NI USB-6218 Measuring Differential voltages across battery pack

[leotheone88]

Hi All,

I am trying to use the USB-6218 to measure voltages across individual cellls in a battery pack, the batteries are connected in series. I want to use differential connections to measure each cell, but I am not confident in regard to how I should connect the batteries to the DAQ. IS there any suggestions for a external circuitry to prevent the DAQ from drawing current? Or can I just connect the DAQ directly to the batteries(li-ion 3.6Volts). 

Thank you,

Lin

[johnsold]

Lin,

The key point is to make sure that the maximum input voltage with respect to AI GND is not violated. The specification for Maximum working voltage for analog inputs (signal + common mode) = +/-10.4 V of AI GND for the USB-6218.

Three Li-ion cells at 3.6 V = 10.8 V. So you cannot have more than two cells in series from AI GND without exceeding the limit.

It does not matter whether you are using differential or single ended input configurations. The input limit still applies. Applying voltages higher than that may result in damage to your DAQ device.

Making measurements of individual cells in large battery packs is a significant challenge. How many cells are in series? Is there any other circuitry in the battery pack, such as cell equalizing circuits? What are the requirements for accuracy and loading? How fast do you need to take readings? Are there fault conditions against which the measurement circuits must be protected?

Lynn