Li-ion Lithium Ion Battery Charging
For Lithium Ion, Li-ion batteries to operate correctly, they must be charged properly otherwise they do not operate correctly.
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Lithium ion, Li-ion batteries offer an excellent level of performance. To gain the best from them, they must be charged correctly.
If lithium ion battery charging is not undertaken in the proper manner, the battery operation can be impaired and they can even be destroyed - so care msut be taken.
Proper charging of Li-ion batteries enables the best performance and longest operational life to be obtained. As a result, li-ion battery charging is normally undertaken in conjunction with a battery management system. This controls the level of charge, discharge and the rates at which these can occur.
Lithium ion charge / discharge chemistry
In very basic terms, the charge and discharge of a lithium ion battery is relatively easy to explain.
When the lithium ion cell or battery is discharging it provides current to an external circuit. Internally the anode releases lithium ions in an oxidation process which pass to the cathode. The electrons from the ions that have been created flow in the opposite direction, flowing out into the electrical or electronic circuit that is being powered. The ions and electrons then reform at the cathode.
This process releases the chemical energy that is stored in the cell in the form of electrical energy.
During the charging cycle, the reactions occur in the reverse direction with lithium ions passing from the cathode through the electrolyte to the anode. The electrons provided by the external circuit then combine with the lithium ions to provide the stored electrical energy.
It should be remembered that the charging process is not totally efficient - some energy is lost as heat, although efficiency levels of around 95% or a little less are typical.
Electronic conditions for lithium ion battery charging
In terms of the electronics for the process, charging lithium ion batteries is very different to charging Ni-Cads or NiMH batteries. It is not possible to use the same electronic circuits to charge them for a variety of reasons.
The charging lithium ion batteries is voltage dependent rather than current based. In this way, the charging of lithium ion batteries is more akin to that of lead acid batteries.
One of the differences with charging lithium ion batteries is that they have a higher voltage per cell - around 3.7 to 4 volts per cell as opposed to 1.2 volts.l
Lithium ion cells also require much tighter voltage tolerance on detecting full charge and once fully charged they do not allow or require to be trickle or float charged. It is particularly important to be able to detect the full charge state accurately because lithium ion batteries do not tolerate overcharging. They over-heat and this reduces their life but in extreme circumstances it can lead to them catching fire of even exploding.
Most consumer orientated lithium ion batteries charge to a voltage of 4.2 volts per cell and this has a tolerance of around ± 50 mV per cell. Charging beyond this causes stress to the cell and results in oxidation that reduces service life and capacity. It can also cause safety issues as well.
The discharge curve shown above would be typical for a consumer lithium cobalt oxide form of lithium ion cell. The different types of lithium ion cell have slightly different voltages, but all will show similar shape discharge curves.
Charging lithium ion batteries can be split into two main stages:
Constant current charge: In the first stage of charging a li-ion battery or cell, the charge current is controlled. Typically this will be between 0.5 and 1.0 C. (NB: for a 2 000 mAh battery the charge rate would be 2 000 mA for a charge rate of C).
For consumer based LCO cells and batteries, a charge rate of a maximum of 0.8C is recommended.
During this stage the voltage across the lithium ion cell increases for the constant current charge. The charge time may be around an hour for this stage.
Saturation charge: After a time the voltage peaks at around 4.2 Volts for an LCO cell. At this point the cell or battery must enter a second stage of charging known as the saturation charge. A constant voltage of 4.2 volts is maintained and the current will steadily fall.
The end of the charge cycle is reached when the current falls to around 10% of the rated current. The charge time may be around two hours for this stage dependent upon the type of cell and the manufacturer, etc.
The charge efficiency, i.e. the amount of charge retained by the battery or cell against the amount of charge entering the cell is high. Charge efficiencies of around 95 to 99% can be achieved. This reflects into relatively low levels of cell temperature rise.
Many cells are now designed for fast charging, although within the ratings for the cell, this process can reduce the life of the battery, and a balance between convenience and lifetime needs to be made.
Lithium ion battery charging precautions
In view of the amount of energy stored in lithium ion batteries and the nature of their chemistry, etc., it is necessary to ensure that the batteries are charged in the appropriate manner and with the appropriate charger and equipment.
Lithium ion battery chargers or battery packs include various mechanisms to prevent damage and danger. Often these mechanisms are provided within the battery pack, which may then be used with a simple charger.
The mechanism required by the lithium ion battery for charge and discharge include:
Charge current: The charge current must limited for li-ion batteries. Typically the maximum value is 0.8C, but lower values are more usually set to give some margin. Some faster charging may be possible for some batteries.
Even for batteries or cells that can withstand higher current charges, there is an impact on the lifetime. If it is possible to keep the charge rate down and not use fast charging, this will improve the useful life of the cell.
Charge temperature: The li-ion battery charge temperature should be monitored. The cell or battery must not be charged when the temperature is lower than 0°C or greater than 45°C.
Lithium ion cells and batteries perform best when operating around room temperature, so charging between the temperature limits mentioned give the best charging and also extends battery life as well.
Discharge current: Discharge current protection is required to prevent damage or explosion as a result of short circuits. There will be a limit for the particular battery pack and this should not be exceeded. In view of the huge amounts of power stored, exceeding the limits can result in fire or even an impressive explosion.
Normally battery packs have charge / discharge management circuitry to ensure that the current capability is not exceeded, but it is always best not to over-stress them.
Different types of Li-ion battery technology are able to provide different capabilities - as a result the actual type of lithium ion battery technology to select will depend upon the application and the current / discharge capability needed.
Over-voltage: Charge over-voltage protection is required to prevent a voltage that is too high being applied across the battery terminals. if the charge voltage is allowed to rise too high then damage can be caused.
- Over-charge protection: The over-charge protection circuitry is required to stop the Li-ion charging process when voltage per cell rises above 4.30 volts. It is extremely important not to overcharge the lithium in battery. The battery management system must provide protection against over charging.
- Reverse polarity protection: Li-ion battery reverse polarity protection is needed to make sure the battery is not charged in the wrong direction as this could lead to serious damage or even explosion.
- Li-Ion over-discharge: Over-discharge protection is required to prevent the battery voltage falling below about 2.3 Volts dependent upon the manufacturer.
- Over temperature: Over-temperature protection is often incorporated to prevent the battery operating if the temperature rises too high. Temperatures above 100°C can cause irreparable damage.
When using a lithium ion battery, it is imperative that the manufacturers charger is used because different elements of protection may be used in the charger and battery pack dependent upon the design.
Li-ion charge discharge cycles
The lifetime of lithium ion cells and batteries is often given in terms f the number of charge discharge cycles they an accommodate before their charge retention capacity falls.
Although lithium ion cells have what is termed a calendar life - their lifetime in terms of elapsed time even if they are not used, another major factor is the number of charge discharge cycles they can accommodate. It is normally this rather than the calendar lifetime that spells the end of the useful life for a lithium ion cell.
Other characteristics of the Li-ion cell show improvements over its competitors. It has been shown to be able to withstand around 1000 charge / discharge cycles if used very carefully and still be able to hold 80% of its initial capacity.
Ni-Cads offer up to around 500 cycles, although this is very dependent upon the way they are used. A badly treated cell may only give 50 or 100. NiMH cells are even worse, and this is one of the main areas receiving development. They are only able to give 500 cycles at the very best before their capacity drops to 80% of the initial charge rating.
It is also found that lithium ion cells and batteries do not suffer from the memory effect that was apparent with Ni-Cads. The memory effect became apparent if cells were only partially discharged each time they were used. With time, they 'remembered' the level of discharge and their capacity was reduced accordingly. As a result it was good to periodically perform a complete discharge of the cells. This is not so for lithium ion cells.
The charging and discharging of lithium ion batteries is key to their operation and long term performance. Typically battery management chips are incorporated into the battery packs. This manages the charging and discharging of the li-ion battery. In this way the user can plug the battery into a charger and leave it to charge in the knowledge that it does not have to be unplugged after a certain time. The battery management chip will also ensure the battery is not discharged too far. The issue is to ensure that the battery management understands the exact state of the battery charge.
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