ni 9205-Grounding Problem

[gaurav841]

Hi,

I am using NI 9205 and NI 9219 Analog input to measure cell voltage of a 20 Cell fuel cell stack. Both modules are being used in Differential mode. 

When I don't ground the signal of these modules, the values of cell voltage are the same as I expect but I get a bump after around 100 s continuously for all channels including the NI 9219 which is completely isolated I guess

When I ground the signal, the bump disappears but then the values of channels 1, 2 , 3 goes down. I measure the cell voltage by multimeter directly on fuel cell and there is no problem at the fuel cell level as voltage is fine. I tried measuring the different cells on these channels and still same problem. 

I am attaching pictures of the graph which I get which maybe of help to understand the difference between grounding and non-grounding. 

Is there a way to get away with this problem? Not sure if this is a software issue or hardware issue.

Thanks,

Gaurav

[OA1]

As far as I know there is no way to stop it however you can reduce it by increasing the interchannel delay in the properties of the Ni 9205.

[johnsold]

Gaurav,

With a 20 cell stack you may be exceeding the common mode specification of the inputs.  This is from the specifications:

---

Maximum working voltage for analog inputs

(signal + common mode)..................Each channel must remain

within ±10.4 V of common

---

The voltages you show in your image are about 0.76 V. With 20 cells that means > 15 V across the stack.  Unless your common reference is near the center of the stack, you will be exceeding the rating.

Lynn

[gaurav841]

Thanks Lynn for pointing our the specification to me.

I tried connecting the COM to middle of stack so that the cell voltages are in the range of +/- 10V of the common. 

Even after doing this, the problem still remains. 

Does (signal + common mode) means the maximum voltage be less than 10.4 V with common being at 0 V. or can this be shifted like by connecting common to maybe 10 V and then the range be (0 to 20 V)?

Thanks

Gaurav

[johnsold]

Gaurav,

The COM terminal is the internal "ground" or common reference voltage internal to the NI 9205 module. All the circuitry connected to the terminals of the device use COM as the reference. So no voltage can be greater than +/-10.4  V away from COM. The module itself is isolated from the chassis or earth ground by up to 250 V. 

This means that the COM on the module can be connected to a voltage in the cell stack which is not at earth ground, so long as that voltage is within 250 V of earth ground.  When you do this, it is important to remember that there may be significant voltage differences between the COM terminals on different modules! Connecting them might damage equipment.

Does the controller for the fuel cell stack do any kind of equalization or reconfiguration which could cause the pulses you see? Those pulses look like something is charging up and then suddenly discharging.

Lynn

[gaurav841]

Lynn,

Thanks for the detailed description. 

My voltage is maximum 18-19V and I am thus I guess I am operating in the safe range.

I can't find any pulses on the controller of system as I am recording the stack voltage at 0.5 Hz frequency using the controller which is working as Load Bank for the fuel cell. 

Not sure where this pulse thing is coming from.

Gaurav