Guest author Mr Neeraj Kumar Singal talks about the Lithium-ion cells nomenclature, quality parameters, key requirements of the cell and cell segregation for grouping.
The article covers:
- Lithium-ion Cell Specifications and data sheets
- Important Terms related to cell/battery performance and their description
- Expectations from a good Lithium-ion cell
- Importance of each cell in a battery pack
- Acceptance parameters of the cells of a purchased lot
- Sorting – the process of grouping of cells expected to perform similarly
Lithium-ion Cell Specifications and data sheets
Cylindrical Cell is designated with a number e.g. 18650 and this cell would be with nominal dimensions of ‘18’ mm dia, ‘65’ mm length and is designated with ‘0’, it being cylindrical in shape.
Similarly, Prismatic Cells are designated with their nominal capacity in Ah e.g. FP55 would indicate that the Standard capacity is 55Ah @1C.
Each lot of cells is supplied with its important “Technical data sheet” or “Specification Sheet”. The important information may include:
1. Rated capacity in mAh or Ah at 1C – 1C is the rate of discharge at which the cell gets discharged fully in 1 hour
2. Nominal capacity in mAh or Ah at —C (e.g. “3000mAh at 0.2 C” means that at the rate of discharge of 3000mAh, the cell gets discharged in 5 hours)
3. Nominal, Charge & discharge voltages: operating – e.g. 3.6V, upper cut off – e.g. 4.2V and lower cut off -e.g. 2.5V
4. Maximum Charge & Discharge current: maximum permissible rate of charging & discharging Constant / variable current (e.g. 1C-2750mAh for charging and 2C-5500mAh for discharging- both at 25oC)
5. Recommended storage temperatures – commensurate with the duration of storage period
6. Cell operating & testing temperature(s)
7. Designed number of lasting cycles (cell cycle life): e.g. 3000 cycles with 80% capacity
8. Discharge capacity at various progressive cycles
9. Discharge capacity at various lower & higher temperatures & storage periods
10. Safety & environmental precautions
Important Terms related to cell/battery performance and their description
The state of charge is usually expressed as a percentage and is the extent of available electrical energy in the cell. As the available electrical energy varies with charge and discharge current, temperature and aging, the state of charge is also defined through use of two terms: Absolute State-Of-Charge (ASOC) and Relative State-of-Charge (RSOC). Relative state of charge remains in the range of 0%-100% (100% when fully charged and 0% when fully discharged). The absolute state of charge is a reference value calculated according to the designed fixed capacity value when the cell is manufactured. The absolute state of charge of a brand new fully charged cell is 100% and even if an aging cell is fully charged, it cannot reach 100% under different charging and discharging conditions.
Relationship between voltage and cell capacity at different discharge rates is represented in the figure below. It may be seen that with the higher discharge rate, the cell capacity lowers. Cell capacity also reduces at lower temperatures.
b) Maximum Charging Voltage
Though the nominal voltage of lithium ion cells with different chemistries varies between 3.2 to 3.7 V (with the exception of Lithium Titanate cell which has the nominal voltage of 2.4 Volts), the charging voltage of lithium cells is usually 4.2V and 4.35V, and this voltage value may change with the different combinations of the cathode and anode materials.
c) Fully Charged
As per widely acceptable norms, when the difference between the cell voltage and the highest charging voltage is less than 100mV, and the charging current drops to C/10, the cell can be considered to be fully charged. The figure below shows a typical lithium cell charging characteristic curve.
d) Minimum Discharging Voltage
The minimum discharge voltage is the cut-off discharge voltage at which the state of charge is 0%. This minimum discharge voltage value is not a fixed value & changes with load, temperature, aging or other factors.
e) Fully Discharge voltage
This voltage is less than or equal to the minimum discharge voltage.
f) Charge and discharge rate (C-Rate)
The charge and discharge rate is an expression of the charge and discharge current relative to the cell capacity which gives the indication of the time period the cell will continue to deliver energy during charging & how much time the cell will take to get charged fully. Term 1C is used to indicate the rate of current which will get the cell discharged fully in one hour. Different charge and discharge rates will result in different capacity of usage. Generally, the greater rate of charging & discharging, smaller is the available capacity.
g) Cycle life
A cycle is process of undergoing complete charge and discharge by a cell which can be estimated from the actual discharge capacity and the design capacity. Generally, after 500 charge-discharge cycles, the capacity of a fully charged cell drops by 10% to 20%.
All cells get self discharged when stored at high temperatures or for long duration. The rate of self-discharge increases with the rise in temperature. Possible reasons could be the deterioration of the ingredients of the electrodes & electrolytes and continuation of chemical reactions at a slow pace (even when not being used). However, improper handling & damage during the manufacturing process can also lead to increased self-discharge. Generally, the self-discharge rate doubles for every 10°C increase in cell temperature. The monthly self-discharge rate of lithium-ion cells is about 1 to 2% as compared to the monthly self-discharge rate of 10-15% for nickel-based cells.
Expectations from a good Lithium-ion cell
Cycle and calendar life determine the value for money for a cell. Cycle life ensures that the capacity of the cell remains above 80% when it has been already subjected to the number of cycles guaranteed by the manufacturer. Similarly, the declared calendar life is the assurance that whether used or not, the cell will have a minimum remaining capacity of 80% within the period of declared cycle life for the cell. With the higher rate of discharge, cell capacity decreases. Similarly, with the slower rate of discharge current, more energy is possible to be extracted from the cell as the deep drop in cell voltage is not encountered.
Optimally, the life of a ternary lithium cell is around 800 cycles, and it is around 2000 and 10000 cycles for lithium iron phosphate & lithium titanate cells respectively. As the Internal Resistance & voltage are different for each of the cells of the battery pack, it becomes very important to group the cells of similar performance while making a battery pack to ensure a good cycle life. As deep charging and discharging may cause the permanent damage to the battery, the recommended SOC (state of charge) range for the discharging – charging cycle is 10 to 90%. Further, if the discharging of lithium ion cells is regularly done at high-rate and high-temperatures, the life can get reduced by as much as 75%.
While purchasing a new battery, the price consideration invariably involves the comparison of initial per kilowatt-hour purchase cost. But logically, the cost should also consider the full life cycle cost, maintenance cost as well as the replacement cost. In simple terms, the full life cycle cost is the multiplication of the average power of the battery & its cycle life (number of cycles of power delivery within the desired & declared range). Cycle life of the battery is highly dependent on the life of each cell of the battery pack and the smart BMS which should be able to identify weak cells, isolate them and run the equalization function effectively. Similarly, the self maintenance of the battery through provision of the self corrective mechanisms without external intervention should work long term while external manual intervention to manually identify cells with low voltage & charge them separately or replace them, should be avoidable.
High energy density/power density
As the energy density (energy available per unit volume or weight) of lithium-ion cells is 2.5 & 1.8 times of nickel-cadmium and nickel-hydrogen cells respectively, they are no doubt superior in this are and consequently Li-ion battery packs have smaller space requirements leaving out more space for functional components of the device. Different Lithium-ion chemistries have variable energy densities and should be used based on the requirement as well as the safety concerns. More the concentration of energy at a single location or in a device / equipment and more the proximity of it to the humans, more intensive will be the safety requirements.
Nickel Metal Hydride cells have a voltage range of 1.4 to 1.6 Volts and nickel-cadmium and nickel-hydrogen cells have a typical voltage of 1.25V. The rate of discharge over a period of storage time is also high for these cells. Comparatively, Li ion cells have higher voltage range & their losses during storage are also lower. For lithium iron phosphate cells the nominal voltage is 3.6V and for ternary lithium & lithium manganate cells, it is 4.2V.
Because of the use of graphite anodes, the voltage of lithium cells is dependent on the cathode materials. Voltage of a cell can be increased through the choice of materials so that the cathode is made up of a material will high positive potential and anode material with a highly negative material. Development of cells with materials resulting in high cell voltage is another way to increase energy density and contain the non useful dead space of the appliance or store higher quantum of energy in bulk energy storage spaces.
High energy efficiency
Energy efficiency is the difference between the energy input during charging and the energy output during discharging. Invariably, the charging voltage is higher than the discharging voltage due to the Coulomb efficiency (charging efficiency – the ratio of cell discharge capacity to charging capacity during a set of cycle). Because of the internal resistance of the cell, the low heat gets generated both during the charging as well as discharging. The input and output electrical energy during charging & discharging mainly gets converted into heat energy. Better cells generate lower heat. Through Coulombic measurement, the difference between the charge & discharge energy capacities can be measured to compute the efficiency and better quality cells can be selected for use.
Good high temperature performance
Lithium cells need to have good high-temperature performance. When the cell core is at the higher temperature, the cathode, anode, separator and electrolyte of these cells should be able to maintain good stability and work normally at high temperatures to ensure the long expected life. Abnormal cell overcharge / over-discharge, fast charge /discharge, short circuit, mechanical abuse / mishandling and high temperature thermal shocks, can easily trigger undesired reactions inside the cells to generate heat and directly damage electrode surfaces. The temperature problem of lithium cells has a great impact on the safety of lithium cells and batteries.
Good low temperature performance
Lithium cells perform well at low temperatures and the lithium ions, electrolyte and electrode materials maintain high activity of chemical reactions, their remaining capacity is high, the discharge capacity is reduced, and the rate of charging remains high.
As the temperature drops, the remaining capacity of the lithium cell reduces faster. Forced charging at low temperatures is very harmful and may lead to thermal runaways. Except for the severe deterioration of discharge capacity, lithium cells cannot be charged at low temperatures. The main reasons for the decrease in the life of lithium-ion cells, when used at low temperatures, are the increase in internal impedance and the degradation of the capacity due to the precipitation of lithium ions.
Therefore, cell specification sheets mostly specify the working temperature range of the cell within which the performance of the cell/battery is expected to perform within the declared parameters.
The good quality input core materials and safe control measures of the cells determine the safe performance of the cells and resultant batteries. The materials of electrodes, electrolytes & separators should have good thermal stability, compatibility and the electrolyte should be good conductor with flame retardant properties. Safety auxiliary measures refer to the safety valve design of the cell, the fuse design, the temperature-sensitive resistance design, and the sensitivity. After a single cell fails, it can prevent the fault from spreading and isolate it.
For a battery pack to perform consistently, it is to be ensured that the cells are from the same source and from the same manufacturing lot so that their performance levels are comparable and variation in their individual performance parameters is very low. The performance parameters to be tested mainly include the internal resistance, capacity, open circuit voltage, time dependent self-discharge and temperature rise. The performance of a battery is highly dependent on the weakest cell and the life of the battery will be at par or less than the actual life span of the weakest cell.
Easy to assemble
For making battery packs, a large number of cells are arranged and connected to make them fit for use. The single cell is formed into a module using processes like welding & crimping and the module is connected through a high-voltage wire to form a battery pack. In this process, ease of single cells soldering, design of connection interface for crimping & suitability of thermal management system each cell of the battery affect the simplicity of the group design and the efficiency of the group. Some battery cells may have high cell density but their shapes may not be design or user friendly. After being processed into battery packs, the energy density is only half of total cells. If the individual cells are not well connected, the energy density of the battery cell will not get effectively utilized.
Importance of each cell in a battery pack
Capacity – While charging, the cell with smallest capacity gets fully charged first and BMS cuts off the charging process as the charging cut-off condition is attained. Similarly while discharging, a cell with small capacity would first release the available energy, and the system will stop discharging as the lower cut-off voltage condition is reached. So, in a battery comprising of many cells, the cells with small capacity are always fully discharged, while cells with large capacity always use part of the capacity and as a result the part capacity of the entire battery pack always remains idle.
Life loss – Life of a battery pack is dependent on the cell with the shortest life and once the shortest life cell deteriorates in performance, the battery output reduces drastically.
Internal resistance and temperature rise – Resistance of each cell is different and same current flows through cells with different internal resistances. The cells with large internal resistance get more heated and increase the battery temperature which, when too high, leads to accelerate the battery deterioration rate. Increased temperature causes the internal resistance to rise further. Internal resistance and temperature rise form a pair of vicious combination which progressively deteriorates the quality & output of the battery and shortens its life.
Cell management in a battery – Currently, engineers mainly consider three aspects to deal with the inconsistency & variability of single cells – sorting of cells to identify ones with similar performance, thermal management after grouping and use of good battery management system (BMS) to provide equalization when a small amount of inconsistency or variation occurs amongst the cells.
Acceptance of the cells of a purchased lot
Generally the Lithium-ion cells are imported directly in big lots or they can be purchased from the bulk importers of the cells. Whatever way procurement is done, first obtain the test certificates of the batch of cells and ascertain the number of batches in the lot. Ensure that the cells are as ordered & are suited for the desired application (e.g. solar, two wheeled mobility, energy storage etc).
To ensure quality of the cells received, develop a sampling plan using which the cell get picked for testing at the in-house or registered laboratory. Further, all imported cells need to get type approved by Bureau of Indian Standards through getting the samples tested in the BIS or BIS registered labs. The cells are tested as per IS 16046 Part 2 of 2018 and IS 16047 Part 3 of 2018. The key testing parameters include:
- Venting (for gas built-up)
- Charging at constant voltage
- Capacity at various temperatures
- External short circuit
- Free fall
- Thermal abuse
- Forced discharge
- Transport tests
The testing is done mostly as per the declared parameters of the cell manufacturer and also as per the requirements specified in the Indian standard. Once the type testing is satisfactorily completed, cells may be tested/processed etc. in India for the intended use.
Next process would be to sort out the cells to identify, segregate and use cells of electrical performance parameters within a specified range for ensuring good, safe and long lasting performance of the cells used in a device or battery.
Sorting – the process of grouping of cells expected to perform similarly
Ideally, different batches of cells should not be used together. Even cells of the same batch are screened and cells within the narrow band of properties are grouped together and in the same battery pack. The purpose of sorting is to select cells with similar parameters. The sorting method has been studied for many years and is mainly divided into static sorting and dynamic sorting.
Static sorting process checks the open circuit voltage, internal resistance, capacity and other parameters of the cells. The target parameters are selected, introduced to statistical algorithms, range of screening criteria is set and the cells of the same batch get divided into several groups.
Dynamic screening is based on the characteristics of the battery cell during the charging and discharging process. Some choose the constant current and constant voltage charging process, some choose the pulse shock charge and discharge process, and some compare their own charging and discharging curves relationships.
Static screening is used for initial grouping and dynamic screening which is supposed to be more precise, is performed later so that more groups with narrower range of parameters can be divided and screening accuracy is much higher. The cost rises as we make the selection range narrower and use better precision equipment for measurements.
Step by step process of sorting
1. Before sorting, cells are to be brought to same voltage value. Initially, they are fully charged to the upper cut-off value and then discharged to Depth of Discharge (DoD) of the lower cut-off value on the cell charge-discharge-testing machine. Thereafter, the State of Charge of 50% is attained for the cells. These cells are then taken to the sticker pasting & sorting machine combination.
2. For preliminary static sorting, sorting machines are used. These machines work on the principle of supplying a fixed voltage to the cell terminals and measuring the internal resistance of the cell.
Based on the ranges of resistance identified for various groups, each cell gets segregated & stored in the designated channels. Normally, 5 to 13 channel sorting machines are commonly used. Based on the chemistry of the cells and the specification sheet, the input voltage is decided and set. The resistance range for each channel is defined and fed in the software input of the equipment. Cells continuously move, their electrical internal resistances get measured and the pusher thrusts the tested cells in the correct channel.
If the sorted cells are to be used in precision devices, the sorted cells of a channel can be further segregated to the next decimal value by repeating the process of charge, discharge, attaining SOC 50% and re-sorting in a narrower resistance range.
Key factors in selection of a sorting machine are:
- Size of cylindrical cell to be tested
- AC 220V supply
- Range of input voltage (suitable for Lithium Ion cells of varied chemistries)
- Range of resistance as well as the sensitivity for capturing & reporting the variation in resistance (say range in 0 to 999ohm and least count 0.1 to 1 milliohm).
- Speed of measurement & output per hour (based on production requirements)
- Range setting options as per requirements
- Number of channels required
- Computer based test data feed system
- Test data storage & extraction facility
- Speed matching with the sticker pasting machine
- Technical & maintenance support
As most of the cells would fall under 5V range, upto 8 to 10V upper range, measurement capability (resolution of 0.1 mV) and internal resistance measurement range of upto 50-60 milliohms with resolution of 0.1mohm should be sufficient to take care of the entire range of cylindrical Lithium Ion cells. After the test, cells of the same batch get segregated on the basis of their internal resistance. Cells with narrow range of internal resistance are expected to have a close range of performance.
Sorting information display varies from machine to machine. However, mostly the voltage range, resistance range, number of cells in each range and % of the total segregated cells in each channel – are displayed on the equipment screen.
3. After segregation, the suitable cells falling within the required range are stored together & identified with batch indication. Such cells are sent to the cell testing & assessment equipment. On cell testing equipment, the cell charge, discharge, capacity, temperature rise, life estimation and loss of capacity due to storage or repeated use may be assessed.
About the Author
This write-up is authored by Neeraj Kumar Singal, Founder and CEO of Semco Group.
He is passionate about Clean Energy and working on various projects to build robust Lithium-Ion ecosystem. One of his ventures, SEMCO Infratech is a solution provider of Lithium-ion cell manufacturing and pack assembly equipments. He can be reached at firstname.lastname@example.org
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