Battery management system

A battery management system ( BMS) is an electronic circuit which is used to monitor and control a rechargeable battery, so a storage battery system. The need for a BMS results from the interconnection of several battery cells of a battery. The BMS is doing the unavoidable production-related variations of various parameters of the battery cells, such as capacitance and leakage currents detect, monitor, and compensate. Partly also operating data are displayed or stored for service purposes.

Use

BMS can be found in a wide variety of interconnections of battery cells, such as traction batteries for electric cars, uninterruptible power supply or notebook.

The simplest form is a charge controller for a few cells. However, modern systems often have complex control systems that monitor a variety of battery cells and provide information on their status.

One of the major challenges is to monitor the state of charge, which is difficult to determine in many battery types. Brand new relevance learns a battery management system in conjunction with increasingly cheaper electronic power components and the advent of lithium-based batteries, in which the individual cells must be closely monitored.

BMS used for various battery types

Use in lead-acid batteries

Lead-acid batteries with a cell voltage of 2 V / cell have a simple charge curve, and are relatively robust against overcharging. The non- storable energy is converted into heat. In conventional lead-acid batteries with 3,6,12 cells ( 6,12,24 V) is therefore usually no need for a BMS. When used as traction battery becomes noticeable by the cyclic loading / unloading the lack of a BMS in the drifting apart of the cells and blocks noticeable. It leads to deep discharge and subsequent failure of the defective cells.

Trials on the use of circuits for cell balancing, eg the BMS, it was with the development of electric cars from about 1990. However, the solutions were also not successful due to the time to expensive electronics.

Use with NiCd / NiMh batteries

These battery cells with only 1.2 V cell voltage and a non-proportional loading characteristics require special charging method (delta peak) with higher control effort and safer Ladeendabschaltung to prevent overheating. The use of BMS is complicated by the low cell voltage. In traction mode with 180 V 150 individual cells were monitored.

Use in lithium -ion batteries

Lithium-based batteries have a simple, proportional charge curve, similar to the lead-acid batteries. However, they are very sensitive to over-or under-voltage condition and in series a plurality of cells perform surveillance in order to prevent premature failure or overheating safe with overload. The development of the BMS was therefore forced to the dissemination of Lixx batteries.

When using the BMS for lithium -ion batteries comes in addition to the temperature control, the diagnosis and the state of charge determination, therefore, especially the charge and discharge control, including balancing to bear, the latter provides for unequal charge states of the individual cells of a convergence.

In-vehicle use

When used in automotive engineering a BMS also serves as an interface between the vehicle and the built- in battery electronic components. Thereby functions are controlled via a BMS, which are necessary for the current operating conditions of the vehicle. Thus, in a switched-off vehicle, the battery system is put into sleep mode. The BMS is thereby started shortly after a regularly programmed in the BMS software cycle time. During this " awake time " is performed by the BMS, a check of the battery system to examine all data (voltages, temperatures, etc.) is in error. At the same time controls a BMS measures to optimize the performance and service life of the battery. This means, for example, to ensure, through cooling and heating if necessary for an appropriate temperature level.

If the vehicle is started, a command from the control unit of the vehicle is sent to the BMS, which then controls the state of the drive battery and the contactors of the battery system closes to energize the motor in the specially programmed with current cases. If, during operation, a fault in the battery system, so this is processed by the BMS and associated with an error category - according to the error, a message will be sent to the driver and in the worst case, the battery system is brought to a safe state ( the contactors open ).

Functions of battery management systems

The standard functions for battery management systems can apply:

Standardization issues

After the service life of the cells is lower than that of the vehicle, and to allow the customer to select from a plurality of peers, a standardized interface of the BMS of the vehicle is desirable. This could develop the cell chemistry independently on at integrating the charging management in the BMS.