Q: How is current interrupt (iR) performed?
A: The current interrupt technique periodically (10 times per second) does the following:
1. Measures the cell voltage (V1)
2. Rapidly removes the cell current
3. Measures the cell voltage a short time after removing the cell current (V2)
4. Reapplies the cell current
5. Calculates the cell iR resistance as follows: iR = (V2 – V1)/I (cell current before interrupt)
The data is displayed and valid only when a current is applied that is large enough to make the voltage difference between steps 1 and 3 above a minimum value. The current interrupt process is suspended during impedance measurement experiments.
*Update! – In response to user questions:
Is the ohmic overvoltage measured automatically by the iR technique during an experiment?
Does E_iR_Stack Read in FCView correspond to the ohmic overvoltage?
The current interrupt function built into the 850 quickly removes load current and measures cell voltage immediately after the interruption. This voltage (iR Comp Potential) has the Stack Potential voltage subtracted from it and this difference (iR Drop) is divided by cell current before the interrupt to obtain iR resistance in milliohms (iR mOhm):
iR in milliohms = 1000 x (iR Comp Potential – Stack Potential)/ Current
The current interrupt cell resistance measurement is running and displaying data whenever the current is large enough to produce a measurable voltage difference during the interrupt. This is the case during Background operation of the software or if an experiment is running. For the current interrupt data to be saved to data files during background recording or during an experiment, the iR Comp Potential, iR Drop, or iR (mOhm) data values (in the right side of the FuelCell software window) must be checked. These values with checked boxes will be visible in FCView. Ask us or refer to the FuelCell Help menu for more details.