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The Battery Management System in the Context of Functional Safety

Accurate temperature readings are very important to the proper functionality of the battery and the safety of the system.

Adrian Michael

The automotive industry is changing. The load that must be carried today by internal combustion engines will, in the future, be handled by hybrid, electric, or even fuel cell-driven vehicles. In the past, many manufacturers concentrated on the mechanical components necessary for conventional internal combustion engines and the drive train. In the future, attention will be given to other components.

It may be the development of new types of solid-state batteries that allow for ranges — and the increased charging and discharging behavior — that cannot be achieved with current rechargeable lithium batteries, or it could be the development of high performance chargers, DC/DC converters, and electric motors.

The battery management system (BMS), as a core component, takes over the proper management and monitoring of the battery. At this time, lithium-ion cells are being used in electric vehicles. They are connected to form a cell assembly until the required total voltage has been reached.

With single-cell voltages of about 3.6 V to 3.7 V available today, around 140 to 250 individual cells are needed to produce 520 V or 900 V for the high voltage system used for a traction battery. In this configuration, the cells must be monitored with regard to their temperature, impedance (internal cell resistance), voltage, and charge and discharge current.

Details of the BMS

A BMS usually consists of several components, the cell management controller (CMC), and the master central unit or the battery management controller (BMC). Here, the CMCs use multi-channel ICs (currently equipped with up to 16 channels) to perform the monitoring function while the BMC handles the control function of the individual
CMCs (Fig. 1).

Conceptual-structure of a battery management system
Fig. 1 – Conceptual structure of a battery management system in advanced vehicle architectures, with a description of the interfaces; source: Vishay

Monitoring Cell Parameters (Temperature, Impedance, Voltage, and Current)

Temperature Monitoring

As a rule, NTC thermistors are attached closely to cell or module walls, or to electrical connecting points, to measure their temperature. As the thermistor heats up, the electrical resistance decreases with a high sensitivity (because of the large negative temperature coefficient of resistance). The temperature can be determined by measuring a voltage in a resistor-thermistor network by means of an integrated analog / digital converter (ADC) of the IC.

Accurate temperature readings are very important to the proper functionality of the battery and the safety of the system. For precise temperature measurement, the NTC and the measuring circuit resistances are very important.

In Fig. 2, the NTC can be the NTCS0603E3103FLT monolithic ceramic NTC SMD thermistor (as shown in Fig. 3), with an R25 value of 10 kΩ, ± 1 %, and a B value of 3435 K, ± 1 %. Such a component has better mechanical resistance to bending compared to some competing multi-layer structures when mounted, for example, on
a flexible PCB (FPC).

This thermistor also has a high thermal cycling withstand capability, and low resistance drift when subjected to high temperatures. The NTC thermistor can be placed in a fixed resistor network with the TNPW / TNPU — which provide ultra precision tolerances and low thermal coefficients of resistivity down to ± 0.1 % and ± 25 ppm/— or ACAS network resistors, which can support ± 0.05 % relative tolerance with 0.1 % of absolute tolerance (Fig. 4). A control IC will sample the voltage produced by the NTC thermistor (Vntc) and will detect both low and high thresholds.

NTC thermistor

 divider bridge
              Fig. 2 – Source: LTspice XVII simulation of a resistor / thermistor divider bridge voltage
                              for IC or ADC input, intended for battery cell temperature detection
NTCAFLEX05 series flex foil sensors
          Fig. 3 – Reference designs for NTCs, performance comparison by technologies, and reference
                         designs for NTCAFLEX05 series flex foil sensors; source: Vishay
Performance comparison
            Fig. 4 – Performance comparison of discrete gain resistors vs a network resistor; source:
                                                                            Vishay

Impedance Monitoring

Impedance measurement is not being used to its full extent. The advantage to this measurement is that it provides a more accurate estimate of the state of charge (SOC) and the state of health (SOH).Expressed in simple terms, the method used here is to apply an alternating current at different frequencies. Then, the complex voltages can be converted and interpreted like currents using software based models.

Single-Cell Voltage Monitoring

The single-cell voltage is usually measured with the integrated ADC of the IC. In this method, a multiplexer measures the individual voltages in sequence and converts them to digital signals in the ADC. These digital signals can then be evaluated.

Current Monitoring

The current (charging or discharging) is not measured for each individual cell, but rather for the cell assembly. The background for this is that the battery pack is “topped off” by means of a central charger, either as an alternating current charge via an integrated charger (the onboard charger, or OBC) or as a direct current charge from an external charger. By connecting the cells in series, the same current flows through all cells and the current in the system needs to only be measured once. For this, either Hall effect current sensors or low
resistance shunt resistors are used.

Another core task of the BMS is to balance the individual cells. During production of the individual cells, the capacity and internal resistance are subject to fluctuations because of the process. As a result, there is non-uniformity in charging or discharging the cell assembly. However, so that all the energy (range) of the battery can be used, the individual cells are balanced with regard to capacity and voltage. There are two basic philosophies here to achieve charge balancing – active balancing and passive balancing.

Balancing

With active balancing, the excess energy of a cell is transferred into a coil by way of an electronic circuit in a first switching operation of a field effect transistor. In a second switching operation, the energy in the coil is fed to the next cell by way of a diode. This method continues until all cells have reached their full charging voltage (Fig. 5).

active balancing
                         Fig. 5 – Conceptual operation of active balancing; source: Vishay

In passive balancing, the excess energy of a cell is converted into heat using a bleed resistor. The IC measures the cell voltage while charging the battery and connects the resistor as soon as a threshold is reached. This process can occur on one or more cells at the same time (Fig. 6). The resistors used here are usually fabricated using thick film technology. They have a relatively high temperature coefficient and a high initial tolerance. Vishay offers a very different approach. Doubleprinted CRCW-HP resistors and specially trimmed RCS resistors permit
double to triple the continuous power compared to conventional thick film resistors, with the same footprint. Or, with the same power requirements, using these series can reduce the space required on the printed circuit board to save money.

Another possibility is the RCL series, which also allows higher continuous power and better thermal cycling performance as a result of being terminated on the long side. With requirements in the automotive industry for making stable solder connections between the component and the printed circuit board from -55 °C to +125 °C and under increased cycles, these form another criterion for selecting suitable components.

passive balancing
Fig. 6 – Conceptual operation of passive balancing; source: Vishay

Due to the high circuitry costs for active balancing and the narrower manufacturing tolerance for the internal resistance and capacitance of the individual cells, passive balancing is mainly used in advanced applications in the automotive area.

Functional Safety (ISO 26262, ASIL-D)

Batteries and their monitoring systems are safety-critical. For this reason, the components used in the system, and the entire system itself, must be developed according to ISO 26262 to satisfy the requirements directed by ASIL-D. As a result, the BMS with the functions of voltage measurement, temperature measurement, and current measurement (except internal resistance measurement) are named in one level with airbag systems, brake systems, and power steering systems, etc. If these systems fail or behave in a defective manner, there is an immediate danger to life and limb.

Redundant, Independent Measuring Methods Can Minimize Risk

Monitoring the cell voltage, in this case, is included among the most critical parameters because the overcharging or deep discharge of individual cells can cause internal short circuits that result in thermal runaway when the cell is charged the next time.

Redundant cell voltage measurement can be performed using two battery ICs. The disadvantages here are, first, that the voltage measurement uses the same method, and second, that the solution used is relatively costly.

Another solution would be to measure the cell voltage in an analog manner using the bleed resistors and compare it to the results of the cell voltage measurement from the IC. This provides an independent method of measurement that can be achieved in a cost-effective manner. The previously described thick film bleed resistors are not suitable here. Rather, thin film resistors should be used because they guarantee precise measurements over the entire service life, even under demanding use.

Vishay offers many options for this, too. First, there is the MC-HP series, which is fabricated in special thin film technology. It combines the advantages of long term stability (≤ 0.2 %; P70, 1000 h) with twice the performance of standard thin film resistors. Second, the MCW series uses thin film technology (in the 0406 and 0612 sizes) with terminations on the long side. This series satisfies the requirements for long term stability (≤ 0.2 %; P70, 1000 h) and continuous power versus space, with virtually the same continuous power at one third of the conventional space required (Fig. 7) and increased thermal cycling performance (3000 cycles). With these
features, these series are suited for use in the BMS as bleed resistors or as cell voltage measuring resistors to implement future requirements directed by ASIL-D in the overall system.

Thermographic comparison
Fig. 7 – Thermographic comparison of thin film resistors terminated on the long side with higher performance and one third the required space compared to conventional terminals; source: Vishay

Components, especially for electrifying the drive train, can no longer be selected without a deep engineering understanding of the overall system, because of the increasing requirements on the performance of the individual components, the space required, the estimates for service life, and drift and more stringent safety requirements. Here, Vishay offers numerous, very different products and solutions that contribute to the efficient
and safe design of the overall system.

Author: Adrian Michael holds the position of manager of product marketing, and automotive, at Vishay Intertechnology. He holds a Dipl.Ing. from Westsächsische Hochschule Zwickau, and has previously served at Axxellon GmbH.

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