Many industrial batteries, for example fork truck batteries, contain an electrolyte solution used for storing and conducting electrical power.  Over time water in the electrolyte solution evaporates from the battery causing the electrolyte solution level (the “electrolyte level”) to fall.  If the electrolyte level falls below a minimal acceptable level in a battery, serious problems can occur to the battery such as reduced electrical power output and/or permanent damage.  For example, if the electrolyte level drops below the top edge of a negative plate in the cell of a battery, the negative plate is exposed to air, which rapidly causes the negative plate to oxidize. 
To address this problem, numerous devices have been proposed for monitoring the electrolyte level in the battery to ensure that the water is replenished before the electrolyte level drops below the minimal acceptable level.  For instance, devices mounted outside the cell of a battery that indicate when the electrolyte level is low are now in common use, see e.g., U.S. Pat.  No.  5,936,382, entitled Battery Electrolyte Level Monitor, issued Aug. 10, 1999, incorporated herein by reference.  The common principle for most of these devices is a metal probe inserted through the cover of a pilot cell in the battery.  Typically, when the tip of the probe touches the electrolyte, the probe sends a signal (via electrical circuitry) to an indicator, such as an alarm or a light, indicating that the electrolyte level in the battery is satisfactory.  On the other hand, when the electrolyte level drops below the tip of the probe, it sends another signal to the indicator that the time for re-watering the battery is imminent. 
One drawback with these probe-based devices is they cannot easily read the electrolyte level below the top edges of battery separators.  Separators are porous plastic sheets that keep the plates apart electronically, but permit ionic current flow between them.  If the metal tip of the probe should touch the wet separator in general, or have any ionic contact with the separator whatsoever, for example through a droplet of electrolyte, or tarry substance, or wet particulate matter, etc.,  then this may cause the probe to continue sending a signal indicating that the electrolyte level is satisfactory, even though the electrolyte may have fallen below the acceptable level.  In other words, the probe causes the indicator to illicit a false indication that the electrolyte level is satisfactory, when in fact, it is too low. 
As a result, most battery manufacturers have kept their probe tips above the separators and require watering more frequently than is actually needed.  However, now there is a demand for batteries that are designed for very low maintenance, i.e., very long watering intervals.  That is, there is a desire to allow the electrolyte levels to drop to a level that is well below the level of the separators, such as to the tops of the plates. 
To make the probes more accurate at measuring the electrolyte levels below the separators without touching them, a mechanical “spreader” or shield is used to wedge the separators apart so that the probe can descend between them without touching them or having any ionic tracking paths to the separators. 
One limitation with this mechanical solution is the tight tolerances involved.  For example, the separators even in a large battery cell may be only a few millimeters apart, and much less on smaller cells.  Therefore, the risk of ionic contact with the separators is quite high, which results in a false signal. 
Still another limitation with spreader designs is that a hole must be provided in the cell's cover which is aligned perfectly above the positive plate; otherwise, the probe will not fit precisely and may damage the separators.  Existing punch-out holes in many batter cell covers, used routinely for level probes—generally do not line up with the plates and cannot be used in conjunction with the spreader designs.  The result is a second set of holes must be drilled into the cell covers, which adds labor cost and inconvenience.  Thus, there is currently no inexpensive and accurate way to measure electrolyte levels in batteries once the electrolyte levels fall below the top of the separators. 
Another drawback associated with current probe designs is their failure to recognize when the electrolyte level in a battery cell falls below the level of the probe.  Many times an indirect current path can still exist from the tip of the probe, along the length of the probe, around the inside of a battery cell and finally down the cell wall to the lowered electrolyte level.  Although this path is of a higher resistance than a direct current path from the probe tip submerged in the electrolyte, the indirect current path may still cause a false indication.