Battery management system
The battery management system's most important task is to protect the drive battery's individual cells and to increase service life as well as cycle numbers. This is especially important for lithium-ion technology because the batteries must be protected from overcharging and excess temperature (Fig. 14.1) to prevent cell destruction.
14.2 Battery management system tasks
The battery management system (BMS) measures the control parameters cell voltage, temperature, and battery current. A typical battery cell has a nominal voltage of 3.6 V at a maximum end-of-charging voltage of 4.2 V and a minimum end-of-discharge voltage of 2.5 V. High discharging (< 2.5 V) causes irreversible damage such as capacity loss and increased self-discharging. Overvoltage (> 4.2 V) results in spontaneous self-ignition and thus poses a safety risk.
Capacity loss is mainly caused during charging when temperatures and voltages are too high. If properly used, a standard battery is good for around 500 to 1,000 cycles before it loses around 20 % of its initial capacity.
Monitoring cell voltage, current, and temperature enables forecasting of a battery’s state of charge (SOC) and state of health (SOH). SOC describes the current state of charge compared to the battery’s maximum capacity. SOH describes the current state of health compared to that of a new battery.
The battery is simulated by an analogous electrical model. The individual components’ parameters are adapted to the material characteristics in terms of capacitive, inductive, and purely ohmic properties, while taking into consideration age-related changes.
This simulation is based on the battery cells’ chemical characteristics in order to model the electrical properties.
Kalman filter method
This method constantly adapts the filtering parameters, which should reflect the battery’s properties, to the current conditions.
14.3 Battery management system components
Typically, battery modules comprise several stacked cells. Cell voltage and cell temperature are monitored in these modules, and the values are transmitted to the control unit. Also, a charge equalization between the individual cells is performed in the modules to reduce battery wiring outlay. Charge equalization and cell supervision are mostly controlled by an ASIC, the cell supervisory circuit (CSC). Drive batteries comprise several modules connected in series and have an output voltage of several hundred volts.
The control unit calculates SOC and SOH and controls charge equalization. Standard automobile interfaces such as the CAN bus or FlexRay bus are used to communicate with the vehicle to determine the SOF. The interface also controls battery charging from the power grid. This is why the control unit must also control the battery’s performance management and should reduce its power requirement in passive state to a minimum.
The HS contactor isolates the battery cells from the vehicle in passive state to prevent unnecessary losses or hazards. It can also isolate the system in the event of extreme malfunctions such as short circuits, excess temperature, or accidents. The battery is also protected by a fuse in the event of short circuits.
The current is generally measured with a special sensor directly at the battery. Two independent systems are used for safety reasons. State-of-the-art systems use a measuring resistor as sensor or they use the electromagnetic field.
Temperature management ensures that the drive battery operates at optimum temperatures. This is especially important to ensure that the cells age evenly. Service life, availability, and safety considerably depend on this.
14.4 Cell supervision and charge equalization
This leads to capacity loss caused by incomplete charging of all cells. The weakest cell determines the battery system’s characteristics.
High-precision measurement is used for each battery cell. The CSC with its accuracy of 2 mV plays an important role for determining SOC and SOH. Current BMSs use different cell voltage monitoring approaches, which each have advantages and disadvantages. Cell voltage is measured with analog-to-digital converters (ADC) as a rule. They are connected to the battery pack cells directly or via a multiplexer (MUX).
Topology choice depends on whether the measured values of all cells should be measured synchronously or whether sequential measurement suffices. The SH circuit greatly influences measurement accuracy and performance. Both values directly determine the chip surface and the costs of monitoring circuits. To detect errors, systems with just one SH circuit are advantageous. However, these solutions pose high demands on the sample rate and input signal filtering. They generally use higher-order input filters than solutions with sigma-delta converters. First-order filters suffice for the latter. The voltage offset caused by battery cell stacking can be adapted to the logic by a digital signal converter. A cascaded reference concept is used here for the individual input comparators.
14.5 Charge equalization
Passive balancing does not improve the discharging process. The capacity discrepancy of cells will increase over time and further decrease maximum capacity utilization.
Active charge equalization approaches are an alternative to today’s passive technologies. DC-DC converters transfer charges between the cells. This is possible during charging and discharging as well as in passive state. More effective charge transfers enable higher compensating currents. This means that larger cells with higher capacities can be used and cell equalization is quicker.
Capacitive and inductive approaches load the cell charge into a storage element and transfer it to an insufficiently charged cell. A switching matrix consisting of transistors connects the storage element with several cells. To maintain high efficiency, this technology is mainly used for lower cell numbers. If cell voltages differ only slightly, the efficiency of capacitive “shuffling” technology decreases.A transformer-isolated charge transfer remedies this situation. Battery cells are connected via switches with transformer windings that enable energy transfers between the battery cells.
All active processes also open up potential for cutting cooling costs. Battery modules of up to 12 battery cells are generally compatible with bidirectional transformer concepts.
ASICs are mostly used to control charge equalization. The procedure described above can easily be extended to charge equalization between battery blocks. In this case, an additional transformer selectively feeds energy into the 12-V on-board electrical system.
14.6 Internal battery communication bus
Measurements in the cell set are synchronized and all errors such as increased cell voltage, communication disruption, disconnected sampling lines, and increased cell temperature are recorded and transmitted to the main controller. Today’s drive batteries essentially use two approaches to exchange data between individual modules and the central control unit.
A vertical communication interface is an alternative to a large number of cell supervisory circuits (CSC) (Fig. 14.9b). Its main task is to transmit voltage and temperature data from the stacked modules to the lowest module, which then communicates with the control unit by means of galvanic isolation. Commands for cell voltage balancing are transmitted in the reverse direction. The data rate of such a system is considerable. The data protocol consists of the address, the measured values for each cell of each module, a cyclic redundancy check (CRC) for serial data transmission, and module programming.
Data transmission must have a certain guaranteed redundancy to ensure a limp-home mode for the battery-powered vehicle. This can be achieved by means of a differential signal, which should be able to maintain communication in the event of an error. The reduced spectrum of radiation is another advantage of differential data transmission. In general, twisted-pair lines without shielding suffice for bus wiring. Failure analysis of a vertical interface must, similar to that of star wiring, take into account high-voltage short circuits. Erroneous measurement results or destruction of the CSC must be prevented.
This is why the communication lines are mostly connected capacitively or via inductive transformers. However, such measures reduce the cost advantages derived from vertical communication.
Radio transmission is another possibility for exchanging data within the battery pack. These wireless systems are currently being researched.
14.7 Battery control unit
This is especially important for the central CPU, whose task is to protect the entire system. Modern microcontroller designs are perfect for this task. They are equipped with two processor cores, monitor each other in the so-called lock-step mode, and integrate error correction and a comprehensive BIST (built-in self-test). These systems have already been tried and tested in safety-relevant automobile applications such as brakes, steering, and airbag.
The main CPU also calculates the battery model in order to determine SOC and SOH. It processes battery cell voltage, current, and temperature data. The computer used should have a floating-point function to fulfill the requirements of the monitoring circuitry. It also controls cell charge equalization and ensures that the battery temperature profile is as homogeneous as possible, providing the battery has controllable temperature management.