Solar Controller 101

Not all controllers are created equal

This brief technical note applies primarily to the solar powered LED controllers used in the Urban Solar photovoltaic (PV), LED lighting designs.  It provides an introduction to the basics of solar controller technology, key features and performance attributes.

Solar controllers provide two primary functions for your solar powered LED lighting system:

  1. Regulate or charge your batteries through your solar panel(s)
  2. Operate your LED lights

Additional functions and features can include photo cells for on and off transitioning, low voltage disconnect (to prevent damage to batteries as a result of a deep discharge), solar system diagnostic tools, temperature compensation, load or LED on time settings, data collection and remote monitoring.

LED Operation – on and off

Traditional solar controllers rely on the solar module voltage to turn the load or LEDs off and on. As the sun goes down the solar module voltage decreases.  Once a predetermined set point is reached, the controller recognizes it is dark and turns the LED lights on.  This process is reversed in the morning.  While functional, there are some drawbacks to this ‘switch’.  For example, overhead lighting may cause the controller to think it is still day time when in fact it is not.  Additionally, accuracy is a concern as the amount of light on the solar module can vary as can the tolerance in the voltage set point.

Timers and motion sensors are also options to be coupled with a solar controller.  The drawback of a timer, such as a sprinkler timer, is accuracy.  Simple timers require manual adjustments for seasonal variations as well as daylight savings time, and will consume power.  Additionally pairing one technology with another technology, such as a sprinkler timer has resulted in reliability issues leading to system failure.  Motion sensors that trigger the LEDs to come on are susceptible to false transitions (also leads to reliability issues) and are a common point of vandalism.

Real time clocks are the most efficient on/off transition technology available in today’s solar LED controllers.  They can also be factory set to the second to enable the LEDs on a programmed schedule, which accounts for seasonal variations in night length and daylight savings adjustments.  A well-designed solar system is based on the worst day of the year; usually on or about December 21, winter solstice.  The balance of the year, the weather is presumably better, which would require a smaller solar panel for the same performance as designed for December 21.

In essence, during the bulk of the year, there is an abundance of solar power.  A smart controller will recognize this and proactively adjust the power levels based on the increased availability of the sun and shorter nights.

Data collection and remote monitoring

Controllers with data logging capabilities allow users to confirm system operation and review operation history.  This may also assist a field technician with troubleshooting and help to determine component (battery) end of life state.  Through cell modems and recent technology advancements, controllers can also provide users with a real time view of their solar system state of health.  Solar charging conditions, the battery state of charge and load (LED) monitoring, are all functions that can be monitored remotely.  This ultimately reduces operating costs through reduced field visits.

Current solar controller technology offers these features and should be strongly considered when selecting a controller for any solar solution.

Diagnostic functionality

Most controllers are equipped with a self-test feature indicating battery state of charge and load or LED operation.  Typically a simple magnet test (swiping a magnet over or near the controller will automatically trigger the test sequence with visual or audio data feedback) and is all a technician has to do.  A deeper diagnostic would involve more tools and consultation with the manufacturer.

Charging methods

A solar controller’s primary function is to harness the higher solar module or panel voltage and regulate it to efficiently charge the battery.  Simple 1 or 2 stage controls rely on relays or shunt transistors to control the voltage in one or two steps.  These essentially short or disconnect the solar panel when a certain voltage is reached. For all practical purposes these are dinosaurs, but you still see a few on old systems – and some of the basic controllers for sale on the internet are of this type.

The two types of charge controllers most commonly used in today’s solar power systems are pulse width modulation (PWM) and maximum power point tracking (MPPT).  Both adjust charging rates depending on the battery’s charge level to allow charging closer to the battery’s maximum capacity as well as monitoring battery temperature to prevent overheating.  If maximizing charging capacity were the only factor considered when specifying a solar controller, everyone would use a MPPT controller.  But the two technologies are different, each with its own advantages.  The decision depends on site conditions, system components, size of array and load, and finally the cost for a particular solar power system.

PWM – Pulse Width Modulation

The PWM charge controller is a good low cost solution for small systems only, when solar cell temperature is moderate to high (between 45°C and 75°C).

The PWM controller is in essence a switch that connects a solar array to a battery. The result is that the voltage of the array will be pulled down to near that of the battery.

MPPT – Maximum Power Point Tracking

To fully exploit the potential of the MPPT controller, the array voltage should be substantially higher than the battery voltage. The MPPT controller is the solution of choice for higher power systems (because of the lowest overall system cost due to smaller cable cross sectional areas). The MPPT controller will also harvest substantially more power when the solar cell temperature is low (below 45°C), or very high (above 75°C), or when irradiance is very low.

The MPPT controller is more sophisticated: it will adjust its input voltage to harvest the maximum power from the solar array and then transform this power to supply the varying voltage requirement of the battery plus load.  It essentially decouples the array and battery voltages so that there can be, for example, a 12 volt battery on one side of the MPPT charge controller and a large number of cells wired in series to produce 36 volts on the other.

Summary

Not all solar lighting controllers are created equal.  Solar lighting controllers are the central component that ties all other sub components together.

Having a controller that can be programmed to efficiently manage the solar power and charge the system while also effectively operating the LED light output is critical.

Investing in a smart controller for your solar powered LED lighting system will maximize your return on investment while ensuring your lighting solution is reliable and deliver peak performance year round.