Battery monitors and application for different types of batteries
and battery monitoring
Use with lead accumulators
Lead-acid accumulators with a cell voltage of 2 V / cell have a simple charging characteristic and are relatively robust against overcharging. The non-storable energy is converted into heat and oxyhydrogen. In conventional lead-acid batteries with 3, 6 or 12 cells (6, 12, 24 V), therefore, a battery monitor system is usually dispensed with. When used as a traction battery, the cyclic charging and discharging makes the absence of a BMS noticeable in the drifting of the cells and blocks. It leads to the deep discharge and subsequent failure of the defective cells. Attempts to use circuits for cell balancing, e.g. Of a BMS, there was the development of electric cars from about 1990 onwards. However, the solutions could not be achieved because of the electronics that were too expensive at the time.
Use with NiCd / NiMh accumulators
These cells with only 1.2 V cell voltage and a non-proportional charging characteristic require special charging processes(delta peak) with a higher regulating voltage and a safe charging end switch to prevent overheating. However, the use of BMS is made difficult by the low cell voltage. In traction mode with 180 V 150 individual cells would be monitored.
Use with lithium-ion batteries
Lithium-based accumulators have more complicated charging characteristics than lead-acid accumulators. Depending on the chemical processes in the cells, nonlinearities occur (among others in LiMn2O4) or the cell voltage is nearly constant over wide charging states (as in LiFePO4). In addition, the cells react very sensitively to over- and undervoltage. This requires monitoring in single cells as well as in the interconnection of several cells in order to reliably prevent premature failure or overheating of individual cells during overloading. The development of the BMS was therefore stepped up with the proliferation of lithium batteries.
When BMS is used with lithium-ion batteries, charging and discharging control, including balancing, is used on the basis of temperature control, voltage diagnostics and charge-state determination, the latter providing for equalization in unequal charge states of the individual cells.
Use in the vehicle
When used in vehicle technology, a battery monitor system also serves as an interface between the vehicle and the electronic components installed in the battery. By means of a land battery monitoring or marine battery monitoring, functions are controlled which are necessary for the current operating conditions of the vehicle or a ship. Thus, when the vehicle is switched off, the battery system is set to sleep mode. The BMS is then started regularly after a software cycle time programmed in the BMS. During this “wake-up time” the BMS checks the battery system to check all data (voltages, temperatures, etc.) for possible faults. At the same time, a battery monitor or system controls and measures to optimize the performance and life of the accumulator. This means, for example, by providing a suitable temperature level by cooling and, if appropriate, heating.
When the vehicle or a boat is started, a command from the control unit of the vehicle is sent to the BMS, which then checks the condition of the battery and closes the contactors of the battery system to supply the motor with power. If an error occurs in the battery monitor during operation, it is processed by the BMS and assigned to an error category. This error is stored for later maintenance in the workshop or displayed to the driver. In the worst case, the monitoring battery system is put into a safe state by interrupting the traction current or by completely turning off the battery.