Cell voltage

The potential across a single electrolysis cell, and the primary indicator of its condition.

Cell voltage is the sum of the thermodynamic minimum, the activation overpotentials at both electrodes, and ohmic losses through the electrolyte, membrane and hardware. Every degradation mechanism eventually shows up as an increase in one of those terms.

A stack contains hundreds of cells in series and normally reports one aggregate voltage. Because the aggregate is an average, a single cell drifting by a hundred millivolts changes it by a fraction of a per cent, which is well inside normal variation. The condition information is in the distribution across cells rather than in the mean.

Comparing cell voltages is only meaningful at a stated current density and temperature, since both affect it strongly. This is why a polarisation curve captured under controlled conditions is more useful as a baseline than any single reading.