Today’s improved solar-powered lights are better than ever

Tag : NIMH Battery
By Alan J. Heavens

The Philadelphia Inquirer
Solar-powered lights have improved as the technology they use hasadvanced — today’s models give off more light thantheir ancestors did. Plus, they never go dark when the electricallines go down.
Need to know: Obviously, solar lights store power from the sunduring the day, then release it at night. But to get the brightestand most consistent light, placement is important. Put them in fullsunlight, most manufacturers promise, and you can have illuminationwithin 15 minutes. Put them in shaded areas and it takes longer forthe lights’ solar cells to charge — and theillumination given off won’t be as strong.
Operating manual: A solar lamp consists of a solar panel orphotovoltaic cells that collect and convert energy from the sun, arechargeable battery that stores the electricity, a light-emittingdiode (LED) lamp, and some sort of sensor or timing device. Duringthe day — except in the winter and on cloudy days — thebattery reaches maximum charge. When the sun goes down, the solarcells stop producing power, and the sensor or timer turns on theLED lamp.
Power up: Remove the cover of a light fixture and you’lllikely find the solar cells on top. A single solar cell produces amaximum of 0.45 volts and a varying amount of current, depending onits size and the amount of light striking the surface. Most outdoorlights have four cells, which produce 1.8 volts and about 100milliamps of current in bright, direct sunlight. The cells arewired directly to the battery through the diode, which keepscurrent from the battery from returning to the cells at night.
Battery pack: Older solar-lamp models and some models made todayuse a standard AA NiCad (nickel-cadium) battery that produces about1.2 volts and can store a maximum of 700 milliamp-hours. But manynewer models use NiMH (nickel-metal hydride) power packs, which canhave two to three times the capacity of a NiCad battery ofcomparable size.
Thanks for the memory: The difference between NiCad and NiMH is howthe battery retains its power over time. In a NiCad battery, thecharge begins degrading almost immediately. A NiMH battery isdesigned to operate from fully charged to fully discharged.
A brighter light: LEDs are five times brighter than the bulbspreviously used in solar lamps, drawing about 45 milliamps. And,unlike conventional bulbs, they emit light but not heat whenelectricity passes through them. One LED emits roughly the samebrightness as a two-watt bulb. Still, the light produced is abouthalf that produced by a candle, which isn’t that much. As oneexpert notes, it’s enough light for you to differentiate yoursidewalk from your lawn, but not enough to keep you from fallingover your kid’s bicycle.
Some solar lights use both halogen bulbs and LEDs, with the LEDproviding consistent lighting and the bulb turning on when itdetects motion. LEDs are designed to never burn out over thelifetime of the product, usually 25 years, which means theyaren’t replaceable in the light fixture.
What will it cost: The cheapest solar yard lights are $10 to $20per lamp, and the better models run even higher. That’sbecause solar cells are still expensive to produce —they’re manufactured from silicon crystals in clean-roomconditions — and NiCad and NiMH batteries also are expensive.
If you have a long driveway or garden path that will need a coupledozen lamps, going solar will be pricey up front. But once thelights are installed, they’ll cost you nothing to operate.