Samlexpower Evolution Series - Page 20

20 | SAMLEX AMERICA INC.
1.4.4.1 Sulfation: Section 1.4.1.2 above gives details of basic electrochemical reactions during charging and
discharging. If the charging process is not complete due to the inability of the charger to provide the required voltage
levels or if the battery is left uncharged for a long duration of time, the soft Lead Sulfate (PbSO
4
) crystals on the
Positive and Negative plates that are formed during discharging / self discharge are not fully converted back to Lead
Dioxide (PbO
2
) on the Positive plate and Sponge Lead on the Negative plate and get hardened and are difcult to
dislodge through normal charging. These crystals are less-conducting and hence, introduce increased internal resistance
in the battery. This increased internal resistance introduces internal voltage drop during charging and discharging.
Voltage drop during charging results in overheating and undercharging and formation of more Lead Sulfate (PbSO
4
)
crystals. Voltage drop on discharging results in overheating and excessive voltage drop in the terminal voltage of the
battery. Overall, this results in poor performance of the battery. To dislodge these hardened Lead Sulfate crystals, some
chargers are designed to detect a sulfated condition at the start of the charging process and go through an initial
De-sulfation Mode that sends high frequency, high voltage pulses at the natural oscillation frequency of the crystals to
dislodge the hardened crystals. Sulfation may also be reduced partially by the stirring / mixing action of the electrolyte
due to gassing and bubbling because of intentional overcharging during the Equalization Stage.
1.4.4.2 Electrolyte Stratication: Electrolyte stratication can occur in all types of ooded batteries. As the
battery is discharged and charged, the concentration of Sulfuric Acid becomes higher at the bottom of the cell
and lower at the top of the cell. The low acid concentration reduces capacity at the top of the plates, and the high
acid concentration accelerates corrosion at the bottom of the plates and shortens battery life. Stratication can be
minimized by the Equalization Stage by raising the charging voltage so that the increased gassing and bubbling
agitates / stirs the electrolyte and ensures that the electrolyte has uniform concentration from top to bottom. The
stirring action also helps to break up any Lead Sulfate crystals, which may remain after normal charging.
1.4.4.3 Unequal charging of cells: During normal charging, temperature and chemical imbalances prevent
some cells from reaching full charge. As a battery is discharged, the cells with lower voltage will be drained further
than the cells at higher voltage. When recharged, the cells with the higher voltage will be fully charged before
the cells with the lower voltage. The more a battery is cycled, the more cell voltage separation takes place. In a
healthy battery, all the individual cells will have the same voltage and same specic gravity. If there is a substantial
difference in the cell voltages (0.2 V or more) and in the specic gravities (0.015 or more) of the individual cells,
the cells will require equalization. Equalizing batteries helps to bring all the cells of a battery to the same voltage.
During the Equalization Stage, fully charged cells will dissipate the charging energy by gassing while incompletely
charged cells continue to charge.
1.4.5 Temperature Compensation To Prevent Over And Under Charging
1.4.5.1.1 Electrochemical reactions during charging / discharging of Lead Acid / Nickel Zinc (Ni-Zn) Batteries are
affected by changes in the temperature of the electrolyte. These type of batteries have a Negative Temperature
Coefcient of Voltage i.e. the battery charging / discharging voltages will fall due to rise in electrolyte temperature and
will rise due to fall in electrolyte temperature. Battery manufacturers, therefore, specify battery voltages and capacity at
Standard Room Temperature of 77º F / 25º C. The Negative Temperature Coefcient is normally within a range of -3 to
-5mV/ ºC/Cell or (i) -18 to -30mV / ºC for a 6-cell, 12V battery or (ii) -36 to -60mV / ºC for 12-cell, 24V battery.
1.4.5.1.2 Lithium Ion charging voltages are not affected by temperature and hence, do not require temperature
compensation.
SECTION 1.4 | General Information – Battery Related