How to form a battery management system
To design a monitor circuit for a new and battery-based power system, what strategy would you take to optimize the cost and manufacturability of the design? The initial consideration will be to determine the preferred structure of the system and the location of the battery and related electronic components. After the basic structure is clear, the next issue that must be considered is the trade-off and coordination of circuit topology, for example, how to optimize the communication and interconnection of the final product.
The external dimensions of the battery will have a significant impact on the structure of the power system. Do you want to use a large number of small batteries to fit a battery module (or battery pack) with a complicated shape? Or use a battery with a large size, which limits the number of batteries or causes other size restrictions due to weight issues?
This may be the most variable part of the design, because new-looking batteries are constantly on the market, and people are constantly working hard to ensure that the battery module or battery pack is integrated into the product to be more consistent with the entire product concept. For example, in the case of car design, batteries may eventually be scattered in certain spaces on the vehicle. If these spaces are not equipped with batteries, the utilization efficiency is very low.
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 shell can be made to integrate some data acquisition circuits in the battery module or battery pack, so that if it needs to be replaced, important information such as production ID, calibration, and specifications can be brought along with the replaceable components. go. This type of information may be useful for battery management systems (BMS) or maintenance equipment, and minimizes the number of high-voltage rated wires required in the wiring harness.
Next, in terms of a given mechanical concept design, the monitoring hardware topology is determined by the number of batteries that are precisely defined and supported. In automotive applications, there are generally more than 100 battery measurement points in total, and the modularity of the system will determine how many batteries are measured for a given circuit system. The most common situation is to divide all batteries into at least two subgroups by means of safe disconnection of "service plugs".
By keeping the voltage below 200V in the event of a fault, this method minimizes the risk of electric shock that maintenance personnel may encounter. The larger size of the battery pack means the use of two isolated data acquisition systems, each of which may support 50 battery taps. In some cases, all electronic components are on an affordable printed circuit board, but this requires a large number of interconnections, as shown in Figure 1(a).
Alternatively, the electronic components can also be scattered and integrated in the battery module more closely, but this requires the use of telemetry link methods. In order to achieve reliable data integrity, the remote measurement function circuit built into the automotive wiring harness must use a rugged protocol, such as the widely used CAN bus.
Although the real CAN bus interface involves several network layers, the PHY layer can be easily used to form a BMSLAN structure to efficiently communicate within the module. This kind 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 the EMI problems that may be caused by the LAN method. The final BMS application interface is likely to be a CAN bus connection to a main system management processor, and will need to define (or specify at the beginning) specific information transaction processing.
Other factors may also affect the physical structure and monitoring circuit. In the case of lithium-ion batteries, the battery capacity needs to be balanced, which leads to additional heat management problems (removal of heat), and if active balancing is required, a power conversion circuit is also required. Temperature probes are often distributed over the entire module to provide a way to correlate voltage readings with charging status, which requires some supporting circuits and connection schemes. A consideration that is often overlooked in the design is that when the product is left unused or stored on the shelf before installation, the battery leakage should be the lowest. In some cases, additional control wiring is necessary.
In these structures implemented above, there is a common measurement function component, which includes a multi-channel ADC, a safety isolation barrier, and a certain degree of local processing capability. The circuit in Figure 2 shows an extensible design platform for data acquisition.
In this picture, the core component that realizes the function is Linear Technology's LTC6803 battery pack monitor IC. Also shown is an SPI data isolator and some optional special-purpose circuits. This circuit includes input filter and passive balance function, forming a complete 12-cell data acquisition solution. If necessary, this type of circuit can be simply copied to support more battery measurement schemes, while sharing the local SPI port of the main microcontroller, which in turn provides external CAN bus or other LAN-type data links. need.
Compared with the previous generation of monitoring devices, the main improvement of the LTC6803 is to support power shutdown and/or to be powered by a battery pack alone. When the power is removed from the V+ pin, the battery load will drop to zero (only nA semiconductor leakage). The operating power can be provided by the voltage of the battery pack that is connected, or supplied to V+ from a separate power source, 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 this case, the lowest power state (ie standby) will only consume 12uA current.
LTM2883 data isolator is powered from the main processor through an internally isolated DC-DC converter, so the device will automatically power off with the main processor. A very useful function of the LTM2883 is that it can also provide a large amount of power from the host to the isolated electronic component (ie, the battery end).
A small step-up power supply function component (LT3495-1 in Figure 2) is driven in this way to independently power the LTC6803 so that the battery only provides ADC measurement input current (that is, the average value "200nA during effective conversion). This circuit has the absolute lowest parasitic battery leakage, while eliminating any battery operating current mismatch, otherwise this mismatch may gradually lead to battery capacity imbalance.
A convenient feature of LTC6803 is that there are two free ADC inputs with similar accuracy to battery inputs. This convenient function allows for auxiliary measurements with very few additional circuits, including temperature, calibration signals, or load current measurements. A particularly useful measurement is to use a gated resistor divider to measure the voltage of the entire battery pack, as shown in Figure 2 (using a 12:1 ratio, connected to the VTEMP1 input).
When the circuit is de-energized, the related FET is disconnected, so that the current measurement will not unnecessarily burden the battery. Since the filtering of this port can be customized independently of the battery input, a real Nyquist (Nyquist) sampling rate of up to 200 sps is possible for accurate charging current calculations. You can use the measurement of a single battery to periodically provide software calibration for the voltage divider of the entire battery pack, so that expensive resistors are not required.
Another very useful usage of auxiliary input is a calibration power supply with high measurement accuracy (such as Linear Technology’s LT6655-3.3, a benchmark with an accuracy of 0.025%). In this usage, the software is allowed to rely on the channel To the inherent matching of the channel, correct all other channels. Please note that the thermistor temperature probe does not need to be based on the potential of the battery, and these probes generally do not require 12-bit resolution. This type of probe is usually suitable for direct connection with a microcontroller, thereby leaving the auxiliary input of the high-performance LTC6803 to achieve more demanding functions.
In short, there are many factors that need to be considered in the battery management system circuit, especially those that determine the packaging constraints. When packaging design ideas come together, it is also important to consider the structure of electronic circuits and information flow that may also have a mechanical impact (for example: connectorization and the number of wires).