Battery management system

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A battery management system ( BMS ) or simply battery management is a measure, but mostly an electronic circuit that is used for monitoring, regulating and protecting accumulators .

Battery box of an electric car model Hotzenblitz with 56 lithium iron phosphate battery cells from Winston Battery , BMS module for each individual cell and bus cabling

These are z. B. to charge status detection, deep discharge protection, overcharge protection or complex systems with data interfaces. In many mobile devices, the BMS also includes the automatic switchover of the power supply depending on whether a charger or a data interface ( USB ) with limited power supply is connected.

In some cases, operating data is also displayed or saved for service purposes. In the case of open lead or NiCd cells, battery management may also include manual replenishment of the missing water.

BMS can be necessary in particular when several accumulator cells are connected in series to form one battery and here in particular with lithium accumulators.

use

BMS can be found in various interconnections of accumulator cells , for example traction batteries of electric cars , uninterruptible power supplies , mobile phones or notebooks . One of the major challenges is determining the state of charge, which for many types of batteries can only be determined imprecisely on the basis of the cell voltage and the internal resistance. The display of the battery status or the remaining operating time has been significantly improved, but is often not reliable.

The simplest form of battery management is a charge controller . Systems for larger lithium batteries in particular, however, often have assemblies that individually monitor all battery cells connected in series and provide information about their status.

In the mass market, battery management systems became relevant in the wake of increasingly cheaper electronic power components and with the advent of lithium-ion accumulators. With these, overcharging and deep discharge of the individual cells must be avoided in order to avoid failures and further damage up to and including the risk of fire to the battery pack.

Lead accumulators

Due to the charging characteristic, lead-acid batteries can be protected against overcharging with a voltage limiter. In the case of starter batteries in automobiles with internal combustion engines, the battery management consists of the alternator regulator, which supplies a constant voltage. Open lead-acid batteries are more robust against overcharging - the energy that cannot be stored is converted into heat and oxyhydrogen, but water is lost. When used as a traction battery , the cyclical charging and discharging makes the lack of a BMS noticeable when the cells and blocks drift apart. It leads to deep discharge and subsequent failure of the defective cells. Lead-acid batteries (especially lead-gel batteries) require deep discharge protection , which prevents the cell voltage from falling below a certain value specified by the manufacturer due to the load or self-discharge.

NiCd / NiMh accumulators

Nickel-cadmium batteries and nickel-metal hydride batteries with a nominal 1.2 V cell voltage and a non-proportional charging characteristic require special charging processes (delta peak) with greater control effort and safe end-of-charge shutdown to prevent overheating or gassing. In contrast, deep discharge is not harmful. Driverless transport vehicles with NiCd batteries are monitored and logged down to cell level in one example and z. B. limited to a final charge voltage of 1.55 V to avoid excessive water loss. This can significantly extend the life of the batteries.

Lithium-ion accumulator

Lithium-ion batteries have more complicated charging characteristics than lead-acid batteries, the end-of-charge voltage must be adhered to very precisely, deep discharge leads to failure. Lithium batteries require deep discharge protection, which prevents the cell voltage from falling below a certain value specified by the manufacturer due to the load or self-discharge.

Depending on the chemical processes in the cells, non-linearities occur, for example with lithium-manganese batteries , or the cell voltage is almost constant over wide states of charge, such as with lithium-iron phosphate batteries . This requires monitoring both in the case of single cells and when connecting several cells in order to reliably prevent premature failure or overheating of individual cells in the event of overcharging or deep discharge. The development of the BMS was therefore driven forward with the spread of lithium batteries.

When using BMS with lithium-ion batteries, a charge and discharge control including balancing is used on the basis of temperature control, voltage diagnostics and the determination of the state of charge , whereby the latter ensures an equalization in the event of unequal charge states of the individual cells.

Use in the vehicle

When used in vehicle technology , a BMS also serves as an interface between the vehicle and the electronic components built into the battery. The BMS controls functions that are necessary for the current operating status of the vehicle. When the vehicle is switched off, the battery system is put into sleep mode. The BMS is regularly started briefly according to a software cycle time programmed in the BMS. During this “waking time”, the BMS carries out a test of the battery system in order to examine all data ( voltages , temperatures, etc.) for possible errors. At the same time, a BMS controls measures to optimize the performance and service life of the accumulator. This means, for example: to ensure a suitable temperature level through cooling and, if necessary, heating.

When the vehicle is started, a command is sent from the vehicle's control unit to the BMS, which then checks the status of the drive battery and closes the contactors of the battery system in order to supply the motor with power. If an error occurs in the battery system during operation, this is processed by the BMS and assigned to an error category - depending on the error, it is saved for later maintenance or displayed to the driver. In the worst case, the battery system is brought into a safe state by interrupting the traction current or switching off the battery completely.

Functions of battery management systems

Simple BMS for 4-cell battery pack: short-circuit protection, individual cell monitoring against overvoltage and undervoltage, balancer

The following can be regarded as standard functions in battery management systems:

  1. Cell protection
  2. Loading control
  3. Load management
  4. Determination of the state of charge
  5. Determination of the "cell health" (aging, residual capacity, internal resistance etc.)
  6. Balancing the cells
  7. history
  8. Authentication and identification
  9. communication
  10. Temperature monitoring and adjustment of the end-of-charge voltage

Standardization issues

Since the service life of the cells is shorter than that of the vehicle, and in order to enable the customer to choose from several competitors, a standardized interface from the BMS to the vehicle would be desirable. This means that the cell chemistry could develop independently when charging management is integrated into the BMS .

Individual evidence

  1. Battery system and charging strategy at the Audi e-tron heise.de, on April 20, 2018
  2. a b http://www.flurfoerderzeuge.de/batterieueberwachungssystem-fuer-fahrerlose-transportsysteme-nicd-batterienblei-akkumulatoren.html Wolfgang Degenhard: Battery management system ensures more economic efficiency, accessed on Jan. 21, 2018
  3. a b Simulated lithium-ion batteries. ( Memento from October 11, 2010 in the Internet Archive ) elektroniknet.de, June 16, 2009
  4. Battery Management Systems (BMS) www.mpoweruk.com Internet portal, "Electropaedia" section, "Battery and Energy Technologies" section (English)