YOUR GUIDE TO OUTDOOR SOLAR LIGHTS


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THE BASICS



Devices that can generate electricity from sunlight, store it during the day and release it at night to produce light are casually called solar lights. They can be used to illuminate a garden, deck, pathway, or other home outdoor areas.
They are also being used in public places such as parks, campgrounds and highways, where it is inconvenient to run electrical wiring. Solar powered devices are not so cheap, but their advantage is that you don't have to run any electrical wiring for them in your garden, the energy is free, and you can install them in seconds.

A REVIEW OF AVAILABLE TYPES



A typical outdoor lighting device contains the following main parts mounted in a plastic case: a small photovoltaic (PV) panel, one or more rechargeable batteries, an electronic circuit and a source of light. Most of the fixtures are self-contained units that need to be mounted in a sunny location. There are also the models that have a PV array separate, so that you can mount the light where you need it and orient the panel toward the sun for the best exposure. The source of light is usually light emitting diodes (LED). More expensive fixtures (such as motion detectors) may have a combination of an LED and a small halogen flashlight bulb. Today's ultra-bright LEDs can produce a focused beam of light equivalent to a 40W incandescent bulb. The batteries, depending on the model, are usually nickel cadmium (NiCad), lead acid, Nickel-Metal Hydride (NiMH), and Nickel Zinc(NiZN). NiCad, lead acid and NiMH batteries operate typically at 1.2V, NiZN - at 1.6V. There are also Lithium-ion (Li-ion) batteries that provide 3.6-3.7 V per cell and technically speaking are the best. However, they require special chargers that carefully monitor the charge process and practically are not used in cheap solar lightings.
Most models have replaceable batteries, however some models use built-in batteries that are not intended to be replaced. Although usually they can be charged and discharged hundreds of times, eventually they will need a replacement. In case of built-in batteries you would need to replace the entire fixture, unless you are able to open it up, unsolder the batteries and solder in the new ones. Most fixtures have an electronic circuit that automatically turns the illumination on at dark and turns it off at sunrise, although some may have a manual on-off switch.



When you buy an outdoor light, check that its batteries are removable and it has an automatic operation. If the spot where you plan to install the unit is shaded, consider a system in which a PV module is separated from the light.




HOW SOLAR LIGHTS WORK



A solar-powered lighting device converts the sun's radiated energy into electric energy by using photovoltaic (PV) cells, stores it in re-chargeable batteries, and then uses this energy to emit light when it is dark. Note that a single PV cell can produce up to 0.5-0.6 V at an open circuit under bright sun, the charge voltage of rechargeable AA NiCad, lead acid and NiMH batteries is 1.3-1.4 V per cell, and a high intensity LED has a forward voltage 3-3.5 V. To overcome this voltage mismatch various methods are used. The simplest way is to connect in series a number of PV cells and a number of the batteries to get the required voltage for an LED. Note that the solar cells are connected directly to the battery through a diode which prevents the battery discharge from a reverse current flowing back through the cells at night. Therefore, the solar module has to produce at least 0.7 V more than the rated battery bank voltage. So, to power a single high-brightness LED directly, you would need about 8 series-connected PV cells and 3 series-connected rechargeable AA batteries. Solar light schematic
An alternative approach uses a boost regulator that steps up the input voltage to a higher level compatible with the LED. There are LED drivers that can operate from input voltages as low as 0.8V and are therefore compatible with single AA cells. For high power LEDs you would still need at least 2 series-connected cells. This schematic diagram shows a boost converter circuit for an outdoor light with 1 W LED, which is based on Zetex Design Note DN61. The number of PV cells should be selected to produce a voltage one diode drop higher than the charging voltage for battery pack (which is typically 1.3-1.5V per cell). So, for two AA cells you need 3.1-3.5 V from the PC array, which would require a PV module with 6-8 series connected PV cells. Since the driver continues operating down to 0.8V input, this circuit needs a low voltage cut off (not shown on the schematic) to prevent over-discharging of the cells.