Pwm Charging Control - Samlexpower SCC-1208L Manuel Du Propriétaire

SECTION 3 |
3.1 PRINCIPLE OF OPERATION OF PWM (PULSE WIDTH MODULATION)
BATTERY CHARGING
A Lead Acid Battery is normally charged to full capacity using the following 3 stages of
charging (See Fig. 2.1):
• First Stage: Constant Current Bulk Charge Stage
• Second Stage: Constant Voltage Absorption Stage
• Third Stage: Constant Voltage Float Stage
PWM (Pulse Width Modulation) battery charging is the most efficient and effective
method for recharging a battery in a solar system. In this controller, the solar panel is
connected to the battery through a series connected MOSFET Switch. During the Con-
stant Current Bulk Charge Stage,the MOSFET Switch is "on" continuously till the next
Absorption Stage is reached. As the MOSFET switch is in"on"condition continuously, the
Solar Panel is connected to the battery continuously and it delivers a constant current
nearly equal to the Short Circuit Current Isc. (actually equal to the current on the V-I
curve of the panel corresponding to the battery voltage).
The subsequent Absorption and Float Stages are constant voltage stages. The MOSFET
switch can NOT be kept in "ON" condition continuously as it will result in continuation
of the Constant Current Stage due to the V-I characteristic of the Solar Panel. In order
to convert the constant current characteristic of the Solar Panel to a constant voltage
characteristic for battery charging purposes, the current from the Solar Panel is fed to
the battery not continuously but in pulses. The MOSFET Switch turns"on" and "off" at
a fixed frequency where the width of the "On Time" is controlled (in SCC-1208L, the
frequency is 30 Hz). This is called PWM or Pulse Width Modulation. The ratio of On Time
of the MOSFET Switch to the sum of On and Off Times is called the Duty Cycle and is
specified in percentage. For example, if the MOSFET Switch remains in "on" condition
continuously, the Duty Cycle will be 100% and if it remains in "on" condition for half
the time out of the sum of "on" and "off" times, the Duty Cycle will be 50%. When the
MOSFET switch is in the "on" condition, a short pulse of constant current will be fed to
the battery corresponding to the voltage of the battery at that time. During this pulse
of constant current, the battery voltage will rise to a higher level if the duration of the
"On Time" pulse of the MOSFET Switch is longer and will rise to a lower level if the du-
ration of the "On Time" pulse of the MOSFET Switch is shorter.The controller checks the
voltage of the battery during the "Off Time" of the MOSFET Switch and adjusts the next
Duty Cycle (width of the "On Time") to ensure that the battery voltage is kept constant.
This technique allows the current to be effectively "tapered" as the battery gets charged
and the result is equivalent to constant voltage charging (e.g. constant voltage charging
in Absorption and Float stages shown in Fig 2.1).
For a battery entering the Absorption Stage at around 80% charged state (Fig 2.1), the
"On Time" pulses would be very long and almost continuous. As the charge state ap-
proaches the end of Absorption and beginning of the Float Stage (Fig 2.1), it may just
"tick" and send very short pulses to the battery.
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PWM Charging Control