Li-ion Lithium Ion Battery Charging
- details of the methods and precautions for charging lithium ion batteries and cells and basic concepts of how to charge a li-ion cell or battery.
In order to obtain the best from lithium ion batteries and to ensure they operate safely, it is necessary to charge them in the correct manner.
By charging lithium ion batteries in the correct manner, and also discharging them in the correct manner, it is possible to ensure longer life and better performance.
However to charge lithium ion batteries correctly requires additional circuitry around them. Often lithium ion batteries have associated charging circuitry built into the overall battery pack to ensure they will at all times be operated within the required envelope.
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 must limited. Typically the maximum value is 0.8C, but lower values are more usually set to give some margin.
- Charge temperature monitoring - the cell or battery must not be charged when the temperature is lower than 0°C or greater than 45°C.
- Discharge current protection is required to prevent damage or explosion as a result of short circuits.
- Charge over-voltage protection - protection is required to prevent a voltage that is too high being applied across the battery terminals.
- Over-charge protection - stops charge when voltage per cell rises above 4.30 volts.
- Reverse polarity protection.
- Over-discharge protection is required to prevent the battery voltage falling below about 2.3 Volts dependent upon the manufacturer.
- 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.
By Ian Poole
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