Here is a brief tutorial on battery management system, how charge balancing works in it, and what's required for its functional safety compliance.
The automotive industry is changing. The load that must be carried today by internal combustion engines (ICEs) 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 ICUs and the drive train. In the future, attention will be given to other components.
It includes developing new types of solid-state batteries that allow for longer ranges—and the increased charging and discharging behavior—that cannot be achieved with current rechargeable lithium batteries. That, in turn, could lead to the development of high-performance chargers, DC/DC converters, and electric motors.
Here, at this technology crossroads, battery management system (BMS), as a core component, is crucial in proper management and monitoring of the battery.
Currently, 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 for their temperature, impedance or internal cell resistance, voltage, and charge and discharge current.
BMS design details
The BMS design comprises several components, including cell management controller (CMC) and battery management controller (BMC). Here, CMCs use multi-channel ICs—currently equipped with up to 16 channels—to perform the monitoring function while BMC handles the control function of the individual CMCs (Figure 1).
Figure 1 The conceptual structure of a battery management system is shown with a description of the interfaces. Source: Vishay
As a rule, external NTC resistors are attached directly to the cell to measure the temperature. As the resistor heats up, the electrical conductivity improves because of the negative temperature coefficient. So, the cell temperature can be determined using evaluation in the IC.
At this time, 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 applies an alternating current at different frequencies. Then, the complex voltages can be converted and interpreted like currents using software-based models.
The single-cell voltage is usually measured with the analog-to-digital converter (ADC) integrated into the IC. In this method, a multiplexer measures the individual voltages in sequence and converts them to digital signals using the ADC. These digital signals can then be evaluated.
The charging or discharging current is not measured for each individual cell but rather for the cell assembly. The background for this approach is that the battery pack is “topped off” by means of a central charger, either as an alternating current charge via an integrated charger in the form of onboard charger (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 be measured only once. For this, either Hall effect current sensors or low resistance shunt resistors are used.
Another core task of BMS is to balance the individual cells. During production of the individual cells, the capacity and the 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, to ensure that all the energy a.k.a. 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.
How charge balancing works
With active balancing, the excess energy of a cell is transferred into a coil by way of an electronic circuit in a switching operation using a field-effect transistor (FET). In the following 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 (Figure 2).
Figure 2 Here is a broad view of the 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 (Figure 3). The resistors used here are usually fabricated using thick-film technology. They have a relatively high temperature coefficient and a high initial tolerance.
Figure 3 The image shows the conceptual operation of passive balancing. Source: Vishay
However, alternative approaches, such as based on double-coated 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. So, with the same power requirements, using these resistors can save money and reduce the space required on the PCB.
Another possibility is enabled by the RCL Series, which also allows higher continuous power and better thermal cycling performance as a result of being terminated on the long side. Given the automotive industry requirements for stable solder connections between the component and the PCB from 55°C to +125°C and under increased cycles, these resistors can also prove highly suitable.
However, due to the high circuitry costs for active balancing and narrower manufacturing tolerance for the internal resistance and capacitance of the individual cells, passive balancing is mainly used in advanced applications in automotive designs.
Functional safety compliance
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 mandated by ASIL-D. In BMS, voltage, temperature and current measurement, except internal resistance measurement, are considered at the same level as airbag systems, brake systems, and power steering systems. If these systems fail or behave in a defective manner, there is an immediate danger to vehicles and passengers.
Here, redundant measurement methods can minimize risks.
Monitoring the cell voltage in this case is 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 first disadvantage of this approach is that the voltage measurements employ the same method. Second, the solution 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 mechanism of measurement that can be achieved in a cost-effective manner. The previously described thick-film bleed resistors are not suitable though. Rather, thin-film resistors should be used because they guarantee precise measurements over the entire service life, even under demanding use.
Take 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. Next, the MCW Series uses thin-film technology in 0406 and 0612 sizes with terminations on the long side. It satisfies the requirements for long-term stability (≤ 0.2 %; P70, 1000 h) and continuous power versus space, virtually the same continuous power at one third of the conventional space required (Figure 4).
Figure 4 Thin-film resistors terminate on the long side with higher performance and require one-third of space than conventional terminals. Source: Vishay
Moreover, these resistors feature increased thermal cycling performance of 3,000 cycles. With these features, these resistors 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.
Components, especially for electrifying the drive train, can no longer be selected without a deep engineering understanding of the overall system. That’s because of the increasing requirements on the performance of the individual components, smaller form factor demands, the estimates for service life, and more stringent safety requirements.
This article was originally published on Planet Analog.
Adrian Michael manager of product marketing for automotive at Vishay Intertechnology.
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