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-optimized 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 analog front-ends (AFEs), the analog-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-optimized architecture. This design features a high-accuracy battery measurement analog front-end (AFE), a high-accuracy ADC, an external reference with excellent initial accuracy and temperature drift, and an independent microcontroller.
By selecting an external reference and an ADC separately, however, additional cost is incurred, and the designer must marginally increase the price of the system as compared to other architectures in order to achieve the greatest accuracy.
The Important Parameters
For optimal accuracy this design needs a high-accuracy battery-measurement AFE that provides the excellent accuracy for monitoring cell chemistries with nearly flat discharge curves, such as lithium phosphate cells. An AFE that monitors multiple cells simultaneously is best.
When selecting the ADC, integral nonlinearity (INL), offset and gain errors, and the temperature coefficients associated with offset and gain errors are the most significant specifications. The offset error is most important, since it will cause the greatest change in accuracy relative to the other parameters. The ADC’s gain error ranks second in importance, followed by INL.
References that are external to the ADC are better suited for high-accuracy systems, because they have better initial output voltage accuracy and output