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Thread: Confused about my LED flashlight battery recommendations

  1. #1

    Default Confused about my LED flashlight battery recommendations

    New here and a bit confused about my LED flashlights battery recommendations. All of them came with 3 AAA cells and options to use #18650 and #123 batteries. With three new 1.5 volt AAA batteries you can get 1.5 volt if connected in parallel or 4.5 volt if connected in series. Neither of these numbers matches the 3.7 volt of the #18650. With two of the #123 in series you can get 6 volt. Just how many volts does it take to run a flashlight that is advertised to run on each of the above? I should think two #123 in series would give a much brighter light but, will that over stress the LED's PN junction? Thanks!

  2. #2

    Default Re: Confused about my LED flashlight battery recommendations

    Alkaline and rechargeable batteries use different methods of voltage reference. An alkaline battery that is labeled at 1.5v may be actually 1.6v even while a 3.7v lithium ion battery will measure up to 4.35v (mostly 4.2v max) off the charger. Some flashlights will run in a wider voltage range from about 3.5v to 8v so you can use 3AAA or 2x123 or 1 lithium ion battery to power them but 1x123 is too low of a voltage to get full light output from the light itself. The lights often have buck circuitry and it turns voltages over wha the LED needs to fully operate into higher current which at times helps when the LED is driven harder and some battery combinations struggle to obtain proper current levels.
    It is possible 2x123 would be brighter than 1x18650 but it is not always the cast I believe and too often using alkaline 3AAAs cannot give you the current needed for higher modes on the light at all or if so not very low before dropping.
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  3. #3
    Flashaholic* hiuintahs's Avatar
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    Default Re: Confused about my LED flashlight battery recommendations

    Quote Originally Posted by laocmo View Post
    New here and a bit confused about my LED flashlights battery recommendations. All of them came with 3 AAA cells and options to use #18650 and #123 batteries. With three new 1.5 volt AAA batteries you can get 1.5 volt if connected in parallel or 4.5 volt if connected in series. Neither of these numbers matches the 3.7 volt of the #18650. With two of the #123 in series you can get 6 volt. Just how many volts does it take to run a flashlight that is advertised to run on each of the above? I should think two #123 in series would give a much brighter light but, will that over stress the LED's PN junction? Thanks!
    The forward voltage drop (Vfwd) of an LED is around 3.0v. It varies a little depending on the LED and also on the current that is passing through it. Good LED drivers will regulate the current through the LED with a boost switching type driver if the flashlight is designed for batteries that are below the Vfwd (like a single AA battery). In your case where all the battery types are above 3.0v, then the driver likely has a buck (stepdown) switching type of power converter. Since theses switching type of power converters can be upwards of 90% efficient, then only 10% is lost in the electronic driver. So really what we are talking is power and not volts. So it really doesn't matter that the driver is being run by a 18650 battery, 3 AAA batteries in series or or 2xCR123A batteries in series, an efficient well designed driver circuit will convert the battery power to a level to drive the LED properly. It will step the voltage down to the Vfwd needed to drive the LED at a predetermined level for whatever output mode is selected. In order to do this since there is a vary narrow window of Vfwd from dimm to very bright, the driver will employ feedback to hold a constant Vfwd. That is the beauty of the LED flashlight is the ability to regulate the output to a constant level even as the battery voltage varies during use.
    Last edited by hiuintahs; 04-12-2017 at 10:46 PM.

  4. #4

    Default Re: Confused about my LED flashlight battery recommendations

    As the others have been saying, it's a slightly bigger picture than just batteries.

    What sort of lights are we talking about? Most lights do not easily support lots of different batteries, more due to the physical size than anything.


    Anyhow, most LED lights use an electronic driver (circuit board) to control output and modes on a light. This driver will have an operating voltage range. Exceeding this voltage will normally fry them and too low a voltage and they won't likely function, or have very reduced performance/modes.

    There are many types of driver depending on what it needs to do.


    Most common LEDs require 3.3-3.8v to operate. Although there are some LEDs that require higher voltage and some that require lower.


    On this basis, most lights then fall into either using a:

    -Buck driver, which lowers the battery voltage down to that needed of the LED. Usually multi battery lights or lots of battery lights.
    -Boost driver, which uses a battery below the voltage of the LED and uses more amps to 'boost' the voltage to the correct level.
    -Linear driver, which has a battery of about the right voltage for the LED and simply controls current and modes, rather than voltage.



    As for batteries.

    Li-ion is typically 4.2v when resting and fully charged. However they have a 'nominal' voltage of 3.6 -3.7v, which is why you often see all these figures mentioned. Nominal is the battery under normal load.

    When amps are drawn from any battery the voltage of the battery will drop, the higher the draw, the bigger the drop as a rule. This is why different batteries perform differently and to different levels of performance.


    Physically larger batteries of the same type will typically outperform smaller versions of the same type, e.g. most 26650 Li-ion will out perform 18650's and 18650's will out perform 14500's.

    Same is true with AA's out performing AAA's.




    Anyhow, feeding an LED more volts than it needs will result in the need to 'burn' off the excess voltage as heat. But the positive here is, it gives you very good stable output, as the battery voltage will be higher than the LED needs, so less demand on amp draw of the batteries. A 2xCR123a or a 2xLi-ion for example.
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  5. #5

    Default Re: Confused about my LED flashlight battery recommendations

    Many thanks for clearing all that up! I didn't realize high quality flashlights had such sophisticated voltage regulating circuitry to keep the LED safe.

  6. #6

    Default Re: Confused about my LED flashlight battery recommendations

    There are a lot of cheap 3*AAA flashlights that depend on the battery internal impedance to limit current.

  7. #7

    Default Re: Confused about my LED flashlight battery recommendations

    Quote Originally Posted by PeterH View Post
    There are a lot of cheap 3*AAA flashlights that depend on the battery internal impedance to limit current.
    Actually most cheap lights either have resistors or linear regulators these days I think as they are cheap enough to put on circuit boards containing the LEDs or the batter contacts.
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