Electro-galvanic Fuel Cell - Cell Limitations

Cell Limitations

Oxygen cells behave in a similar way to electrical batteries in that they have a finite lifespan which is dependent upon use. The chemical reaction described above causes the cell to create an electrical output that has a predicted voltage which is dependent on the materials used. In theory they should give that voltage from the day they are made until they are exhausted, except that one component of the planned chemical reaction has been left out of the assembly: oxygen.

Oxygen is one of the fuels of the cell so the more oxygen there is, the more electricity is generated. The chemistry sets the voltage and the fuel, the oxygen, sets how much electric current it can give. If you put an electric circuit on the cell that draws current you can draw up to this current but ask for more and the voltage from the cell fades.

Failures in cells can be life threatening for technical divers and in particular, rebreather divers. The failure modes common to these cells are: failing with a higher than expected output due to electrolyte leaks, current limitation due to exhausted cell life and non linear output across its range. These failures are usually attributable to physical damage, contamination during manufacture or defects in manufacture.

Failing high is invariably a result of a manufacturing fault or mechanical damage. In rebreathers, failing high will result in the rebreather assuming that there is more oxygen in the loop than there actually is which results in hypoxia.

Current limited cells do give a high output in high concentrations of oxygen. The rebreather assumes there is insufficient oxygen in the loop and injects to reach a setpoint the cell will never achieve resulting in hyperoxia.

Non-linear cells do not perform in an expected manner across its range of oxygen partial pressures. Calibration will not pick up this fault which results in inaccurate loop contents of a rebreather. This gives the potential for decompression illness.

Preventing accidents in rebreathers from cell failures is possible in most cases by accurately testing the cells before use. Some divers carry out in-water checks by pushing the oxygen content in the loop to a pressure that is above that of pure oxygen at sea level to indicate if the cell is capable of high outputs. This test is only a spot check and does not accurately assess the quality of prediction of failure of that cell. The only way to accurately test a cell is with a calibrated test chamber which can hold a static pressure without deviation and the ability to log the results and graph them.