right up, use fast charging as they need them to work quickly again after a complete discharge means that the lifespan is rarely as long as it might be. However with many people changing phones every two years as contracts are renewed, and other people choosing to change them a little less frequently, it is still not always a major issue. Typically phones tend not to last much more than four years, although battery replacements are available.
By taking a few precautions with the way lithium ion batteries are used and stored it is possible to gain the maximum lifetime out of them before their performance degrades to such an extent that it is difficult to use them. Sometimes it will not be possible to implement any of them or all of them, but where it is feasible it can provide a useful increase in battery lifetime and a reduction in running costs as they will need replacement less frequently, if at all.
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