solar system charge controllers

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Charge controller Introduction

The charge controllers are is a voltage or current regulators. Its main function is to fully charge a battery without permitting overcharge or reverse current flow (generally during night). The charge controller is singlehandedly responsible for ensuring the smooth functioning of the battery.

Every power system requires a control strategy to describe the interactions between its components. In standalone PV system charge controller is one such control element which manages the energy flow to PV system, batteries and loads by collecting information on the battery voltage and knowing the maximum and minimum values acceptable for the battery voltage.

Types of charge controllers

There are mainly three varieties of Charge controllers

  • Ordinary ON or OFF control (1 or 2-stage control)
  • 3-stage and /or PWM
  • MPPT (Maximum power Point tracking)

Ordinary ON or OFF control (1 or 2-stage control)

Ordinary ON or OFF controllers consist of a relay that opens the charging circuit when a preset high-voltage point is reached and closes the circuit again when a preset low-voltage limit is reached, allowing charging to continue. It is the simplest control strategy to control the charging and discharging of battery.

There are two main operating modes for the controller:

  • Normal operating condition, when the battery voltage fluctuates between maximum and minimum voltages due to variation in solar irradiance e.t.c.
  • Overcharge or over-discharge condition occurs when the battery voltage reaches some critical values.

Over charging Protection

To protect the battery against an excessive charge, the PV arrays are disconnected from the system, when the terminal voltage increases above a certain threshold Vmax off and when the current required by the load is less than the current delivered by the PV arrays. PV arrays are connected again when the terminal voltage decreases below a certain value Vmax on.  If a solar array is connected to a lead-acid battery with no overcharge protection, the battery life will be adversely affected.

Over discharging protection

To protect the battery against an excessive discharge, the PV arrays are disconnected from the system, when the terminal voltage decreases above a certain threshold Vmin off and when the current required by the load is greater than the current delivered by the PV arrays. PV arrays are connected again when the terminal voltage improves above a certain value Vmin on.  

3-stage and /or PWM charge controller

Modern charge controllers use Pulse Width Modulation (PWM) to slowly lower the amount of power applied to the batteries as the batteries get closer and closer to fully charged. This type of controller allows the batteries to be more fully charged with less stress on the battery, extending battery life. It can also keep batteries in a fully charged state (called “float”) indefinitely. PWM is more complex, but does not have any mechanical connections to break.

The 3-stage PV Charge Controller uses the Micro-Processor and PWM (Pulse Width Modulation) to give optimal and safe charging. It makes varying On-Off pulses of electrical energy to connect or disconnect photovoltaic (PV) panel from charging the battery according to the battery state. It has 3 stages of charging, as follows:

Bulk charge

In this mode PWM wave will be absent. Hence a preset maximum constant amount of current (amps) is fed into the battery. As the battery is being charged up, the voltage of the battery increases gradually.

Absorption charge

After the preset voltage on the batteries is reached the voltage is then held constant. As the battery continues to be charged at constant voltage, the charging current slowly decreases. The charging voltage is held at the Bulk Voltage Setting for one full hour with various rapid On-Off pulses (PWM). It then switches to Float Charge Mode.

Float Charge

The controller will maintain the battery voltage at the float voltage setting by giving shorter On-pulse charge to make up for any detected self discharge of the battery. When the battery voltage drops below the Float Voltage Setting for a total period of 10 minutes, a new charging cycle is activated in Bulk or Absorption Charge.

Charging of battery

The three stages charging method works well with the chemical reaction that occurs as a battery is being charged. When a battery is more discharged, a regulated maximum current can be applied, since there is a lot of material available for the reactions to occur.

Discharging of battery

As the battery refills, less and less chemical material is available for the reaction. By using PWM to slowly reducing the charge current, while maintaining a preset high voltage, the battery is more closely refilled at the reaction rate of the chemicals. Finally, the Float voltage keeps the battery fully charged at all times taking care of the self discharge.

MPPT (Maximum Power Point Tracking)

The newer Maximum Power Point Tracking (MPPT) controllers are more power efficient compared to other charge controllers. They match the output of the solar panels to the battery voltage to ensure maximum charge (amps).

For example: A solar panel is rated at 300 watts, you won’t get the full 500 watts unless the battery is at optimum voltage. The Power/Watts delivered to load is equal to Volts times Amps or P=V*I. With a regular charge controller, if your batteries are low at say 20 volts, then your 300 watt solar panel rated at 10 amps at 30 volts (10 amps times 30 volts = 300 watts) will only charge at 10 amps times 20 volts or just 200 watts. You just lost 33% of your capacity! The MPPT controller compensates for the lower battery voltage by delivering closer to 15 amps into the 20 volt battery maintaining the full power of the 300 watt solar panel! 300 watts = 20 volts times 15 amps = 300 (P=E*I).

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