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6v LED brighter with 3v battery

Discussion in 'LEDs and Optoelectronics' started by foggs, Apr 4, 2019.

  1. foggs


    Aug 1, 2017
    I have a cheap headlamp that came with 2 CR2016 (3v) batteries in series (6v).

    If I replace them with a single CR2032 (3v), the LED is significantly brighter (3000 lux vs over 9000 lux!)

    The CR2016's are a little older but i measured all batteries to be very close to 3v.

    Why is it doing this?
  2. Externet


    Aug 24, 2009
    Measure cells voltage when the led is on.
    CR2032 has more current capability; lower internal resistance.
  3. foggs


    Aug 1, 2017
    1 x CR2032: 2.98v (off) -> 2.82v (on)
    2 x CR2016: 5.98v (off) -> 2.63v (on)

    Interesting! Why does it do this?
  4. BobK


    Jan 5, 2010
    Because they are not new, perhaps?

  5. dave9


    Mar 5, 2017
    Because when the circuit is trying to pull more current than the battery internal resistance allows, you get voltage droop.

    The real question is what the LED specs allow for max current, which may also depend on how well it is heatsunk (or in this case, heatsinking an encapsulated bi-lead LED can depend upon a short LED cathode lead soldered to a good amount (thickness) of copper wire or PCB layer.

    It could be that the designers knew the 2 x series CR2016 power supply was inherently current limiting so they didn't (need to) design for higher LED power, so driving the LED at higher current could overheat it.

    To clarify it is not a "6V LED", it's (presumably white) 3.(n)V LED but the tiny coin cells are acting as a current limit, plus as already stated, older cells already at a lower voltage.

    If you power this for long from the single CR2032 cell, you might find that the LED more quickly gets dim and a significant % of the CR2032's capacity is wasted because the partially discharged voltage plus voltage droop is below the forward voltage of the LED. CR2032 cells aren't "good" for much more than 20mA if that, driving a white LED.
  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    You should compare this datasheet for CR2016 with this datasheet for the CR2032 to gain some insight into what is going on. BTW, neither cell was designed to power the relatively high current demands (20 mA and up) of LEDs. The fact that they will work in that application simply lines the pockets of the battery producing companies since you will be replacing the cells quite often. Take the SWAP (Size, Weight, And Power) hit and use a cheap alkaline cell, or better yet, a rechargeable battery. Size matters, but coin cells for flashlights? Well, maybe to find a keyhole in the dark, but you might as well throw yer cash at hearing-aid batteries!
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