Rechargeable batteries are the fundamental components of Battery Energy Storage Systems (BESS). Nowadays, more and more different chemical systems are combined into battery packs consisting of dozens, hundreds, or even thousands of cells, achieving more efficient operation at higher voltages. For battery management system (BMS) designers, this design structure faces many challenges in achieving optimal performance, efficiency, reliability, and safety.
For example, designing or selecting integrated circuits (ICs) that meet application requirements requires a deep understanding of battery chemistry, charging, monitoring, load balancing, isolation, safety, and communication technologies to ensure efficient implementation.
To this end, suppliers have integrated many necessary functions into dedicated ICs that are essentially independent of processors. Many models of this type of IC not only support multiple lithium based battery chemical systems, but are also compatible with non lithium battery cells. This type of IC collects data from battery cells and makes optimal real-time battery management decisions and actions. In addition, these types of ICs also provide data to the system processor regarding the battery cell status and operational status.
This article will briefly introduce the unique technical requirements of multi cell groups. Then, introduce Analog Devices' advanced specialized optimized ICs and elaborate on how to use these ICs to meet the above requirements.
Multiple battery cells will bring more challenges
The basic circuit diagram of a battery pack may seem simple, but it actually includes multiple battery cells that obtain higher voltage through series connection and larger current through parallel connection. This means that such configurations are just a simple extension of single cell/few cell battery packs, requiring almost no additional management. This multi cell battery pack is suitable for electric tools that require 18V or 48V, electric vehicles (EVs) that require 400V or 800V, and BESS systems that typically require 1500V.
The actual situation of these larger battery packs is that their details and complexity far exceed what is shown in their circuit diagrams. As the number of cells and battery packs increases, the difficulty of addressing these challenges grows exponentially.
Firstly, it is necessary to monitor the battery cell to track its terminal voltage, charge discharge curve, state of charge (SoC), temperature, and fault precursor characteristics. In addition, it is necessary to manage different battery cells uniformly and record and consider their differences.
If there is a lack of a universal set of rules, it will further increase the complexity of battery cell management. In addition, the appropriateness of the management strategy adopted depends on the chemical characteristics of the battery cells. The management strategies adopted for different major chemical systems are different (such as lithium-ion (Li ion) and lead-acid batteries), and within the same generalized chemical system (such as various Li ion battery formulations), the management strategies used are also different. Therefore, advanced BMS management strategies must be customized for the chemical characteristics of the managed battery cells.
Due to the large number of battery cells contained in high-voltage and high-capacity battery packs, which must meet numerous safety standards, monitoring and managing local battery cells is currently the most feasible engineering solution. Although the system is usually equipped with a main processor, it can usually only issue advanced regulatory instructions for local cell monitoring and evaluate the overall performance of the battery pack. The monitoring and management of a single battery cell is accomplished by an autonomous electronic system that provides real-time functionality and primarily operates without the need for system level processor intervention.
Passive and active battery balancing
Cell balance is particularly important for maintaining the integrity of multiple cell groups, ensuring that some cells are not damaged due to overload, and avoiding other batteries from being idle due to low utilization. Cell balancing can prevent damage to cells and battery packs, thereby maximizing performance. Cell balancing ensures that all cells in the battery pack reach their maximum capacity simultaneously, preventing overcharging, SoC imbalance, overdischarging, and premature aging, ultimately extending the battery's lifespan.
There are two methods for cell balancing: active and passive balancing. Active equalization is more accurate and faster than passive equalization, but it is more complex to implement. Active balancing uses active circuit technology to redistribute charge between each cell in the battery pack, ensuring that the SoC of all cells remains consistent. This circuit monitors the voltage of each battery cell and adjusts the charging and discharging currents accordingly based on the monitoring results.
In contrast, passive balancing relies on Ohm's law and balancing resistors to adjust the cell to the same SoC state. In addition to low accuracy and slow speed, passive balancing can also dissipate (waste) excess energy in high battery cells.
Starting from multi cell monitoring
Although there are already a large number of ESS solutions on the market, the two core front-end functions of BMS still lie in the monitoring and balancing of battery cells. The ADES1830CCSZ IC shown in Figure 1, as a 16 channel, multi cell, multi chemical system battery monitor, not only achieves the above functions, but also integrates numerous key features that help simplify the overall system design and operation.
Analog Devices' ADES1830CCSZ Cell Monitor with Multiple Cells and Chemical Systems (Click to Enlarge)
Figure 1: ADES1830CCSZ cell monitor with multiple cells and multiple chemical systems is used as the basic building block for a comprehensive BMS. (Image source: Analog Devices)
This multi cell group monitor can measure up to 16 series connected cells, with a total measurement error (TME) of less than 2 mV across the entire temperature range; while the TME of other ADES1831CCSZ with the same specifications is slightly higher, at 5 mV. The measurement input range of -2 V to 5.5 V makes ADES1830 and ADES1831 suitable for most battery chemical materials.
In order to maintain consistency when monitoring battery packs containing a large number of cells, all cells can be redundantly measured synchronously through dual integrated analog-to-digital converters (ADCs). These analog-to-digital converters (ADCs) operate continuously at a high sampling rate of 4.096 megasamples per second (MSPS), thus reducing the use of external analog filters and achieving aliasing free measurement results. If necessary, additional noise reduction can be achieved through downstream programmable infinite impulse response (IIR) filters. ADES1830 and ADES1831 also have passive balancing function - achieved through independent pulse width modulation (PWM) duty cycle control, and support a discharge current of up to 300 mA per cell.
Although a single ADES1830 or ADES1831 device only supports 16 cells in series, multiple devices can be cascaded to simultaneously monitor the cells of a long string high-voltage battery pack. To achieve interconnection between the IC chips, each device is equipped with an isolated serial port interface (isoSPI), which is electrically isolated through user selected capacitors or transformers to achieve long-distance high-speed communication that can resist radio frequency interference.
Through this method, a single main processor connection can read data and monitor the entire battery string. This serial port link enables bidirectional communication, ensuring data integrity even in the event of communication path failures.
To optimize the applicability of these multi cell detectors, Analog Devices has launched the EV-ADES1830CCSZ evaluation board (Figure 2, left). In order to be closer to reality, multiple evaluation boards can be connected through the isoSPI interface to monitor a long string of cells in the battery pack (on the right side of Figure 2).