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Solar charge controller is a device which regulates voltage and/or current to protect batteries from overcharging in the solar photovoltaic system. It has been observed that most solar panels of 12 volts have output voltage of 16 to 20 volts approximately. So, if there is no regulation the batteries will be damaged due to overcharging. In order to get fully charged, most batteries require around 14 to 14.5 volts. A solar charge controller regulates the rate of adding and subtracting of electric current from batteries. It helps to prevent batteries from overcharging and deep discharging. It is also protective against over voltage. This feature of solar charge controller contributes to enhance the lifespan of batteries significantly.

Types of Solar Charge Controller

Various shapes, sizes, features, and price ranges of solar charge controllers are available in the market. They are within the range of 4 amps to up to the 60 to 80 amp MPPT programmable controllers having computer interface. The full abbreviation of MPPT is Maximum Power Point Tracker. It’s an electronic DC to DC converter which optimizes the match between the photovoltaic array (solar panels) and the battery bank or utility grid. They convert a higher voltage DC output from solar panels (and a few wind generators) down to the lower voltage required to charge batteries. If any system requires currents over 60 amps, then two or more 40 to 80 amp units are connected in parallel.

Basically, solar charge controllers are three types. They are as follows:

Relay based charge controllers: These types of charge controllers depend on relays or shunt transistors to control the voltage in one or two steps. When a specific voltage is reached, they necessarily disconnect the solar panel.

PWM charge controllers: PWM stands for Pulse Width Modulation. It is often used as float charging method. Using a very rapid "on-off" switch, it sends out a series of short charging pulses to the battery. The controller always checks the state of the battery to determine how quickly and how long (wide) send pulses should be sent. When the battery is fully charged with no load, it provides "tick" every few seconds and sends a short pulse to the battery. The controller may turn into "full on" mode or the pulse would be almost continuous when battery is discharged. The state of charge on the battery between pulses is checked by the controller and adjusts itself each time. These type of charge controllers now very much industry standard.

Maximum power point tracking (MPPT) charge controllers: MPPT charge controllers are high frequency DC to DC converters. They take DC input from the solar panels, turn it into high frequency AC and convert it back down to a different DC voltage and current for matching the panels to the batteries perfectly. MPPT’s can operate at very high audio frequencies, usually in the range of 20-80 kHz. High frequency circuits are designed with very high efficiency transformers and small components.

Some linear (that is, non-digital) MPPT’s charge controls are also available in the market. They are comparatively cheaper than the digital ones in terms of price and design. Somehow, they can improve efficiency. But overall efficiency of these charge controllers can vary a lot and sometimes lose their "tracking point" when a cloud passed over the panel.

MPPT’s are most effective under these conditions:

·         Solar panels work better at cold temperatures, but without a MPPT you are losing most of that. Cold weather is most likely in winter - the time when sun hours are low and necessary power is required to recharge batteries most.

·         MPPT puts more current into the battery when the state of charge is lower in the battery. This is another condition when the extra power is most required.

·         When panels are 100 feet away and 12 volt battery is charging, the power and voltage drop are considerable unless very large wires are used. But, it’s too much expensive. On the other hand, when four 12 volt panels are connected in series for getting 48 volts, there is very little power loss and the controller can convert that high voltage into 12 volts at the battery. That is, when a high voltage panel setup feeds the controller; comparatively smaller wires should be used.

Most controllers are designed by various type of indicator, a simple LED, a series of LED’s or a digital meter. At present, built in computer interfaces are used in controllers for monitoring and control.  The simplest one has a couple of small LED lamps which shows the available power and charge in the system. Both voltage and the current coming from the panels and the battery voltage are displayed in the meter of this charge controller. Amount of current is being pulled from the load terminals are also displayed in some charge controllers.