Solar Charge Controllers

Solar photovoltaic charge controllers are used in off-grid PV solar systems to control the amount of energy from the solar PV panels going into the batteries.

By monitoring battery voltage they regulate the charging current from the PV array enabling batteries to be fully charged but not overcharged to protect the batteries from subsequent gassing, loss of electrolyte and possible plate damage.

• Control energy flow to the batteries
• Prevent batteries from overcharge
• Regulates voltage and current
• Tapers charge current into battery as it gets full
• MPPT controllers allow maximum power to be harvested

Charge controllers allow solar energy to be fully utilised for battery charging, increasing efficiency, increasing charge acceptance of the battery and increasing battery life.

Optional Features

Charge controllers may offer additional or optional features to enhance the system’s performance.

• Low voltage warning or disconnect function to switch off loads if the batteries become very low, preventing excessive discharge.
• Displays (built in or remote) to indicate charging currents, load currents and battery voltage. One of the purposes of a charge controller is to inform the user whether the system is working properly and to allow monitoring of the system. This is done through a visual indication of battery condition, charging currents, load currents and fault conditions. This can be done using:
• Simple status LEDs (light emitting diodes) showing for example whether a current is flowing from the solar array to the battery or indicating whether a battery is in a low, medium or full condition.
• Meters can be included to display voltage (and hence battery state of charge) and amperage of solar modules or loads. Often a single display is used with selection or scrolling through different parameters.
• Remote connection eg. by Bluetooth for wireless communication with smartphones or tablets.
• Lighting control and timers To allow loads to be switched on automatically and then off again after a certain amount of time. This is often used for lighting systems where lights may be switched on at dusk stay on for a few hours and then be switched off. Some controllers are also able to switch lights on again for a period of time before dawn.
• Temperature Compensation to ensure accurate charging

These work by gradually reducing (or tapering) the flow of energy into the battery as a full charge is reached so achieving a more complete charge. Very fast solid state switching technology is used for this, known as pulse width modulated (PWM) control.

• Solar panel voltage must match battery voltage (36 cells)
• Panel operates at battery voltage
• Tapers charge into battery as it gets charged
• Current into battery is limited to maximum power point current of panel
• Low Cost
• Simple and reliable
• Suitable for smaller systems (<200Wp)
• Can include load control features
• Optional displays (built in or remote)

This allows batteries being charged to reach a regulation set-point and then be maintained at that voltage by automatically tapering the charging current based on the battery’s state of charge in order to always keep the battery charged.

These are needed for use with larger 60 cell PV panels in order to match the PV output voltage with battery voltage.

• They allow PV panels to operate at their maximum power point voltage
• Controller drops this down to battery voltage.
• Current boosted by doing this
• Panels deliver full power to battery

Advantages

• Panel voltage doesn’t need to match battery voltage
• Can use high voltage string configurations
• Smaller cable can be used
• PV can be sited further from battery
• Better for larger systems (>200Wp)
• 60 cell panels are better value than 36 cell 12V panels so the extra power delivered can justify extra cost of the controller even for 12V systems

They allow a PV array to deliver up to 30% more power to a battery than it would if it were connected directly to the battery. The current from PV modules varies with sunlight levels but is fairly constant over a wide range of voltages. This means they can operate at higher voltages than the voltage of the battery bank which they are charging.