Thursday, 11 February 2016

Lithium ion batteries



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Lithium ion batteries,These batteries are now becoming popular especially in telecom sectors,Space projects etc...because of their charging and discharging feature,Compact,energy density and finally the monitoring feature involved in Li-Ion batteries.

Working of a Battery?

The batteries is a storage device,that converts chemical energy.The batteries are having 3 major parts or components.Anode(negatively charged),Cathode(Positively charged) and Electrolytes.

For example,Take one container and fill it with diluted sulfuric acid as the electrolyte. Now immerse zinc and one copper rod in the solution and connect them externally by an electric load. Now your simple voltaic cell is completed. Current will start flowing through the external load.

Zinc in diluted sulfuric acid gives up electrons as below:

                                                  
These Zn + + ions pass into the electrolyte, and their concentration is very high near the zinc electrode. As a result of the above oxidation reaction, the zinc electrode is left negatively charged and hence acts as cathode. The diluted sulfuric acid and water disassociate into hydronium ions as given below:

                                       
Due to the high concentration of Zn + + ions near the cathode, the H3O+ ions are repelled towards the copper electrode and get discharged by removing electrons from the copper atoms. The following reaction takes place at the anode:

                                       
As a result of the reduction reaction taking place at copper electrode, copper is left positively charged and hence it acts as the anode.


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Lithium ion Battery?

These batteries having better features compared to other lead acid or other batteries because there will be a monitoring section provided which can give you all real time data's at that time, means Voltage of individual cells,battery pack,total voltage,cell temperature,Over current or Under current etc...., each and every parameters of the battery including SOH and SOC also tracked using the in-build BMS(Battery Monitoring System).


The Lithium ion batteries have high energy density, means the amount of energy stored per unit volume is higher than the traditional Lead acid batteries so that the size of the battery was compact too.

The Li-Ion is a lightweight metal so that it is easy to form ions at the time of chemical reaction,The Li-ion battery can store as much as twice of the power Lead Acid battery can hold and also these batteries are also environmentally friendly but susceptible to 100% discharge so 80% discharge is recommended.

Let us take an example of 48V/75AH Li-Ion battery,so it has 15 no of 3.2V cells connected in series,thus makes you 48V battery.thus for the same size it give twice the power of other battery types.

These batteries have some modern electronics involved to better understand each and every parameters,so mainly BMS (Battery monitoring system),Contractors,Temperature sensors,Shunt,RS485,Data Logger,CAN.

The BMS may contain MOSFET to replace contractors which are responsible for charging and discharging,The BMS can able to control the battery from fail due to any Over or Under current/Voltage/Temperature,also it has alarms in case of any problems occurred so that these BMS are very helpful to protect batteries by monitoring all possible parameters.


RS485 which is used to monitor the parameters by connecting PC/Laptop.CAN communication is used to communicate between each battery so that the data logger can store the data of all the batteries.Data logger is used to log all the parameters of the battery and generally the Master module hold data logger and CAN communication allow to get information from each battery.


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The commercially used negative electrode is graphite and  positive electrode will be one of these three materials- lithium cobalt oxide, a polyanion-such as lithium iron phosphate or a spinel -such as lithium manganese oxide.The electrolyte will be a mixture of some organic carbonates such as ethyliene carbonate or diethyl carbonate containing complexes of Li ions,

Charging:

The external voltage was given through any external charger with the same polarity is applied between the electrodes, thus the charging process will start. The lithium atoms leave the metal oxide structure and ionize into Li+ ions under the release of an electron of each one. In this process Li+ ions diffuse to the negative electrode(anode). At the surface of the graphite particles the Li+ ions and electrons recombine with each other forming neutral lithium atoms and are reintercalated into the molecular structure of the graphite particles,otherwise Lithium is deposited in the anode.When no more ions will flow, the battery is fully charged and ready to use.

Ref:TUM EES YouTube Channel

Discharging

During discharge, lithium atoms oxidize by forming Li+ ions and electrons, whereas Li+ ions move to the positive electrode diffusing through the electrolyte and the separator. The electrons flow from the negative electrode to the positive on the external circuitry, where the resulting current flow can be used for an application. At the positive electrode the electrons recombine with the Li+ ions and are stored in the molecular structure of the active material, otherwise Lithium is deposited in the cathode.When all the ions have moved back, the battery is fully discharged and needs charge again.