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INTRODUCTION & FEATURES
The individual or combined effects of sun peak hour changes, poor charge control
and the daily load changes can be potentially damaging to the battery. Cheaper
charge control strategies such as simple on/off PV array shedding (Non PWM control)
will generally provide the battery with sufficient charging current to complete the
Bulk Charge Phase which will return the battery to 80% State of Charge. After
the Bulk Charge Phase, the Taper or Absorption Charge Phase is very important in
preventing stratification, hard sulfation and pre-mature capacity loss.
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. 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 with no load,
it may just “tick” and send very short pulses to the battery. In a discharged battery,
the pulses would be very long and 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 the device is not adequately protected.