Application of an efficient AA Ni-MH batteries management system

　　The external dimensions of the AA Ni-MH batteries will have a significant impact on the power system structure. Do you want to use a large number of small cells to fit into a complex-shaped AA Ni-MH batteries module (or AA Ni-MH batteries pack)? Or do you want to use cells with a very large appearance, which will limit the number of cells due to weight issues or cause other size constraints? This may be The biggest design variable is because batteries with novel shapes are constantly coming to the market, and people are constantly working hard to make the AA Ni-MH batteries modules or AA Ni-MH batteries packs more consistent with the entire product concept after being integrated into the product. For example, in the case of a car design, batteries may end up dispersed in certain spaces on the vehicle that would be inefficiently utilized without the batteries.

　　Another consideration is the interconnection of test signals and/or telemetry signals between the AA Ni-MH batteries (or modular AA Ni-MH batteries pack), the AA Ni-MH batteries management system (or its subsystem), and the end application interface. In most cases, a housing can be made to integrate some data acquisition circuitry in the AA Ni-MH batteries module or AA Ni-MH batteries pack, so that if replacement is needed, important information such as production ID, calibration, usage specifications, etc. can be brought with the replaceable components. Walk. This type of information may be useful for AA Ni-MH batteries management systems (BMS) or service equipment and minimizes the number of high-voltage rated wires required in the wiring harness.

　　Next, for a given mechanical concept design, the monitoring hardware topology is determined by the precisely defined number of cells that need to be supported. In automotive applications, there are typically more than 100 AA Ni-MH batteries measurement points in total, and the modularity of the system will determine how many cells a given circuit system measures. The most common situation is to separate all batteries into at least two sub-groups by safely disconnecting "service plugs". By keeping the voltage below 200V during a fault condition, this approach minimizes the risk of electrocution that service personnel may encounter. The larger AA Ni-MH batteries pack size means two separate data acquisition systems, each supporting perhaps 50 AA Ni-MH batteries taps. In some cases, all electronic components are on an affordable printed circuit board, but this requires a large number of interconnects, as shown in Figure 1(a). Alternatively, the electronic components could be dispersed and more tightly integrated within the AA Ni-MH batteries module, but this would require a telemetry linking approach. To achieve reliable data integrity, remote measurement circuits built into automotive wiring harnesses must use a ruggedized 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 for efficient communication within the module. This type of distributed structure is shown in Figure 1(b). This topology allows the computing workload to be distributed among several small processors, thereby reducing the required data transfer rate and mitigating EMI issues that can arise from LAN approaches. 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 outset) specific message transactions.

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

　　Across the structures implemented above, there is a common measurement functionality building block that includes a multi-channel ADC, safety isolation barriers, and some degree of local processing. The circuit in Figure 2 shows a scalable design platform that implements data acquisition functions. In this diagram, the core component that enables the functionality is Linear Technology's LTC6803 AA Ni-MH batteries pack monitor IC, along with an SpI data isolator and some optional special-purpose circuitry shown. This circuit includes the input filter and passive balancing functions, forming a complete 12-cell data acquisition solution. If desired, this type of circuitry can be simply replicated to support more AA Ni-MH batteries 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. need.

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

　　A convenient feature of the LTC6803 is that it has two free ADC inputs with similar accuracy to the AA Ni-MH batteries inputs. This convenient feature allows auxiliary measurements, including temperature, calibration signal, or load current measurements, to be made with little additional circuitry. A particularly useful measurement is to measure the voltage across the AA Ni-MH batteries pack using a gated resistor divider, 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 tax the AA Ni-MH batteries. Since the filtering of this port can be customized independently of the AA Ni-MH batteries input, true Nyquist sampling rates of up to 200sps are possible for accurate charge current calculations. Individual cell measurements can be used to periodically provide software calibration of the voltage divider across the entire AA Ni-MH batteries pack, 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 reference with 0.025% accuracy), in which case the software is allowed to depend on the channel. Inherent match to channel, corrects for all other channels. Note that thermistor temperature probes do not have to be referenced to the AA Ni-MH batteries potential, nor do these probes typically require 12-bit resolution. This type of probe is often suitable for interfacing directly with a microcontroller, leaving the auxiliary input of the high-performance LTC6803 free for more demanding functions.

　　In summary, there are many factors to consider in AA Ni-MH batteries 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 circuitry and information flow (e.g., connectorization and wire count) that may also have mechanical effects. Once these factors are weighed and the package design is mature, simply plug in an LTC6803 platform and you're ready for a well-established, scalable and cost-effective data acquisition solution.

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