Building an efficient battery management system

Assuming you have been tasked with designing a monitor circuit for a new battery-based power system, what strategies would you adopt to optimize the cost and manufacturability of the design? The initial consideration would be to determine the preferred structure of the system and the location of the battery and related electronic components. Once the basic structure is clear, the next issue that must be considered is the trade-offs of the circuit topology, for example, how to optimize the communication and interconnection of the final product.

The battery's form factor will have a significant impact on the power system architecture. Will a large number of small cells be used to fit into a complex-shaped battery module (or pack)? Or will a large-form factor be used, limiting the number of cells due to weight issues or causing other size constraints? This is perhaps the most variable part of the design, as new form factors continue to be introduced and efforts are made to make the battery module or pack more consistent with the overall product concept when integrated into the product. For example, in the case of an automotive design, the batteries may end up being dispersed in certain spaces on the vehicle that would be inefficient if not for the batteries.

Another consideration is the interconnection of test signals and/or telemetry signals between the battery (or modular battery pack), the battery management system (or its subsystems), and the final application interface. In most cases, a housing can be made to integrate some of the data acquisition circuitry in the battery module or battery pack so that if it needs to be replaced, important information such as production ID, calibration, usage specifications, etc. can be taken away with the replaceable component. This type of information may be useful to the battery management system (BMS) or service equipment, and minimizes the number of high-voltage rated wires required in the wiring harness.

Next, the monitoring hardware topology is determined by the precisely defined number of cells that need to be supported for a given mechanical concept design. In automotive applications, there are typically more than 100 total cell measurement points, and the modularity of the system will determine how many cells a given circuit system measures. The most common case is to divide all cells into at least two subgroups in the form of safe disconnect "service plugs". This approach minimizes the risk of electric shock to maintenance personnel by keeping the voltage below 200V in the event of a fault. The larger form factor battery pack means that two isolated data acquisition systems are required, each supporting perhaps 50 cell taps. In some cases, all electronic components are on a single economical printed circuit board, but this requires a large number of interconnects, as shown in Figure 1 (a). Alternatively, the electronic components can be dispersed and more tightly integrated into the battery module, but this requires a telemetry link method. For reliable data integrity, the remote measurement function circuit built into the vehicle wiring harness must use a rugged protocol such as the widely used CAN bus. Although the true CAN bus interface involves several network layers, it is convenient to use the PHY layer to form a BMS LAN structure for efficient intra-module communication. This type of distributed structure is shown in Figure 1 (b). This topology allows the computational workload to be distributed among several small processors, thereby reducing the required data transfer rate and alleviating EMI issues that may be caused by the LAN approach. The final BMS application interface is likely to be a CAN bus connection to a main system management processor, and specific information transactions will need to be defined (or specified at the beginning).

Other factors may also affect the physical structure and monitoring circuits. In the case of lithium-ion batteries, cell balancing is required, resulting in additional thermal management issues (heat removal) and power conversion circuits if active balancing is required. Temperature probes are often distributed throughout the module to provide a method to correlate voltage readings with charge status, requiring some supporting circuitry and connection schemes. An often overlooked consideration in design is that battery leakage should be minimal when idle or stored on a shelf before the product is installed. In some cases, additional control wiring is necessary.

In all of the above implementations, there is a common measurement functional building block that includes a multi-channel ADC, safety isolation barriers, and some degree of local processing capability. The circuit in Figure 2 shows an extensible design platform for implementing data acquisition functions. In this figure, the core component of the implementation function is Linear Technology's LTC6803 battery stack monitor IC, and an SPI data isolator and some optional special purpose circuits are also shown. The circuit includes input filters and passive balancing functions to form a complete 12-cell battery data acquisition solution. If necessary, this type of circuit can be easily replicated to support more battery measurement schemes while sharing the local SPI port of the main microcontroller, which in turn provides the external CAN bus or other LAN type data link required.

The main improvement of the LTC6803 over the previous generation of monitoring devices is that it supports power shutdown and/or power supply from the battery pack alone. When power is removed from the V+ pin, the battery load will drop to zero (only nA semiconductor leakage). Operating power can be provided by the connected battery pack voltage, or from a separate power supply to V+, as long as the voltage is always at least as high as the battery pack. For simplicity, the LTC6803 can also draw power directly from the battery pack, in which case the lowest power state (i.e. standby) will consume only 12uA current. The LTM2883 data isolator is powered from the host processor through an internal isolated DC-DC converter, so the device will automatically power down with the host processor. A very useful feature of the LTM2883 is that it can also provide large and host-derived power to the isolated electronic components (i.e. the battery end). A small boost supply function (LT3495-1 in Figure 2) is driven in this way to independently power the LTC6803 so that the battery provides only the ADC measurement input current (i.e., < 200nA average when actively converting). This circuit has the absolute lowest parasitic battery leakage while eliminating any battery operating current mismatch that could otherwise cause a gradual imbalance in battery capacity.

A convenient feature of the LTC6803 is that there are two free ADC inputs with similar accuracy to the battery inputs. This convenient feature allows auxiliary measurements to be made with very little additional circuitry, including temperature, calibration signals, or load current measurements. One particularly useful measurement is to measure the voltage across the battery stack with a gated resistor divider, implemented as shown in Figure 2 (using a 12:1 ratio connected to the VTEMP1 input). When the circuit is powered down, the associated FET is disconnected so that the current measurement does not unnecessarily stress the battery. Since the filtering of this port can be customized independently of the battery input, true Nyquist sampling rates of up to 200sps are possible to achieve accurate charge current calculations. Software calibration of the voltage divider across the stack can be provided periodically using measurements of individual cells, eliminating the need for expensive resistors. Another very useful use of the auxiliary input is to measure a very accurate calibrated power supply (such as Linear Technology's LT6655-3.3, a 0.025% accurate reference), allowing the software to calibrate all other channels by virtue of the inherent channel-to-channel matching. Note that thermistor temperature probes do not have to be referenced to the potential of the battery, nor do these probes generally require 12-bit resolution. Such probes are often suitable for direct connection to a microcontroller, leaving the high performance LTC6803's auxiliary inputs free for more demanding functions.

In summary, there are many factors to consider in battery management system circuits, especially those that determine packaging limitations. When packaging design ideas come together, it is also important to consider the structure of the electronic circuits and information flow (for example: connectorization and number of wires) that may also have mechanical effects. Once these factors have been weighed and the packaging design ideas have matured, a reputable, scalable and cost-effective data acquisition solution can be directly inserted into a platform using the LTC6803.