Actions
Forum Thread Details
  • State Not Answered
  • Replies 11 replies
  • Subscribers 286 subscribers
  • Views 2446 views
  • Users 0 members are here
Related

Trickle charging a button cell battery using an energy harvester?

Former Member


Hey all from Element 14! My name is Elliott Leinauer and im a highschool student working on a science fair project. My project encompasses using very low voltage and current and harvesting it to create more voltage to power a high lumen led light! To make somthing like a flashlight. My problem as of right now is trying to store this harvested energy. My harvested energy is supplying me anywhere from .700-1.5v depending on conditions. With roughly .15-.20mA. Im curious to know if its possible to somehow trickle charge a 3.5v lets say 120mah button cell battery in order to give my led light constant power so it stays nice and bright over a long duration of time. Is it possible to do this with the current I have? If so can anyone point me in the right direction as to which button cell battery I should buy?

 

Keep in mind im realtivley unexpericend with circuitry,  I was  thinking  to raise my current without loosing to much voltage  I could somehow incoperate a transistor in the circuit before the power is directed to the button cell. Is this atall possible or am I just talking nonsense?

Thanks!!

Parents
  • 0

    Hi Elliott,

    The problem is not the current but rather the voltage. In order to recharge a battery it is necessary to have a voltage that is higher than the battery voltage. The current that is then supplied by the charging voltage is determined by the internal resistances of the battery and the charging voltage supply. The other bump in the road for you will be that most button cell batteries are not designed to recharge. With the level of voltage and current available from your energy harvesting there is no danger but you may not get a very efficient recovery of energy. One solution that you might want to try is to use a super capacitor to store the energy from the harvesting. Here is one that Newark sells.

     

    http://www.newark.com/illinois-capacitor/505dcn2r7q/super-capacitor-aluminum-elect/dp/02P5131

     

    John

  • 0 in reply to jw0752

    John,

       Wouldnt a supercapacitor cause a rapid depletion of power causing the led to become dim very quickly? Or is that a myth?

  • 0 in reply to Former Member

    You can't get more energy out than you put in so the average power available for your LED is going to be about 0.15 X 1.1 = 0.165mW  which is not very bright !

     

    If you used the capacitor John suggests it leaks some current (about 0.03mA according to spec) so you might hope to charge it to 1V but the average charge current will be only about 0.12mA.

    To get up to 1V will take 5/0.00012 seconds = 41666 = 11.57 hours.

     

    Now to work the LED you'll need some sort of voltage boosting circuit - Google "single cell led driver" and avoid anything with the words "joule thief" in the description (why  - because switching up converters are engineering and "joule thief" is snake oil speak !!!).

    The boost circuit will be about 80% efficient (could be better if you do it well).

    Suppose you keep your LED on until the cap voltage drops to 0.5V - the energy in the capacitor = (CxVxV)/2 so it starts with 2.5J and drops to 0.625J giving you 1.875J into the booster and 1.5J into the LED.

    So after 12 hours of charging you can have 1.5W for one second, or perhaps more usefully 50mW for 30seconds.

     

    MK

Reply
  • 0 in reply to Former Member

    You can't get more energy out than you put in so the average power available for your LED is going to be about 0.15 X 1.1 = 0.165mW  which is not very bright !

     

    If you used the capacitor John suggests it leaks some current (about 0.03mA according to spec) so you might hope to charge it to 1V but the average charge current will be only about 0.12mA.

    To get up to 1V will take 5/0.00012 seconds = 41666 = 11.57 hours.

     

    Now to work the LED you'll need some sort of voltage boosting circuit - Google "single cell led driver" and avoid anything with the words "joule thief" in the description (why  - because switching up converters are engineering and "joule thief" is snake oil speak !!!).

    The boost circuit will be about 80% efficient (could be better if you do it well).

    Suppose you keep your LED on until the cap voltage drops to 0.5V - the energy in the capacitor = (CxVxV)/2 so it starts with 2.5J and drops to 0.625J giving you 1.875J into the booster and 1.5J into the LED.

    So after 12 hours of charging you can have 1.5W for one second, or perhaps more usefully 50mW for 30seconds.

     

    MK

Children
No Data