voltage measuremen using Labview and SC-2345

[ptxlinh]

Hi all,

As part of my work, I need to do some voltage measurement for a coin battery. I followed the instruction in the Getting Started with Labview, section Accquiring Data and Communicating with Physical Instruments, however the voltage obtain is very different from the voltage of the battery. As this is my very first time using Labview and the SC-2345, I just simply cannot figure out what the problem is. I hope you can help me with this.

Attached are the program and the set-up I use. I connect a battery to a resistor, and use the first set of cables from SC-2345 to connect to the circuit. The red cable is connected to the positive side of the battery and the blue cable to the negative side of the battery.
In Labview, I select physical channel ai1 to measure the voltage. For the Terminal Setting, as I am not sure what the difference is, I tried for all three available methods. However, the results are all wrong.

In fact, when I disconnect everything, the values are the same when being connected to the battery. My battery is 3V, whereas the reading is somewhere around 0.20V (if I use RSE) and around 5.2 (if I use Differential and set the max input as 5V). Apparently, the system isn't reading the value from the battery.

Do you know what could go wrong in the way I did? Is there anything that I need to set up but I didn’t? I checked the cables connections but all look fine to me.

Thank you very much for your help!

Best wishes,
Linh

[ptxlinh]

This is the image of the set-up

[NI-Bongo]

Hi ptxlinh,

Which modules are you using in the SC-2345 and how are you connecting the signals on that end?  Which DAQ device do you use to connect the SC-2345 to your computer?  The connections for differential, RSE, and NRSE will be different.  With a floating signal source, any of the measurement configurations should work, but we need to be sure it is connected properly to the SCC module, which must in turn be properly connected and configured with the SC-2345.

Also, typically the analog input of a multifunction DAQ device has a very high input impedance that means you don't need a resistor to measure the potential difference between the battery terminals.

Getting Started with NI-DAQmx
http://zone.ni.com/devzone/cda/tut/p/id/5434

[ptxlinh]

Dear John,

Thanks a lot for your help. I'm using SC23435, PCI-6251 card and Labview 8.0.

(1). Here are how the cables connected:

- The cables are connected to SCC-AI01. This module is sitting at slot J1 (ACH0/8).
- The red cable is connected to Port 4, the black cable is connected to Port 3 of the module.

In using Measurement & Automation Explorer, the configuration of SCC is set as seen in the attached image “config.bmp”.

I did a simple test to measure only a battery of 3V using Test-Pannel, using different channel Dev1/ai0, Dev1/ai1, Dev1/ai8. However, the results are still wrong. Please refer to the corresponding images attached.

Do you know what else can go wrong? Thanks a lot.

(2). Actually, I need to do all this because I need to measure how long a battery last if it is used for running a constant current. I can't use the formula because in reality, the hours may be less than the capacity dividing the current. Hence, the engineer suggests me to use a resistor as a load to drain the voltage. When the voltage reaches its cut-off value, I can derive how long the battery run.
Theoretically, this is not true as well because when voltage is decreasing while resistor stays constant, current won't be a constant anymore. Nevertheless, I want to give it a try. Also, I am not sure if there is a better way to do this. If you have any suggestion, I would appreciate very much.

Best wishes,
Linh

[NI-Bongo]

Hi Linh,

Check out page 6 of this manual:
SCC-AI Series Isolated Analog Input Modules User Guide and Specifications
http://www.ni.com/pdf/manuals/371066c.pdf

Let's use the NRSE configuration suggested in Figure 2 of the manual.  Connect pin 3 to AISense as well, which is on pin 62 of the SC 2345 terminal block.  Switch the configuration in Measurement & Automation Explorer to NRSE and see what you get then.

The resistor makes sense to have in the circuit if you are actually interested in the current draw from the battery.  As you mention, it will not remain constant, but if you have a threshold value for the voltage, you should be able to get a plot of votagle vs. time and convert it to current vs. time to see exactly how constant it is.