Design a cell-monitoring system to optimize accuracy, lower costs, or both: Page 9 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.
a reference voltage as close to the full-scale input as possible and make sure that it operates within the capabilities of the microcontroller. This step is quite important. The objective is to lower the IN/OUT ratio as much as possible and, therefore, decrease the amount by which the output must be multiplied to regain the original scale. For example, the IN/OUT ratio for the internal voltage reference of the Freescale K10P64M72SF1 microcontroller is 4V/1.195V = 3.35. However, using a 3.3V external reference produces an IN/OUT ratio of 4V/3.3V = 1.21.
 
Finally, the reference’s initial accuracy and temperature coefficient parameters are important, although they do not influence accuracy as much as the reference voltage.
 
The Component Selection

Once again, the Freescale Semiconductor K10P64M72SF1 microcontroller is used. The MAX6034B voltage reference was selected for its price point as well as its 13mV initial accuracy and 75ppm/°C (max) temperature drift. It also has a 3.3V option, which the selected microcontroller will accept and which greatly improves accuracy compared to a 1.195V reference used in the cost-optimized example above (Figure 5).
 
Selecting R 1 = 1MΩ and R 2 = 213kΩ with 0.1% tolerance, the maximum error introduced by the voltage-divider is 3.55mV.

The cost-optimized, accuracy-enhanced architecture using the Freescale Semiconductor K10P64M72SF1 and the MAX6034B voltage reference achieves six-sigma error as low as 31.154mV and three-sigma error as low as 17.632mV. Compared to the cost-optimized architecture discussed above, this example design offers an 87.6% decrease in six-sigma error with only a minor increase in the cost of the system.  

Conclusion

Four architectures have been outlined to give system designers flexibility in implementing battery-management systems. In all cases the MAX14921 (or MAX14920) was the optimal AFE capable of delivering high-performance battery monitoring and cell-balancing capabilities for multiple design constraints. The selection of ADC, reference, and microcontroller will depend on the accuracy-cost trade-off.

References

1 If the MAX6194 reference is used in

Design category: 

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