Lamar Technologies Alpha C-25 - Manual (Page 8)

3. OPERATING GUIDE
ALPHA C-25 Manual V2.1.doc Page 8
As shown by the curve in figure 3-1 the cell voltage of a discharged battery rises
rapidly when the battery is first placed on charge. The extent of the initial rise
depends on the charging rate. As the charge continues, the voltage rises at a
slower rate and eventually levels off when a full state-of-charge is reached. It can
be seen that the specific gravity reading lags behind the rate of ampere-hour return
during most of the charging cycle. Consequently, the specific gravity is not
indicative of the available ampere-hour capacity until the cell approaches a full-
charge state.
When a battery reaches a full state-of-charge the voltage of the battery will
stabilize and remain constant or decrease (in constant current mode). The
charging should discontinue at this stage. A minimum of 103% of previous
discharge should be placed in the battery.
In general a vented battery may be charged at any rate that will not produce
excessive gassing or electrolyte temperatures above 113°F (45°C). VRLA
batteries should never be charged in a constant-current mode with a current
greater than C
1
/10 (C
1
equals the rated capacity of the battery). During constant
current charging at a rate in excess of C
1
/10, oxygen is produced at an excessive
rate. The resulting increasing pressure will cause the cell to vent. Venting of
gasses results in a depletion of electrolyte. As the electrolyte cannot be replaced in
a sealed battery, the cell will dry out resulting in a decrease in capacity and
eventually battery failure. Therefore constant-potential charging is the
recommended charge method for valve regulated lead acid batteries
3.2 CHARGING METHODS
There are two main methods of charging a battery:
1) Constant current (CC)
2) Constant potential (CP)
In what follows both methods will be described in some detail.
3.2.1 CONSTANT-CURRENT CHARGE METHOD
In this method the current remains at a preset level while the voltage can reach a
high level, e.g. 34-37 volts.
An advantage of the constant-current charge method is that the ampere-hour input
into the battery can be determined precisely by multiplying the charging current
with the charge time in hours. However, it is necessary to ensure that the battery is
not charged at a high rate for an excessive period of time. Such overcharging can
result in overheating, excessive gassing, and possible damage to the battery.