Crude Battery Charger

Discussion in 'The Projects Forum' started by DC_Kid, Jun 2, 2009.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    ok, i did some searching on AAC and didnt find anything that directly answers my questions.

    i am building a large clock (4ft x 4ft). the movement is high-torque quartz unit that is powered by a single C battery. i wish to make the run time much longer. so my idea is to add two more C holders and parallel all 3 batteries. sounds simple enough, but i wish to take it a step further. this clock is big and i do not wish to take it down to change the batteries. i was thinking of using NiMH batteries and building a crude battery charger circuit so when batteries dies i can use a wall wart to charge the batteries back up, etc.

    i was thinking of a auto-off circuit. something real crude and simple. attached schematic shows my 1st stab at it. would this work as a auto-off when bat voltage reaches ~1.5v? figured the zener would be 2.2v (1.5bat + 0.7scr). as the battery gets closer to 1.5v the current should drop off enough for the scr to kick off?

    i'm not even sure if NiMH is right for this application. would NiMH self discharge too fast and i would find myself re-charging often? perhaps sealed lead-acid or lithium-ion would be better? i know lith-ion cell is ~3.7v so i would need a low loss regulator to feed the movement, perhaps defeating my goal of long battery life....?

    any thoughts or suggestions?
     
    Last edited: Jun 3, 2009
  2. Mike Mandaville

    Active Member

    May 27, 2009
    81
    1
    Am I correct in my understanding that it is your intention to charge these batteries in place? If so, then why bother with the batteries at all? Wouldn't it be simpler to just use an AC power adapter directly?
     
  3. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    i am not allowed to have a cord hanging from this clock. it's also a art piece.
     
  4. Mike Mandaville

    Active Member

    May 27, 2009
    81
    1
    Okay, so you don't want to take it down to change the batteries, but you want to take it down to charge the batteries. Is this correct?
     
  5. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    no.

    the frame is made from 1.5"x1.5" aluminum angle. on the bottom of the frame (floor facing) i will add a wal wart jack (mini round) and mini pushbutton switch. when batteries die i plug the clock in for some time (still working out a led to indicate charge done).
     
  6. Mike Mandaville

    Active Member

    May 27, 2009
    81
    1
    Since you don't want any hanging cords, to power your clock, why don't you use Tesla's method for the wireless transmission of power!:


    [​IMG]
     
  7. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    How much current does the clock need?
    How long does it run on a single C-cell?
    How long do you want it to run?


    Did you consider making a battery holder that opened up downwards in the bottom of the frame?


    You have to specify the current drain and your definition of "too fast" and "often", to enable anyone to fully answer that, but in normal sized clock units (AA), the self discharge would be the main issue.


    That might work with a diode in series, but they will gradually die in such an application and self discharge is the main issue here too.

    I'd install a battery cover (hidden) in the bottom of the frame if I had this problem, or perhaps two, for parallel cells if needs be.
    Allways using the best alkaline cells will make a big difference in lifetime too.

    Domestic size clock units usually has a battery life time of more than a year - I have a clock still running on an AA-cell I installed sometimes in 2006.
     
  8. HarveyH42

    Active Member

    Jul 22, 2007
    425
    5
    Four feet by four feet, seems likely it would cover an outlet or light switch (which you could tie into), or get an outlet installed behind the clock...
     
    Double00Buck likes this.
  9. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    soren,
    trap door was considered on the face, and some form of pop-out in the frame, but i wanted to avoid that.

    quartz movements typically draw little current, but this unit is a high torque unit so it probably uses a tad more energy for each movement pulse. as i dont yet have the movement (its been shipped) i cannot measure the current.

    if one regular C gave Xdays then 3C's should give about 3Xdays. i was hoping by using rechargeables i could get close to 3Xdays, and not have to hassle with taking the clock down to change the batteries.


    Mike, yes i thought of using a wifi charging mechanism. i figured there is so much free 802.11abgn in the air i could harness that energy for free to power the clock for eternity.

    perhaps i'll just start with 3 standard C cells and then do some testing with NiMH (555 with freq of clock and a load that matches the movement, etc). but to my original question, would the charger i posted work?
     
    Last edited: Jun 3, 2009
  10. balisong

    Member

    Feb 26, 2008
    27
    0
    Just an idea, could you discretely incorporate a solarcell into the clock? If the room lighting is insufficient to keep the battery(s) topped up, it will at least lengthen the time between recharges. A narrow beam IR emitter nearby might be enough supplemental light.
     
  11. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    solar cell is not possible. just 3 C batteries on the back of the clock.
     
  12. tkng211

    Well-Known Member

    Jan 4, 2008
    65
    2
    Think the max. terminal voltage of the cell is critical. As the commercial tolerance of zener diodes is 5-10%, better use 'PROGRAMMABLE PRECISION REFERENCES' TL431 for the voltage reference. A modified circuit is attached.
     
  13. radiohead

    Active Member

    May 28, 2009
    474
    31
    In your multiple C cell parallel circuit, you may want to consider adding a diode on each cell to prevent charged batteries from trying to charge the lesser charged ones, or go with a higher mAH battery. I do like HarveyH42's idea, though.
     
  14. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    tkng211,
    let me put your cicuit into verbiage so i can follow, please correct me where i'm wrong. and i do have some National programmable zeners.

    so when the cells are dead there is enough potential bias to get the npn going (scr has been triggered, etc). the zener clamps the npn base voltage. as the batteries charge the emitter voltage will increase and when base-to-emitter is just under 0.7v the npn will stop making scr stop and charing is complete. so i should set the zener to 2.9v (1.5v cell + 0.7v npn + 0.7v scr)?

    it looks like the npn is the "switch" as opposed to the scr? if so then i could simplify by removing the scr? without scr would i run into a on-off oscillation issue? i could add a LED in series with npn collector as a indicator?

    i know this is all basic stuff, so forgive me for being brain dead at the moment...
     
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