How are LED lamp run times calculated?

doktorsteve Posts: 112
edited October 2008 in Workshop
Manufacturers claim huge run times for their LED lamps. Typical is the 1 Watt Nova lamp from Smart which claims 20 to 40 hours. My O-level physics (still valid after 30+ years) says otherwise.

Forward drop on a 1 Watt Luxeon LED is about 3.2V (maybe more) so the current for 1 Watt input is 1/3.2 or 340mA. Now if I fit 2500 mAHr NiMH cells in, then the total run time is (theoretically) 2500/340 which is 7.5 hr (roughly).

Looking at it another way: The total capacity of each cell is 1.2Vx2500mAHr which is 3Whr so the total capacity of 4 AA cells is 12Whr i.e. the maximum theoretical run time for a 1 Watt lamp is 12 hours assuming 100% efficiency regulator.
I know that in this lamp a simple dropping resistor is used (see ... t_nova.php) so the lamp is about 60% efficient. I also know that there are no dim or flashing modes for this lamp.

So what am I missing? How do manufacturers get their figures and more to the point how do they get away with quoting them? Do they use a special kind of alkaline battery? Even the fancy lithium iron cells only boast 2900mAhr.
100% ME!
Do you think I would be this bad on drugs?


  • gkerr4
    gkerr4 Posts: 3,408
    i agree with your physics.

    Isn;t it something to do with the way they specify LED "bulbs" (i know they're not)

    as in - a "1 Watt" LED doesn't actually expend 1 watt?

    I think it is in that part of the specification that is misleading - not the physics behind it.

    either that or they could just plain be telling fibs

  • cougie
    cougie Posts: 22,512
    No idea - but the Fenix run times do tie in with what I get - so either they are better at sums, or they dont fib like some others ?
  • A 1 Watt LED dissipates 1 Watt at a certain applied voltage and current. I know this from building lamps for music stands. If a manufacturer quotes 6-10 hrs for 4 AA cells then I believe it. Anything above 10 hrs is either a blatant lie or they are not actually running the LED at 1Watt. This would not surprise me as some LED lamps do not have sufficient heatsink to get rid of 1 Watt so the LEDS would be downrated to cope with the heat. Also as a LED gets hot its effective resistance increases thus reducing the current and power.

    I have a Bikehut/Hella lamp and it lasts around 6 hrs as expected. I think their big un times are for flashing mode which may be only 25% duty.
    100% ME!
    Do you think I would be this bad on drugs?
  • Many LED lights go into a dim low power mode when the batteries are nearly exhausted and they can function in this dim state for ages and it is this time that tends to be quoted.
    I have a head torch that will run for about 4 hrs on full power but will continue to work on reduced power for well over 20 hrs.

    I think manufacturers should clearly state what the FULL POWER run time is.