Measuring voltage of a battery pack and temperature sensors :: NI-USB 6210

[d3jck]

Hi!

I have one LiPo battery (4S, 16.8 V - full charged) and I would like to measure voltages of cells during discharge of this battery with the NI-6210 DAQ (as differential signals). So, I would like to use 8 wires (2 wires for each cell), and connect them to the 4 differential inputs of the DAQ. Battery will be disharged with active load. Can I left Analog Input on DAQ floating (battery is one type of floating source)? I am not sure if it's ok to connect AI GND via resistor to the one wire of battery...OK, max VCM is +-11 V from AI GND. If I connect plus terminal of second terminal (minus terminal of third cell) to the AI GND, I will have +- 2 x Cell voltage on the input of instrumentation amplifier max. (2x(+-4.2V)=+-8.4V) which is OK, because I'm inside of +-11 V.

But I also want to measure temperature on the other inputs (I have 2 temperature sensors which output is current and it is practicaly in linear connection with temperature, so I will measure voltage drop on one resistor). This temperature sensors will have separate (sensors will not be supplied from the battery) but common power supply, and each sensor will be on their own cable with maximum length of 3, 4 meters.

I think, the best option is that I left AI GND unconnected. What do you think?

Thank you for any suggestions!

Best regards,

Dejan.

[d3jck]

I was talking about this example (on the picture).

 

Where would you reccomend to connect AIGND? One option is leave AIGND not connected (instrumentation amplifier will measure voltage referenced to (I know) floating AIGND). I don't know if reverse current is problem in this example because all devices are isolated....

Other option would be to connect A2 or A8 to AIGND via resistor. All voltages will be inside +-11 V, but this can also be dangerous, because GND of temperature sensor will be on AIGND voltage, which is not zero voltage of the battery.