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 basics
Charging lithium ion batteries is very different to charging Ni-Cads or NiMH batteries.
Charging lithium ion batteries is voltage sensitive rather than current based. Charging lithium ion batteries is more akin to charging lead acid batteries.
Differences are found in that the lithium ion batteries have a higher voltage per cell. They 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.
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.
Li-ion battery charging
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 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.
Lithium ion battery charging precautions
In view of the amount of energy stored in lithium ion batteries and te 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.
- 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.
- Charge current: Discharge current protection is required to prevent damage or explosion as a result of short circuits.
- Over-voltage: Charge over-voltage protection is required to prevent a voltage that is too high being applied across the battery terminals.
- 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.
- 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 characteristics
Other characteristics of the Li-ion cell show improvements over its competitors. It has been shown to be able to withstand over 1000 charge/discharge cycles 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.
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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