C&D Technologies RS-1476 Standby Battery Vented Cell - Manual (Page 30)

However, some users of vented batteries have chosen to use the impedance,
conductance or resistance measurement technique for additional analytical
data. It is important that readings be taken periodically in exactly the same
location. The readings can only be compared if the cell electrolyte temperature
is similar to that of previous readings. Values are then compared against both
the initial values, at the time of installation, as well as individual cells of the
same age. See IEEE-450, Annex J for further information.
2.11 Plate polarization measurements
The life of a lead acid battery is optimized when it is properly floated at the
middle of the recommended string float voltage, operated in a temperature
controlled environment and is rarely cycled. Application requirements can
make the ideal operating parameters impossible or compromised. To better
understand and monitor the health of a flooded lead acid battery, it is possible
to measure the plate potentials independent of the float voltage of a given cell.
This technique makes use of an instrument known as a third electrode or
reference electrode. The third electrode is inserted into the electrolyte of the
cell under examination and voltage measurements are made that may be
either direct measurements or converted measurements, depending on the type of third electrode used to record the
polarization potentials of the positive and negative plates. It is interesting to note that although a given cell float voltage may
be above or below the recommended average value of cells in a battery, the cell may be operating properly. This is
because the polarization potentials of the positive and negative plates are within an acceptable range. Third electrode
measurement instruments are available in both miniature lead acid cells and mercury-mercurous sulfate cells.
Although the technique has been used almost exclusively by battery manufacturers and research and development
laboratories for years or under highly controlled field test conditions, some battery testing organizations have added
this test parameter for in-depth customer site evaluations.
Third electrode measurements can inform the experienced battery technician of the following:
1. If the positive plates are properly polarized, excessive plate corrosion will be minimized while still maintaining the
plates at a sufficient potential and state of charge to avoid sulfation
2. It is essential that the negative plates remain fully charged while at the same time not depressing the potential of the
positive plates. The correct negative plate potential is dependent on individual manufacturers’ design parameters
and vary widely due to the addition of organic material (expanders) in the negative plates, depolarizers either added
to the electrolyte, or in the case of lead-antimony cells, the transfer of antimony as the cell ages.
FIGURE 4.2.10 - Measuring internal cell
ohmic values
PART 4
REFERENCE INFORMATION, TROUBLE SHOOTING & EXTENDED MAINTENANCE (CONTINUED)
RS1476/0215/CD 28 www.cdtechno.com
2.12 Sedimentation
As the lead-acid battery ages a phenomena occurs that results in an accumulation
of positive active material in the sediment chamber of the cell. This is called
sedimentation. This occurrence is a natural result of the charging process while the
battery is on float charge. During the charging process, the mechanical agitation
from the generation of oxygen at the positive plate loosens contact at the surface
of the plate of the softer positive active material resulting in shedding. The shed
active material settles to the bottom of the jar where design space has been
provided for the containment of the sediment. As previously mentioned, this is
a natural occurrence. It ceases to be a natural occurrence when the sediment
accumulation becomes excessive. This is typically the result of discharging the
battery too frequently or overcharging the battery for a prolonged time at higher
voltages. An example of excessive sedimentation is shown in Figure 4.2.11.
2.13 Shifted Separators
Shifted separators are the result of a loose element pack that has allowed the
separators movement that may occur during transit. While a cosmetic flaw, the
operability of the cell is not compromised and the cell remains functional. As can be
seen in Figure 4.2.12, there is no danger of short circuiting between the plates even
though the positive plates are nearer to the separators edge than normal.
Figure 4.2.11
Figure 4.2.12