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Boost accuracy with cell-monitoring system

Posted: 14 Mar 2014  Print Version  Bookmark and Share

Keywords:cell measurement system  cell-monitoring  analogue front-ends  analogue-to-digital converter  AFE 

When designing battery-operated systems, developers must deal with the cost, space and accuracy trade-offs. "Accuracy" for us here is not the accuracy of the end application. Rather we are focusing on the accuracy of the cell measurement system—if this fails, so might the application. Systems operating with a lithium phosphate cell require highly accurate monitoring because of the nearly-flat discharge curve of that battery chemistry. Other common chemistries do not have as much flatness on their discharge curves, so the accuracy of measurements can be less precise, less accurate.

The accuracy required for a battery-monitoring system is tightly linked to the battery chemistry and the specific application.

Optimising cell monitoring: The process of accuracy
No one will argue that we need to optimise the accuracy of cell monitoring in battery-powered systems. The task is most easily accomplished when broken into simple steps focused on meeting specific application requirements. The design process can be divided into three stages: first, select the cell architecture for an application; second, determine the critical performance parameters; and third, select the system components.

The system architecture is the primary factor that will drive any effort to optimise cell-monitoring accuracy. As we examine architectures, we will also address the interrelated stages two and three, the critical performance parameters and optimal system components, respectively. The selection of suitable components is wide and each architecture can be used in a range of systems with very different needs. Consequently, the discussion of system components will include examples.

Architectural analysis
The table shows the relative cost, maximum expected six-sigma error, and maximum expected three-sigma error of the four cell architectures considered in this analysis. The six-sigma error denotes the maximum error statistically expected on 99.99966% of all systems built using the respective architecture. The three-sigma error denotes the maximum error statistically expected on 99.73% of all boards built using the same architecture.

Table: Cost/performance comparison of different cell architectures.

Accuracy-optimised architecture
To obtain high accuracy in a cell design, the microcontroller should factor as little as possible into the error of the overall system. While many microcontrollers integrate ADCs and references, these components generally do not have the resolution or accuracy required for reliable sub-millivolt measurements. Consequently, these types of microcontrollers should be avoided for applications that require the highest cell-monitoring accuracy.

The accuracy-optimised architecture, shown in figure 1, provides the greatest accuracy because it offers flexibility for selecting the main components that contribute to system precision: high-accuracy, battery-measurement analogue front-ends (AFEs), the analogue-to-digital converter (ADC), and the reference. You will note immediately that the ADC and reference are separate components. This is important and deserves more attention.

Figure 1: Accuracy-optimised architecture. This design features a high-accuracy battery measurement analogue front-end (AFE), a high-accuracy ADC, an external reference with excellent initial accuracy and temperature drift, and an independent microcontroller.


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