Lead acid batteries used in today's uninterruptible power supplies (UPS) and telecommunications applications are a far cry from Planté's improvements on Sinstede's original lead and sulphuric acid secondary cells. Or are they?Despite all the enhancements to material used in electrolytes, separators, plates and cases, the introduction of valve regulated lead acid (VRLA) cells in the early 80s only confirmed that there was life yet in a technology fundamentally developed in the 1850s.
The traditional methods used for maintenance of the lead acid cells, measurement of depth and specific gravity of electrolyte, also became impractical with the introduction of the VRLA cell. Then the cell case (either as a 'jar' or mono-bloc') was sealed except for a pressure release valve. Electrolyte became a gel, and maintenance was reduced to quarterly voltage, and if they were lucky, load test.
However, out of this an industry established itself, particularly in the USA, to provide 'connected' means of electronic battery monitoring. This was initially accomplished by regularly measuring voltage of each cell in the battery string through a large wiring loom. There are many systems in use today all over the world that continue to successfully do this.
However, degradation of the cell can take place that can be masked from a voltage only test. The best test still proved to be a periodic discharge test. But what happens in between the quarterly tests? Can a cell fail that quickly. The answer is yes they can and yes they do. A VRLA cell, on a MegaVA UPS can become faulty in just a matter of days following a routine discharge test. This is caused by chemical changes taking place in the cell brought about by the UPS charge and discharge cycles itself. Cycles that take place even though the UPS never comes on line. In addition, corrosion can occur in all of the connecting hardware of the battery string, internal to the cell or external. All these affects are very significantly affected by temperature
Consequently a method had to be found that enhanced the voltage only test. The source resistance or impedance of the cell was isolated as a good indicator of cell condition, indicating both chemical degradation as well as physical damage.
Today there are a number of companies all over the world providing instruments that are either hand held, semi-permanent or permanently connected that provide electronic data to reliably predict cell failure using ohmic value measurement. This term was coined by the IEEE in the USA to replace the often confusing terms; resistance, impedance and conductance which have been repeatedly shown to be equally good indicators of performance and life expectancy.
