IEEE 1188a:2014 pdf free download – IEEE Recommended Practice forMaintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications
Measuring plate potentials in a vented(flooded) cell is relatively easy, but it is much more complicated in aVRLA cell. This difficulty is why one of the simplest field measurement to determine the presence ofnegative self-discharge is the float current of the string. Open circuit voltage of the cell is another method.In a vented(flooded) cell it is easy to check plate voltages separately by placing a reference electrode intothe cell electrolyte and reading the“half-cellvoltage directly with a voltmeter.But a VRLA cell is sealedto prevent oxygen from entering, so if the vent is removed to insert a reference electrode, the oxygen fromthe outside air will oxidize the negative plate, depressing its voltage and thereby disturbing the reading.
Installing a reference electrode into a VRLA cell in a lab is certainly possible, if done with great care, butextremely difficult to do so in the field with the battery in service. That is why the easiest and best fieldmeasurement to determine the presence of self-discharge in the field is to measure the float current, sincefloat current tends to increase with time and the dry-out in all cells that are prone to negative discharge.However, this reading is not instantaneous, but rather a matter of multiple readings to record and establishtrends.
Another factor that helped conceal the self-discharge phenomenon from the battery industry for so long isthat plate voltage readings will generally be quite satisfactory on all VRLA cells when they are new.Thisresult is because new cells are generally shipped very wet with electrolyte so that they behave essentiallylike vented (flooded) cells for the first period to time(perhaps a year or more). After this time frame thebattery will continue to dry out, become true VRLA cells, and will begin to deteriorate. At this point thebattery will either have a problem of negative self-discharge or not depending upon the purity of theirpaste.
With vented (flooded) cells, the negatives are submerged in acid so that the oxygen in the headspace of thecell can ‘t get to the negative plate to discharge it. In VRLA cels, on the other hand, oxygen is channeleddirectly to the negatives through voids in the AGM separator.(Indeed the principle of the VRLA celldepends upon this process). If the negative plate also has a propensity to self-discharge chemically simplydue to its own impurity, the two effects taken together, oxygen and impurity, can lead to negative plate self-discharging during float charge.
There are methods available to recover reversible PCL. These are catalysts，rehydration and high-ratecharging. These are described below.
Loss of water means loss of ionic contact between the plates.Conversely, restoration of the water that hasbeen lost from the electrolyte will re-establish ionic contact between the plates. This phenomenon has beenreferred to by some manufacturers as“lack of compression”because when this occurs there is usuallyinadequate compression between some parts of the plates and the glass mat separator which leads to theportions of the plates that are not in adequate contact with the glass mat to become undercharged andsulfated.This condition, in turn, does not allow that portion of the plate to participate in the discharge or for it to accept a charge.These conditions will lead to a degradation of the internal ohmic values of the cellswhich will impact the capacity of the cells.