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questions about led with small 3v batteries

Discussion in 'General Electronics Chat' started by Shay, Jun 19, 2017.

  1. Shay

    Shay New Member

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    hi
    so i bought this 3v LED and i'm going to power it with small watch 3v battery.
    the first thought was to connect to 3V batteries (to get 6V) and then connect a 150 Resistor (6v-3v devided by 20/1000) to that and then to the LED
    the miliamper for my battery (maxwell cr2032) is: Nominal Capacity (mAh)* 220

    my question is how much time will that battery in average. hold that LED before it will have to be replaced ?
    if we take the average 3v led lifespan.

    (how do we even calculate that ?)

    and if i will use 2 batteries and a resistor, will that double the lifespend of the led before the battery will have to be replaced ?
     
  2. Shay

    Shay New Member

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    my guess and correct me if i'm wrong on average it will hold it for around 7h, and two batteries will hold it for 14h ?
     
  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    You have correctly calculated that with two 3V cells in series you have 6V and that with a 150Ω resistor 20mA will flow through a LED which has a 3V forward voltage. That 20mA also flows through both cells, so both will run down at the same rate. If each cell capacity is 220mAh (an optimistic figure) then the cell lifetime will theoretically be 220mAh/20mA = 11h for each cell. Expect somewhat less in practice.
     
  4. dave

    Dave New Member

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  5. Cicero

    Cicero Active Member

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    No way, those batteries are not designed for that kind of 'high' (relatively speaking) current.

    mAh ratings are not a 1-size-fits-all spec unfortunately. For example different batteries can handle current spikes better than others, etc etc. So even though 2 batteries may have the same mAh rating, they wont perform the same even under the same load conditions.

    Those button batteries are meant to deliver very low currents (<200uA for example) for a very long time. If you're pulling a continuous 20mA, those batteries will hardly last any time at all. I'd be surprised if you get a couple of hours out of them, but there's no real way to know how they'll perform under those conditions without trying it.
     
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Instead of "somewhat" in my post, perhaps I should have said "a lot". :)
     
  7. Externet

    Externet Member

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    You can always run the LED at 5 mA or less instead of pushing it to the limits of its capability and stressing the battery, shortening life.
     
  8. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    I have used a Red Ultra-bright LED (Vf ~2V) as an on-off indicator on a battery-powered test box which is used outdoors. I found that at less than 1mA, the LED was still bright enough to tell if the equipment is turned on, even in bright sun...
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Why don't you l;ook at the datasheet of a cR2032 battery cell? it shows that it is designed for a 15k load which is a current of only 0.2mA, not 20mA which is 100 times more. The voltage drops as the battery discharges and its spec's are measured when it has dropped to 2V.

    The manufacturer of your battery is Hitachi Maxell, not Maxwell and you will use two cells in series, not "to" cells.
    A graph on the datasheet shows that with a load of 3.5mA then its mAh is only 160 then its voltage will be 2V in 160mAh/3.5mA= 45.7 hours. The graph does not show currents higher than 3.5mA where the mAh is dropping fast.
     
  10. Colin

    Colin Member

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    Two mistakes above. Using 2 cells will not double the hours of illumination. It will be the same as one cell. Basically you are wasting one cell by heating the resistor.
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    No. The LED is 3V and the battery is also 3V. Will one tiny cell light the LED dimly for a few seconds before its voltage drops too low? Two cells produce 6V when new and slowly drop to 2V each when the LED will still produce plenty of light.
    One cell= a few seconds. Two cells= an hour or two.
     
  12. Colin

    Colin Member

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    One cell= a few seconds. Two cells= an hour or two.

    Not really. I have a 3v keyhole torch with a white LED. It lasts for over 12 months when used a few nights a week for about 10 seconds looking for the keyhole and also as a torch.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Colin, he said his LED has a forward voltage of 3.0V, the same as a new battery cell. Maybe your white LED has a forward voltage of 2.5V so it takes longer for the battery voltage to drop 0.5V.
     

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