Li-ion batteries

Li-ion batteries are the powerhouse for the digital electronic revolution in this modern mobile society. Lithium ion batteries are now popular in majority of electronic portable devices like Mobile phone, Laptop, Digital Camera, etc due to their long lasting power efficiency. These are the most popular rechargeable batteries with advantages like best energy density, negligible charge loss and no memory effect.

Cell Construction

A Li-ion battery is constructed by connected basic Li-ion cells in parallel (to increase current), in series (to increase voltage) or combined configurations. Multiple battery cells can be integrated into a module. Multiple modules can be integrated into a battery pack.

The best known lithium ion product is the cylindrical ‘18650’ cell used for many consumer applications. The numerical designation indicates that the cell is 18 mm in diameter and 65.0 mm long. For such cells the active materials are coated onto both sides of metal foils: aluminum for the positive and copper for the negative. Lengths of these foils are interleaved with a separator and wound around a circular mandrel or spindle to produce a ‘jelly roll.’ The jelly roll is placed inside a metal tube, forming the basis of the cell. There are also pseudo-prismatic (rectangular) cells that are made in the same way except that the jelly roll is produced by winding the electrodes around a flat mandrel.

Components

A basic Li-ion cell consists of a cathode (positive electrode) and an anode (negative electrode) which are contacted by an electrolyte containing lithium ions. The electrodes are isolated from each other by a separator, typically microporous polymer membrane, which allows the exchange of lithium ions between the two electrodes but not electrons. In addition to liquid electrolyte, polymer, gel, and ceramic electrolyte have also been explored for applications in Li-ion batteries.

Operation

Li-Ion battery uses Lithium ions as the charge carriers which move from the negative electrode to the positive electrode during discharge and back when charging. During charging, the external current from the charger applies an over voltage than that in the battery. This forces the current to pass in the reverse direction from the positive to the negative electrode where the lithium ions get embedded in the porous electrode material through a process called intercalation. The Li- Ions pass through the non aqueous electrolyte and a separator diaphragm. The electrode material is intercalated lithium compound.

The negative electrode of the Li-Ion battery is made up of carbon and the positive electrode is a metal oxide. The most commonly used material in the negative electrode is Graphite while that in the positive electrode may be Lithium cobalt oxide, Lithium iron phosphate or Lithium manganese oxide. Lithium salt in an organic solvent is used as the electrolyte.

The electrolyte is typically a mixture of organic carbonates like Ethylene carbonate or Diethyl carbonate containing lithium ions. The electrolyte uses anion salts like Lithium hexa fluoro phosphate, Lithium hexa fluoro arsenate monohydrate, Lithium per chlorate, Lithium hexa fluoro borate etc. Depending upon the salt used, the voltage, capacity and life of the battery varies. Pure lithium reacts with water vigorously to form lithium hydroxide and hydrogen ions. So the electrolyte used is non aqueous organic solvent. The electrochemical role of the electrodes charge between anode and cathode depends on the direction of current flow.

In the Li-Ion battery, both the electrodes can accept and release lithium ions. During the Intercalation process, the lithium ions move into the electrode. During the reverse process called de intercalation, the lithium ions move back. During discharging, the positive lithium ions will be extracted from the negative electrodes and inserted into the positive electrode. During the charging process, the reverse movement of lithium ions takes place.

Hulikkal’s Lithium ion Battery

Lithium ion batteries are preferred in electric vehicles due to the improved performances, such as high acceleration rate and long driving distance from a single charging at a lesser time duration. Hulikkal batteries can be charged 100% within one and a half hours without compromising on the performance and life of the battery. Hulikkal has designed and successfully tested various batteries such as, 48V-8Ah, 48V-12Ah, 48V-16Ah, 48V-20Ah, 48V-50Ah and 48V-75Ah for various vehicular applications. Hulikkal is proud to announce that 1000s of vehicles employing lithium-ion batteries reporting trouble free since their practical operation in field.

Need for Battery Management System (BMS)

The battery management system (BMS) is a critical component of a lithium-ion battery that is used in electric and hybrid electric vehicles. The purpose of the BMS is to guarantee safe and reliable battery operation. Charge control, cell balancing and state-of-health monitoring are the main functionalities that have been incorporated in a BMS. As an electrochemical product, a battery acts differently under different operational and environmental conditions. BMS helps the battery to perform under different conditions, as per the application requirement, without any battery abuse and thus extends the life of the battery. BMS is powered by the battery and the BMS idle power must be significantly less than the self-discharge of the battery cells.

Hulikkal’s BMS

BMS design and manufacturing is a specialised field of Power Electronics; our experience in the field has helped us to custom-build our own BMS, as per the local usage conditions. Basic design of the BMS is relatively easy to achieve based only on voltage and current, which are often called as PCBs. However, only well experienced BMS manufacturer will understand the following intricacies like self discharge properties of cells with respect to age and temperature, rate of charge and discharge optimization through BMS, power delivery for next few seconds, accurate coloumb counts (SOC) with respect to battery pack age, BMS calibration with battery-package, cost performance optimisation, re-design iterations based on field inputs. With our rich working experience over the years with our prestigious technical partners, we are able to create accurate chip level designs, firmware support, BMS diagnostic tool and software, and specific BMS design as per the cell source based on its chemistry and type, as per local usage requirements. This enables ease of repair on battery pack (cell replacement irrespective of cell ageing), series and parallel optimisation with smaller cells, etc.

Charging any Li-ion cell to 100% of its state-of-charge (SOC) or discharging to 0% SOC will degrade its capacity at a faster rate over the period of usage. Therefore, a small portion of a cell’s capacity should not be used to have a longer life. With very accurate control of the state of charge of each Li-Ion cell by using a good BMS, battery pack life can be maximized while its degradation is minimized. However, controlling hundreds of series-parallel connected cells is quite challenging. However, Hulikkal has an edge over other players in the field due to its strategic relationship with Internationally reputed BMS Partners.

Modularity

Hulikkal has access to a magic module that is built with a certain group of individual cells after long years of R&D and testing. This module enables to build any battery pack (from e-scooter to e-busses) by using these modules as building blocks to connect them in series-parallel. This modularity enables easy battery pack design, product launch, manual handling, repairs / servicing & battery swapping.