The use of flooded battery cells in electrical power backup systems has always necessitated a high use of resources and personnel. Although long lasting (25 years), the batteries themselves are expensive, and must be maintained properly if this lifetime is to be achieved.
Proper maintenance dictates visits by service personnel on a monthly basis, to check the levels of the acid electrolyte in the cells, which must not be allowed to fall below the level of the plates or internal group (bus) bars.
Also, in several countries legislation dictates that the level of the electrolyte must be monitored at all times, either by remote means or by regular personnel visits.
Remote sensing of the electrolyte level in batteries has been available for some time, usually by inserting a sensor physically into the electrolyte via one of the cell filler caps. The sensor then uses either a float switch or sensing electrodes to determine the level. Insertion into the electrolyte, however, is not the most desirable method since the designs of many cell types make this difficult if not impossible.
It is, therefore, desirable to determine the electrolyte level from outside the cell container, without any intrusion into the container or the electrolyte.