What is EV BMS and how does it work

Battery management system (BMS) is a technology specially used to supervise the battery pack. The battery pack is a component of the battery unit, which is electrically organized in a row x column matrix configuration to provide the expected load of voltage and current in the target range over a period of time. The supervision provided by EV BMS usually includes:

Monitoring battery, Provide battery protection, Estimate the operating state of the battery.

Continuously optimize battery performance, Report operation status to external equipment.

BMS

How does the battery management system work?

The battery management system does not have a set of fixed or unique standards that must be adopted. The scope of technical design and the functions realized are usually related to: cost, complexity and size of battery pack, battery application and any safety, life and warranty issues.

According to the certification requirements of various government regulations, if the functional safety measures are not in place, the cost and punishment are the most important.

EV BMS has many design functions, including battery pack protection management and capacity management. We will discuss here how these two functions work. Battery pack protection management has two key areas: electrical protection, which means that the battery is not allowed to be damaged due to use outside its safe operating area (SOA), and thermal protection, involving passive and / or active temperature control to maintain or bring the battery pack into its SOA.

Electrical Management Protection: Current

Monitoring battery pack current and cell or module voltages is the road to electrical protection. The electrical SOA of any battery cell is bound by current and voltage. A well-designed EV BMS will protect the battery pack by preventing operations that exceed the manufacturer’s battery rating. In many cases, to extend battery life, it is possible to further derate to reside within the SOA security zone.

Lithium-ion batteries have different current limits for charging and discharging, and both modes can handle higher peak currents, albeit for short periods of time. Cell manufacturers typically specify maximum continuous charge and discharge current limits, as well as peak charge and discharge current limits. EV BMS that provide current protection will definitely apply maximum continuous current. However, sudden changes in load conditions may be taken into account prior to this; For example, sudden acceleration in electric cars. EV BMS can combine peak current monitoring by integrating current and increment time to determine whether to reduce available current or cut off battery pack current altogether. This makes the EV BMS almost instantaneous sensitive to extreme current spikes, such as short circuits that have not attracted the attention of any permanent fuses, but can tolerate peak demands as long as they are not excessively long.

Electrical management protection: voltage

Lithium ion batteries must operate within a certain voltage range. These SOA boundaries will ultimately be determined by the internal chemical properties of the selected lithium-ion battery and the battery temperature at any given time. In addition, since any battery pack will experience a large number of current cycles, discharge due to load demand and charge from various energy sources, these SOA voltage limits are usually further limited to optimize battery life. EV BMS must know what these limits are and will make decisions based on how close they are to these thresholds. For example, when approaching the high voltage limit, the EV BMS may require a gradual reduction in the charging current, or a complete termination of the charging current if the upper limit is reached. However, this limitation is usually accompanied by additional inherent voltage hysteresis considerations to prevent control chattering with respect to the shutdown threshold. On the other hand, when approaching the low voltage limit, EV BMS will require critical active violation loads to reduce their current demand. In the case of electric vehicles, this can be achieved by reducing the allowable torque available to the traction motor. Of course, EV BMS must put the driver’s safety first and protect the battery pack from permanent damage.

Thermal management protection: temperature

On the surface, the operating temperature range of lithium-ion battery seems to be very wide, but the overall battery capacity will decrease at low temperature due to the significant slowdown of chemical reaction rate. In terms of low temperature performance, their performance is much better than that of lead-acid or Ni MH batteries; However, temperature management is important because charging below 0 ° C (32 ° f) can cause physical problems. When charging at low temperature, metal lithium coating will appear on the positive electrode. This is permanent damage, which will not only reduce the capacity, but also make the battery more prone to failure if subjected to vibration or other pressure conditions. EV BMS can control the temperature of the battery pack by heating and cooling.

Importance of battery management system

A complete battery energy storage system, can consist of dozens, hundreds or even thousands of lithium-ion batteries, depending on the application. The rated voltage of these systems may be less than 100V, but may be as high as 800V, and the power supply current of the battery pack may be as high as 300A or higher. Any mismanagement of the high-pressure group may lead to life-threatening catastrophic disasters. Therefore, EV BMS is very important to ensure safe operation. 

Conclusion:

Introducing EV BMS into battery system will increase the cost, and the battery pack is expensive and potentially dangerous. The more complex the system, the higher the safety requirements, resulting in the need for more EV BMS supervision. However, EV BMS protection and preventive maintenance in terms of functional safety, life and reliability, performance and scope, diagnosis, etc. can ensure to reduce the overall cost, including warranty related costs. If you want to know more about lithium battery, please contact Maxworld Power.

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