I am building a DAQ system to measure differential voltages across 50 battery cells that are connected in series. I plan on using NI 9205 modules in a cDAQ-9178 chassis. The voltage across each cell will never exceed 1.3V, giving an absolute maximum series voltage of 65V.
I read through Grounding Considerations for Improved Measurements (lets call this GCIM) and the NI 9205 manual but I am still a bit lost.
TL;DR: Can I use four NI 9205 modules to measure 1.3V differential voltages across each of 50 battery cells in series without damaging anything? If not, how many cells can I measure with this equipment?
Each AI+ line will be directly connected to the AI- line of the next channel to use as it's reference voltage. I will be using the bias resistor configuration shown in Figure 7 of the GCIM. I used equation 1 from GCIM based on 50 cells. Is this correct?
- V_CM = (65V + 63.7V) / 2 = 64.35V
Here is everything else I've found that seems relevant to my question:
- The only direct mention of common mode voltage is in the first paragraph of the first page of the manual, " In addition, the NI 9205 also includes [...], and high common-mode voltage range." But what is that voltage range?
- In the "Safety Voltages" section it says, "Connect only voltages that are within the following limits: +-30V channel-to-COM", but differential signals are floating relative to COM, so does this even apply?
- We are using the DSUB variants of the module, and in the "NI 9205 with DSUB Isolation Voltages" section it says, "60 V DC, Measurement Category I continuous channel-to-earth ground." What does this mean? Is that the common mode voltage limit?
Any guidance is highly appreciated!
Bonus question: How are the values of bias resistors calculated? What should I use for this setup?
