There has been a lot of discussion about LED based lighting systems in these forums of late, since they are considerably more efficient than incandescent light bulbs (2to4 times as energy efficient for the same light output) However LED's are very different devices,for instance applying an ac voltage can destroy them instantly therefore it would appear worthwhile to review their properties and application potential. LED stands for light emitting diode,it is a particular type of junction diode,which when forward biased,emits photons,with energy levels (color that is) dependent on the bandgap of the junction material.If the diode has a reverse voltage applied to it, no current flows and no light is emitted and if this reverse voltage is large enough, reverse breakdown occurs, resulting in instantaneous destruction of the device.That voltage is usually around 3 V,in the same range as the normal forward bias voltage The moral of this story, don't apply ac voltages to LED's unless the reverse voltage is limited (by another diode or LED). Conduction properties:LED diodes behave like normal diodes in that respect,above a certain voltage threshold the current increases rapidly with increasing voltage and the device will be destroyed due to thermal breakdown unless the current is limited somehow,for instance by connecting a series resistor between the voltage source and the diode.The increased voltage drop across the resistor will keep the the current from increasing too rapidly.This of course depends on the expected voltage variation and on the size of the resistor (the larger the better),however this is pretty wasteful since a lot of power would be wasted in that voltage dropping resistor.Ideally LED's should be operated from a "current source",that is a source that supplies a constant preset current regardless of the load,for instance it puts out 0.1 Amp and 3V with one LED,with two in series 6 V. but still at 0.1 Amp. Lead acid or Nicad batteries behave more or less like constant voltage sources,that is the output voltage does not decrease much with increasing load,as the battery gets fully discharged the internal resistance increases and the output voltage under load goes down rapidly,so you can probably get away with using those type of sources and dispense with the power robbing resistor. The operating voltage of LED's varies quite a bit depending on color and manufacturer.For white LED's it in the 3-3.5 V range.They should NOT be put in parallel,the threshold voltage can vary somewhat between individual diodes resulting in uneven currents and light output,they can be strung in series all sharing the same current. Heat sinking,high temperatures are the death knell for semiconductor devices and LED's are no exception, therefore heatsinking of high power devices is essential.Apart from thermal runaway high operating Temp. can lead to rapid device aging,but under normal temp conditions they will practically last forever though.In hot regions of this country this is even more of a concern LED's are instantaneous on off devices they turn off in a fraction of a microsecond,this leads to a flicker effect if pulsed at low frequencies.