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Monday, 26 December 2016

About the Solar Charge Controller

A solar PV system has several components. Apart from the solar PV panels, it has charge controller, inverter, battery bank, supporting structure, protective fuses, breakers & surge protectors, cables etc. All the components other than solar PV panel are collectively called Balance of System (BoS). A well designed system is required for the smooth and reliable operation of the Solar PV plant.

What is Solar Charge Controller and how it works?

A Solar Charge Controllers (S.C.C.) or simply Charge Controller monitors and controls the power output from the solar PV panels and the battery. The Controller controls the flow of charge into the battery while charging from the solar PV system, thus prevents overcharging of batteries. It detects the status of battery charge by measuring the terminal voltage of the battery.

In overcharge condition, the battery terminal voltage increases above a certain level. When the battery reaches overcharge status, as shown by the measured voltage, the charge controller cuts off the solar PV supply to the battery.

As the connected load continues to use the battery energy, the terminal voltage of the battery drops down. This drop in voltage is also detected by the charge controller as it is continuously monitoring the activities. Whenever the battery voltage reaches the normal operating range, the battery is connected back to the solar PV system, by the charge controller, for regular charging.    

Similarly, the charge controller also controls the discharge from the battery. It prevents deep discharge of the battery. In deep discharge condition, the battery terminal voltage decreases below a certain level. This happens because of excessive drainage of charge from the battery, probably due to prolonged use of significant load. As the battery gets into deep discharge state, the charge controller detects it from the measured voltage and disconnects the battery from the circuit, so that no current can be further drawn from the battery. The life of a Lead-Acid battery very much depends on the Depth of Discharge (DoD).

"For example, as per Luminous, a renowned solar battery manufacturer in India, solar Flooded Lead-acid Tubular batteries have a life of 1500 cycles at 80% DoD, 3000 cycles at 50% DoD and 5000 cycles at 20% DoD."
Thus, both overcharging and deep discharging of battery or battery bank must be avoided to enhance the battery life.

Features available in Charge Controllers:

A Charge Controller usually/ may have the following features:

1.       LED or LCD display of various parameters or functions,
2. Automatic Temperature Compensation feature; automatically adjusts the battery charging according to the ambient temperature,
3.  User defined setting of battery voltage and type,
4.  Protection for low voltage, overvoltage and reverse connection,
5.  Automatic priority selection feature,
6.  Equalization feature.
Omission or addition of certain feature may happen, and it depends on the manufacturer.

Types of Charge Controllers:

The two commonly available types of Charge Controllers are:

     Pulse Width Modulation (PWM) Charge Controller, and
2.  Maximum Power Point Tracking (MPPT) Charge Controller.

Pulse Width Modulation (PWM) Charge Controller:

PWM Charge Controller is a solid state controller that usually works on three step charging algorithm. It has a semi-conductor switch which is switched ON and off by PWM at a variable frequency to maintain the battery voltage. When the battery voltage reaches the pre-specified value, the charging current is reduced as per the charging algorithm to avoid heating and gassing of the battery.

The PWM charge controller adjusts the charging according to the battery condition and requirement by controlling the speed of the switching element, which breaks the PV output current into pulses at some constant frequency and varies the width and time of pulses to regulate the amount of charge flowing into the battery.

Pulses of current helps the battery as it mixes the electrolyte, clears the lead electrode and prevents sulphation.

PWM charge controller maintains the battery capacities to 90-95% and has the ability to recover lost battery capacity. This helps in equalizing drifting of battery cells, automatically adjusts battery aging, increase the charge acceptance of the battery and self regulates the voltage drops and temperature effects of the solar PV system. These charge controllers are cheap and available in a wider range of capacities.

PWM charge controller is a good low cost option for small roof top solar PV systems where the ambient temperature is moderate or high.

Drawbacks of PWM charge controller:
1. The controller voltage must match the battery bank voltage, and
2. Usually the maximum current capacity of PWM charge controller is limited to 60 A.

Maximum Power Point Tracking (MPPT) controller:

Maximum Power Point Tracking (MPPT) controller is a charge controller with an additional device called the Maximum Power Point Tracker. These controllers provide a digital tracking of the PV panel output and compare it with the battery voltage. It then works out the maximum power that the panel can flush out to charge the battery or to the load. It tracks the optimum voltage so as to get the maximum amperage to charge the battery. Actually it is the amperage which makes sense for the battery.

MPPT charge controllers have higher efficiency, thus higher output power and overall better battery management than PWM charge controllers. These charge controllers continuously adjust the load on the solar PV system under varying operating conditions and keeps it operating at the maximum power point. As mentioned earlier, the controller checks the output of the PV array and compares to the battery voltage. It then calculates the maximum power that the PV array can produce. Accordingly, the controller converts the PV output voltage and converts it to the optimum value that fetches the maximum current into the battery or the load.

MPPT power varies according to the weather conditions, i.e. solar radiation, ambient temperature, and cell temperature. The voltage point at which the PV system produces maximum power is called Maximum Power Point (M.P.P.). Thus, the MPPT charge controller acts as a DC voltage converter which converts the PV array voltage into a voltage that fetches maximum power. The converter converts the DC input from the PV into a AC voltage and converts it back to DC matching the battery voltage. A Buck converter is used to step down the voltage, whereas a Boost converter is used to step up the voltage. They are used in PV systems of higher capacity, although MPPT charge controllers with smaller capacity say 17 A, is also available in the market.

For a PWM charge controller, the output current is the same as the input current, whereas for a MPPT charge controller, the output power is the difference of the input power and controller losses. With an MPPT charge controller employed, the system can deliver 20 to 45% more power in winter and 10 to 15% more in summer. The actual gain may vary depending on the weather temperature, state of the battery charge etc.

Figure 1 shows a 12 V, 17 A MPPT solar charge controller of Su-Kam make which is available in approximately 2,300 INR (in Bhopal, MP).

Fig.1: 12 V, 17 A MPPT solar charge controller of Su-Kam make 
    
MPPT charge controller has the following advantages:

1.  More effective in low temperature and cloudy days,
2. Have better efficiency in the range 93-97%. ( much better than PWM controller).
The drawbacks are complex circuit and comparatively higher cost.

In some solar PV systems, the charge controller and the inverter are collectively housed in a single unit and are called the Power Conditioning Unit (P.C.U.). Batteries are also among the key elements used in off-grid solar PV systems and hence one should know a bit about these storage devices.

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