Lithium batteries are preferred today for many energy applications. Much of the success of lithium batteries is due to electric vehicle market, which in recent years has significantly boosted research and development in battery technologies and also in the area of electronics. Lithium batteries have superior characteristics to their lead-acid predecessors, which for a long time dominated the energy storage market.
Some advantages of lithium batteries are their high energy density, their ability to charge and recharge quickly, their high efficiency (96%, considering a complete charge and discharge cycle) and their ability to operate with partial states of charge. However, the advantages of lithium batteries are accompanied by some precautions that require the use of BMS (battery management systems) for safe operation.
Safe operating area
Lithium batteries have a SOA (safety operation area) that imposes some restrictions such as:
Overvoltage: Overvoltage, which is an overcharging situation, should be avoided in lithium batteries. Overvoltage can cause an abrupt increase in current, which causes the lithium ions to no longer be able to accommodate themselves properly in the anode intercalation layers. This leads to the formation of metallic lithium in the anode, which can result in permanent loss of capacity or internal short circuit.
Undervoltage: A very low state of charge, corresponding to a very low voltage level, can cause the anode's copper collector to dissolve in the electrolyte, causing an internal short circuit. The cathode material may also break down, resulting in permanent loss of storage capacity.
Overtemperature: Excessive heating of the lithium battery can disintegrate the passivation layer and cause the electrolyte to react with the anode. This leads to an uncontrolled increase in the internal temperature through an exothermic reaction. As a consequence of overheating, the organic solvents inside the electrolyte can break down, releasing flammable gases and increasing the internal pressure of the battery. Finally, the separator can melt, causing an internal short circuit.
Undertemperature: Excessive cooling of the battery is also not a desirable condition, as this reduces the speed of chemical reactions (reducing the charging capacity and slowing down recharging) and causes the creation of dendrites (which can cause an internal short circuit). Undertemperature is unlikely to occur in Brazil, and is a more common problem in countries with cold climates, where temperatures can be very low in winter.
BMS: management system
The BMS (battery management system) is an electronic system responsible for monitoring and controlling lithium batteries. The BMS monitors the voltage, current, temperature and state of charge of the batteries. In addition, the system is responsible for limiting these variables, preventing overcharging, overvoltage, undervoltage and overtemperature.
In addition to monitoring, the BMS also has the important function of equalizing the cells in the battery banks. Lithium batteries are always found in the form of banks that contain a certain number of cells connected in series. In series connections, it is not enough to monitor the total voltage; it is important to monitor and control the voltage of each cell individually.
Figure 2 shows a schematic diagram of a BMS system, and Figure 3 illustrates two possible schemes used for cell balancing. Passive balancing dissipates energy from overcharged cells, bringing them to the same state of charge as the others. Active balancing allows energy to be redistributed among cells, and is the most efficient and preferred option in most modern BMS systems.
The BMS, in addition to its monitoring, protection and control functions (cell balancing), must also have a communication interface for exchanging information with external equipment (inverters, charge controllers, etc.).
Examples of BMS applications
Unlike traditional lead-acid batteries, lithium batteries require all the care mentioned above and cannot simply be connected to an inverter or charge controller. Furthermore, lithium batteries are rarely supplied separately; they are more commonly found in the form of battery packs or banks with a built-in BMS.
Typically, battery bank manufacturers already develop and supply integrated solutions that contain the cells and all the electronics required for the safe operation of the batteries. Figure 4 illustrates the electronic board of the BMS system of a Tesla Motors vehicle, USA.
Figure 5 illustrates a lithium battery bank with integrated BMS developed in Brazil by PHB Eletrônica, in partnership with CPqD (Telecommunications Research and Development Center). Figure 6 illustrates a system similar to that in Figure 5, produced by the international company Pylontech and marketed in Brazil by PHB Eletrônica. Each module has a nominal voltage of 48 V and a nominal capacity of 50 AH.
