Safe operation of lithium batteries with BMS

Lithium batteries need some care that requires the use of BMS for safe operation

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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 greatly 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 have long dominated the energy storage market.

Some advantages of lithium batteries are their high energy density, their fast charge and recharge capacity, 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 the lithium battery are accompanied by some precautions that require the use of BMS (battery management systems) for safe operation.

Safe operating area

Lithium batteries have an SOA (safety operations area) which imposes some restrictions such as:

Overvoltage: Overvoltage, which is an overcharge situation, must be avoided in lithium batteries. Overvoltage can cause the current to increase abruptly, which means that lithium ions can no longer accommodate properly in the anode's intercalation layers. This leads to the formation of metallic lithium on the anode, which can result in permanent loss of capacity or an internal short circuit.

Undervoltage: A very low state of charge, corresponding to a very low voltage level, can cause the anode copper collector to dissolve in the electrolyte, causing an internal short circuit. The cathode material may also break down, causing permanent loss of storage capacity.

Overtemperature: Excessive heating of the lithium battery can disintegrate the passivation layer and cause a reaction between the electrolyte and the anode. This leads to an uncontrolled increase in internal temperature through an exothermic reaction. As a consequence of overheating, the internal organic solvents of the electrolyte may break down, producing the release of flammable gases and increasing the internal pressure of the battery. Finally, the separator may melt, causing an internal short circuit.

Subtemperature: Excessive cooling of the battery is also not a desirable condition, as this reduces the speed of chemical reactions (reducing the charge capacity and slowing down recharging) and causes the creation of dendrites (which can cause an internal short circuit). Subtemperature is difficult to occur in Brazilian territory, being a more common problem in countries with cold climates, where temperatures can be very low in winter.

Figure 1: Lithium battery safe operating area (SOA). Source: PHB Eletrônica, SEPOC Tutorial, 2018

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.

Furthermore, the system is responsible for limiting these variables, preventing overload, overvoltage, undervoltage and overtemperature.

In addition to monitoring, the BMS also has the important function of equalizing the battery bank cells. 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 the schematic diagram of a BMS system and Figure 3 illustrates two possible schemes used to balance the cells. Passive balancing dissipates energy from overloaded cells, bringing them to the same state of charge as the others.

Active balancing allows the redistribution of energy between cells, making it the most efficient and preferred option in most modern BMS systems.

Figura 2: Diagrama esquemático de um sistema BMS. Fonte: PHB Eletrônica, Tutorial do SEPOC, 2018 — Figure 2: Schematic diagram of a BMS system. Source: PHB Electronics, SEPOC Tutorial, 2018

Figura 3: Estratégias de balanceamento de células empregadas pelos sistemas de gerenciamento de baterias (BMS). Fonte: PHB Eletrônica, Tutorial do SEPOC, 2018 — Figure 3: Cell balancing strategies employed by battery management systems (BMS). Source: PHB Eletrônica, SEPOC Tutorial, 2018

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 need all the care mentioned above and cannot simply be connected to an inverter or charge controller.

Furthermore, lithium batteries are rarely supplied individually, being more common to find them in the form of packs or battery banks with a built-in BMS.

Typically, battery bank manufacturers already develop and supply integrated solutions, which contain the cells and all the electronics necessary for the safe operation of the batteries.

Figure 4 illustrates the electronic board of the BMS system of a Tesla Motors vehicle, USA.

Figura 4: BMS eletrônico (acima) e bancos de baterias (abaixo) de um veículo elétrico da Tesla Motors. Fonte: PHB Eletrônica, Tutorial do SEPOC, 2018 — Figure 4: Electronic BMS (top) and battery banks (bottom) of a Tesla Motors electric vehicle. Source: PHB Eletrônica, SEPOC Tutorial, 2018

Figure 5 illustrates a lithium battery bank with integrated BMS developed in Brazil by PHB Eletrônica, in partnership with CPqD (Center for Research and Development in Telecommunications).

Figure 6 illustrates a system similar to that in Figure 5, produced by the international company Pylontech and sold in Brazil by PHB Eletrônica. Each module has a nominal voltage of 48 V and a nominal capacity of 50 AH.