What is a Li-Ion or lithium ion battery?
A lithium ion (Li-Ion) battery is a type of rechargeable battery commonly used in portable electronic devices, such as mobile phones, laptop computers, digital cameras, electric bicycles, hand tools, and energy storage devices, among others.
These batteries work through an electrochemical process in which lithium ions move between a negative graphite electrode and a positive lithium metal oxide electrode during charging and discharging. This movement of lithium ions is what generates the electrical current used to power the devices.
Lithium ion cells have a nominal voltage of around 3.7 volts. However, during charging and discharging, the actual voltage can vary from around 4.2 volts at full charge to around 3.0 volts at full discharge, depending on the exact chemistry of the battery and the manufacturer's specifications.
Lithium-ion batteries are popular due to their high energy density, meaning they can store large amounts of energy in a relatively small and lightweight size compared to other types of rechargeable batteries. Additionally, they have a low self-discharge rate compared to other rechargeable battery technologies, meaning they can retain their charge for long periods when not in use.
However, lithium-ion batteries also have some limitations, such as their sensitivity to overcharging and deep discharge, which can reduce their lifespan if not handled correctly. They can also be prone to inflammation or even explosion if physically damaged or used incorrectly. Furthermore, another problem that arises when building battery packs made up of individual elements is the imbalance of the cells.
What is lithium ion battery cell imbalance?
Lithium-ion battery cell imbalance refers to disparities in the state of charge or discharge between the different individual cells that make up the battery. This imbalance can arise due to a variety of factors, including:
- Manufacturing Variations: Individual cells in a lithium-ion battery may have slight differences in terms of capacity, internal resistance, or other characteristics due to variations in the manufacturing process.
- Uneven use conditions: Individual cells may experience different charge and discharge conditions due to factors such as ambient temperature, applied charge and discharge current, frequency of use, and level of mechanical stress.
Cell imbalance can have several negative effects, including:
- Reduced effective capacity: If some cells reach their maximum voltage before others during charging, full battery charging will stop before all cells are fully charged, resulting in reduced effective battery capacity.
- Permanent Damage: If some cells are discharged below their minimum voltage during discharge, they may suffer permanent damage, such as dendrite formation or loss of capacity, which can reduce battery life and affect long-term performance. term.
- Operational instability: Cell imbalance can cause unstable battery operation, which could result in uneven current distribution during charging and discharging, increased temperature and risk of failure or dangerous situations, such as overheating. or inflammation.
To address these issues, the battery management system (BMS) actively monitors and controls the charge and discharge status of each individual cell, balancing the cells to ensure optimal and safe performance of the battery as a whole. This involves redistributing charge between cells during charging and discharging to keep them all within safe and uniform voltage and capacity ranges.
What is a battery's BMS and what is it for?
A Battery Management System (BMS) is a crucial component in lithium-ion batteries and is used to monitor and control various aspects of their operation. Some of the main functions of a BMS in this type of batteries are:
- Overcharge Protection: The BMS monitors the voltage of each battery cell and prevents the battery from being charged above its maximum safe voltage. This is essential to prevent damage to the cells and to ensure safety during charging.
- Over discharge protection: The BMS also monitors cell voltage during discharge and prevents the battery from discharging below a safe level. Discharging a lithium-ion battery below a certain level can permanently damage the cells and reduce their lifespan.
- Cell Balancing: Differences in capacity or state of charge between individual cells may arise due to manufacturing variations or uneven use conditions. The BMS controls the charging and discharging of each cell to keep all cells balanced in terms of voltage and capacity.
- Temperature monitoring: Lithium-ion batteries can become unstable and dangerous if they become too hot during charging or discharging. The BMS monitors the temperature of the battery and, if an abnormal temperature rise is detected, it can take measures to reduce the current or interrupt charging to prevent damage.
- Short Circuit and Overcurrent Protection: The BMS can detect short circuit conditions or abnormally high currents and disconnect the battery to prevent damage or safety risks.
In summary, the BMS is essential to ensure safe and efficient performance of lithium-ion batteries by monitoring and controlling factors such as charge, discharge, cell balance and temperature.
What types of BMS for lithium ion battery exist?
There are several types of battery management systems (BMS) for lithium-ion batteries, and the choice of BMS type depends on several factors, such as the size and complexity of the battery system, monitoring and control needs, and the desired level of integration. Some of the common types of BMS include:
- BMS integrated into the battery: In this design, the BMS is integrated directly into the battery and is responsible for monitoring and controlling the status of each individual cell. This type of BMS is common in smaller scale lithium-ion batteries, such as those used in portable electronic devices. This is the type of BMS that we generally find in this section of our website.
- Standalone BMS: In this approach, the BMS is a separate unit that connects to the battery and is responsible for monitoring and controlling the health of the battery as a whole. This type of BMS is used in larger scale battery systems, such as those used in electric vehicles or stationary energy storage systems.
- Distributed BMS: In this design, each battery module has its own BMS that monitors and controls the status of that specific module. These BMSs can communicate with each other to coordinate operations and share information about the health of the battery as a whole.
How are lithium ion batteries connected to make battery packs?
When it comes to connecting lithium-ion batteries to form battery packs, there are several common approaches:
- Series Configuration: In this configuration, individual cells are connected in series, meaning that the positive terminal of one cell connects to the negative terminal of the next cell. This increases the total voltage of the pack while keeping the battery capacity equal to that of a single cell.
- Parallel Configuration: In this configuration, individual cells are connected in parallel, meaning that the positive terminals of all the cells are connected to each other, as are the negative terminals. This increases the total capacity of the pack while maintaining the voltage equal to that of a single cell.
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When you connect two lithium-ion batteries in parallel, they are not necessarily balanced with each other in terms of voltage and capacity. Parallel connection simply means that the positive terminals of both batteries are connected to each other, as are the negative terminals. This increases the total capacity of the battery system while maintaining the same voltage.
However, balance between individual batteries in a parallel system is not automatically guaranteed. Differences in capacity and state of charge between individual batteries, different aging, can result in charge and discharge imbalance, which can lead to uneven current distribution and affect overall system performance.
- Series and parallel combination: This is a combination of the series and parallel configurations, where groups of cells are connected in series and then these groups are connected in parallel with each other. This approach is used to increase both the voltage and the total capacity of the pack.
In any configuration, it is important to consider the need for proper balance between cells to avoid voltage and capacity imbalances, which can be managed by the corresponding BMS.
There is a standard nomenclature to identify the assembly of individual cells to form larger battery packs using the letter S and the letter P. The letter S is used to determine the series connection, while the letter P indicates a connection in series. parallel.
- 2S Li-Ion: It means that there are two individual cells connected in series. This format increases the total voltage of the battery pack, keeping the capacity the same.
- 3S Li-Ion: Indicates that there are three individual cells connected in series. This increases the total voltage of the battery pack to approximately three times the voltage of a single cell, while keeping the capacity the same.
- 4S Li-Ion: It means there are four individual cells connected in series. This increases the total voltage of the battery pack to approximately four times the voltage of a single cell, while keeping the capacity the same.
- And so on: The letter "S" followed by a number indicates the number of cells connected in series in the battery pack. The more cells that are connected in series, the higher the total voltage of the battery pack.
These notations are important for quickly understanding the nominal voltage of the battery pack and are widely used in the industry to describe the specifications of lithium-ion batteries and their applications.
Example: 4S2P Lithium Ion Battery
The "4S2P" notation refers to the configuration of a lithium-ion battery where the individual cells are connected in both series (S) and parallel (P).
"4S" indicates that there are four individual cells connected in series. When the cells are connected in series, the total voltage of the battery pack is the sum of the voltages of each cell. Therefore, if each individual cell has a nominal voltage of around 3.7 volts, then a "4S" battery pack would have a total nominal voltage of around 14.8 volts (3.7V/cell * 4 cells).
"2P" indicates that there are two groups of cells connected in parallel. Each group of cells in parallel provides the same capacity as a single cell. This is done to increase the total capacity of the battery pack while maintaining the same voltage.
In short, in a "4S2P" configuration, you have four lithium-ion cells connected in series, providing a total voltage of around 14.8 volts, and two of these groups of cells are connected in parallel to increase the capacity of the pack. of batteries.
To make the different battery packs, nickel foil is used, which is welded using a spot welding station.
