Samlex Solar SCC-30AB Mode D'emploi page 29

PWM Battery Charging
PWM (Pulse Width Modulation) battery charging is the most efficient and effective
method for recharging a battery in a solar system. The PV array is connected to the
battery through a series or parallel (called Shunt) connected MOSFET Switch.
During the initial constant current, Bulk Charge Stage (see Fig. 4.1) the Series
connected MOSFET Switch (in a Series Type of controller like SCC-30AB) is kept
on continuously till the PWM Absorption Voltage is reached. During this time the
solar panel delivers nearly constant current that is equal to or nearly equal to the
Short Circuit Current I
(see Fig. 2.9). For example, for a panel with Short Circuit Current of 2.7 A and
discharged battery voltage of 11 V, this current will 2.7 A.
Once the battery voltage reaches the Absorption Voltage (Fig. 4.1), further charging
takes place at constant voltage in the Absorption / Equalization / Float stages. During
the above constant voltage stages, the PWM control circuit switches on and switches
off the MOSFET Switch at a frequency of several hundred cycles per second. Thus,
instead of a steady output from the controller, it sends out a series of short charging
pulses to the battery – like a very rapid "on-off" switch. The controller constantly
checks the state of the battery to determine how long (wide) the pulses will be. In
a fully charged battery in Float stage with no load, it may just "tick" and send very
short pulses to the battery. In Equalization / Absorption stages, the pulses would
have longer duration and will be almost continuous. The controller checks the state
of charge on the battery between pulses and adjusts itself each time. This technique
allows the current to be effectively "tapered" and the result is equivalent to "constant
voltage" charging.
Series and Shunt Type of Charge Controller
When the MOSFET Switch is connected in series with the PV Array and the battery,
the Controller is called Series Type. When it is connected in parallel across the PV Array
/ the Battery, it is called Shunt Type. In Series Type, the MOSFET Switch is kept open
when the battery is fully charged. The PV Array stops supplying current during this
period. In the Shunt Type, when the battery is fully charged, the MOSFET Switch is
kept closed to shunt (divert) the full Short Circuit Current of the PV Array away from
the battery.
Advantages of Series Type of Charge Controller
A series Type of Charge Controller has the following advantages over a Shunt Type:
• Power systems experience temporary over voltage conditions. For example, when lightning
strikes, extremely high electrostatic energy is discharged. This energy induces damaging
high voltage transients in exposed and un-protected electrical circuit elements like cables
etc, and these high voltage transients are fed to the electrical devices and cause damage if
corresponding to the voltage of the discharged battery
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IntroductIon & features
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