Design a cell-monitoring system to optimize accuracy, lower costs, or both: Page 3 of 10

December 02, 2013 //By Jeremy Georges
Design a cell-monitoring system to optimize accuracy, lower costs, or both
Jeremy Georges, MTS, Maxim Integrated discusses cell-measurement architectures for cell balancing and battery-measurement applications and presents example designs that meet diverse accuracy and cost requirements.
voltage temperature coefficient values than references integrated in an ADC. Find an external reference with excellent initial accuracy and temperature drift, because that performance will enhance system accuracy at room temperature and the cell’s accuracy over the whole operating temperature range.
 
An Error Measurements spreadsheet is available so you can enter the above parameters in a Total Error Calculations table. This tool will produce an expected maximum six-sigma error over temperature and an expected maximum three-sigma error over temperature for the system. Using this calculated error as a guideline for meeting design requirements is advised, but actual accuracy is typically much better than even the three-sigma value.
 
The Component Selection

Since accuracy is the main goal in this system, the designer must select system components accordingly. The system requires a battery-monitoring analog front-end (AFE), a capable microcontroller, an ADC with reasonably high resolution, and a reliable voltage reference. Figure 2 diagrams a solution optimized for maximum performance and high accuracy. Note that some lithium cell chemistries have a full-charge voltage up to 4.4V per cell. For such chemistries the precision, micropower
MAX6194A reference is recommended. 1
 

 

Figure 2. Block diagram of an accuracy-optimized architecture for battery monitoring. This design uses the MAX14921 AFE that simultaneously samples up to 16 battery cell voltages, MAX11163 ADC, and MAX6126 reference.

This design uses a high-accuracy, battery-measurement AFE that monitors up to 16 battery cell voltages 2 and has a unique sample-and-hold architecture for monitoring cell chemistries with nearly flat discharge curves. The MAX14921 measurement
accuracy is well within 500µV of the battery voltage. Bench measurements have shown that average measurement errors from the IC alone are well under 300µV for most cell voltages. 3
 
The 16-bit ADC is selected for its excellent offset error, gain error, and INL. The reference provides0.02% initial accuracy and excellent temperature coefficient for reliable performance over the entire -40°C to +85°C temperature range

Design category: 

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