Samlexpower Evolution Series - Page 18

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1.4.3 Battery Charging Stages:
General descriptions of 4 stages of battery charging are given at Sections 1.4.3.1 to 1.4.3.4 below. Depending upon
the type of battery and its application, different Charging Proles can be created using appropriate charging stages.
NOTE:
7 types of Charging Proles are available in EVO through programming parameter "CHARGING PROFILE". Refer to
Section 5.6 for details.
1.4.3.1 Stage 1 - Constant Current Bulk Charge Stage: In the rst stage, known as the Bulk Charge Stage,
the charger delivers a constant, maximum charging current that can be safely handled as specied by the battery
manufacturer. The value of the Bulk Charge Current depends upon the total Ampere Hour (Ah) capacity of the battery
or bank of batteries. A battery should never be charged at very high charging current as very high rate of charging will
not return the full 100% capacity as the Gassing Voltage rises with higher charging current due to “Peukert Effect”.
Also, very high charging current produces higher temperature in the active material of the plates resulting in loss of
cohesion and shedding of the active material that settles on the bottom of the plates. Shedding of the active material
results in loss of capacity. If the quantity of the shedded active material at the bottom of the plates rises, it may short
the cells.. As a general thumb rule, the Bulk Charging Current should be limited to 10% to 13% of the Ah capacity of
the battery (20 Hour discharge rate). Higher charging current may be used if permitted by the battery manufacturer.
This current is delivered to the batteries until the battery voltage approaches its Gassing Voltage of around 2.4 V per
cell at 77º F / 25º C or 14.4 V for a 12 V battery and 28.8 volts for a 24 volt battery. The Bulk Charge Stage restores
about 75% of the battery's capacity. The Gassing Voltage is the voltage at which the electrolyte in the battery begins
to break down into Hydrogen and Oxygen gases. Under normal circumstances, a battery should not be charged at a
voltage above its Gassing Voltage (except during Equalization Stage) since this will cause the battery to lose electrolyte
and dry out over time. Once the Gassing Voltage is approached, the charger transfers to the next stage, known as the
Absorption Stage.
i
INFO
As the Bulk Charge Stage is a constant current stage, the charger does not control the voltage and the
voltage seen at the output terminals of the charger will be the actual battery voltage (this will rise slowly
towards the Gassing Voltage under the inuence of the constant charging current).
1.4.3.2 Stage 2 - Constant Voltage Absorption Stage During the Absorption Stage, the charger changes from
constant current to constant voltage charging. The charging voltage is held constant near the Gassing Voltage to
ensure that the battery is further charged to the full capacity without overcharging. The Absorption Stage feeds
additional 40% of the capacity that adds up to a total charged capacity of around 115% to take care of around
15% loss of charging efciency. As the output voltage of the charger is held constant, the battery absorbs the charge
slowly and the current reduces gradually till all of the soft Lead Sulfate (PbSO
4
) crystals have been converted to Lead
Dioxide (PbO
2
) on the Positive Plates and Sponge Lead (Pb) on the Negative Plates. The time the charger is held in the
Absorption Stage before it transitions to the next Float Stage is determined in one or more of the following conditions:
a) By a xed timer (e.g. 4 to 8 Hours). This may result in overcharging of almost fully charged batteries.
b) When charge current drops to specied threshold: Switching over to the Float Stage when the charge
current drops below a certain threshold (e.g. 10% of the charger Bulk Charge Current). This may result in
overcharging and locking in the Absorption Stage if the battery is feeding an external load that has a value > the
specied threshold.
SECTION 1.4 | General Information – Battery Related