simplest Solar AAA battery charger

Discussion in 'General Electronics Chat' started by PsiCo, Jan 17, 2013.

  1. PsiCo

    Thread Starter New Member

    Jan 17, 2013
    Hi all
    I need help with a concept i have been working on Please - my understanding of electronics is very basic (non exsistant)
    I want to make a simple Solar 1.5 V AAA battery charger. Emphasis is on size + weight.
    just 1 single AAA NiMh rechargable battery charged by (i am thinking ....) 2 of those little solar powered calculator solar strips (ave 3.6Vdc @ 1.5 Watts Max)
    in my mind the unit would be tiny-like a Tic-tac Container(60x35x15mm-fits 3x AAA batt) - or even smaller, to just charge a single AAA battery - it would come in very handy when camping
    theres always a "flattest" battery in my bunch - with this i could solve the problem by just leaving it on a rock in th sun at my camp ,while I enjoy my day. charger working quietly...
    When i get back to camp - my flat battery is fully charged.YAY!!
    A standard AAA battery charges @ 2.8Vdc with 120 - 150 mA - says so on the back of my normal Energizer Mains powered AA / AAA Charger
    In a perfect world I would love if it could also charge an AA battery ,but at 2.8Vdc with 260-300mA - i dont think 2 solar calc strips will be enough - again from the back of my Energizer mains Charger - its max power use is 8 Watt - with 4 AA batteries in - im plan on using 1 x AAA
    I live in South Africa - Cape Town. Here we have almost cloudless days for about 9 months of the year - ive found info that states :960 watts per squre meter is what the sun puts out on a sunny day - a calculator solar strip(12% efficiency) is about 1 square inch = 1.58 Watts - WE GET FULL SUN HERE ....
    A friend suggested just wiring the 2 solar strips in parallel ,connecting the battery in parrallel with these + a 5K / 10 K resistor to limit current (in series on the Positive....right??) this seems viable, but very "bushmechanic" like to me
    Should there maybe be a 2.8V diode (zener?) in series with the resistor - to limit the voltage as well?
    would this work - or is the resistor size wrong? to little power from 2 strips?
    Also - what sort of power (heat) would the resistor + diode have to deal with?
    Small + cheap + hopefully very easy to make - a usefull addition to any campers gear / "bug out" bag
    adding a third solar strip is viable - if needed - just makes the unit a bit bigger possibly - not preffered...
    Hopes this gives some people ideas..
    thanx in advance
  2. Audioguru

    New Member

    Dec 20, 2007
    A Ni-MH cell is 1.4V to 1.5v when it is fully charged.
    The capacity of the latest one from Energizer is 700mAh and they recommend charging it at 70mA then the charger should shut off when it detects a full charge.

    The tiny solar strip on a calculator is much too weak to charge an AAA cell.
    A calculator is Cmos with an LCD display so its current is extremely low.
    The solar panel on a cheap solar garden light should be used.

    If you use a 5k or 10k series resistor then the charging current will be almost ZERO!
    The current from the solar panel on a solar garden light is only about 60mA so it doesn't need a series resistor. The charging battery cell limits the voltage.

    My Energizer charger also says "2.8V at 120mA" for AAA because it charges two 1.4V cells in series and has a timer to turn it off after about 6 hours.
  3. PsiCo

    Thread Starter New Member

    Jan 17, 2013

    thanx for that - its so difficult to make sense of all the info we are bombarded with , so thats why i asked. I didnt know that the Energiser charges 2 at a time - thankx for that

    So lets take it to its logical conclusion : how many strips would i need (realistically) to charge 1 AAA battery. I can fit 6 or 8 Strips onto a tiny little box - (its a Tic-Tac container ,about the size of a match box) + 1 x AAA fits so perfect is almost too good to be tru
    i can place 8/9 strips on all the surfaces except the lid + then stand it on its lid in the sun ALL Day - to get max solar radiation - can even lie a small mirror at the back to improve the efficiency of the rear-facing(away from sun) strips

    i figured the resistor was way too big - like i said :bushmechanic....

    i am convinced this concept will work - i dont expect it to be Super efficient - garden solar lights are 2 expensive - i can buy about 50 cheep solar powered calculators (5 rand) for the price of 1 Solar garden Light (R250) - buying 1 new just too rip it appart isnt viable.
    if I just get the little strips - Lord knows how cheap theyll be...probably talking in cents (not sense...) here. i have looked - there are no suppliers of individual solar cells here - we can only buy manufactured Rooftop type panels here...In South Africa.and again-buying 1 just to rip it up isnt viable. plus all the 1's i looked at the individual cells were toooo big (10cmx10cm - or more)

    Even if it takes all day to charge just 1 battery - thats fine.Thatll be 1 more usable battery while i am camping,that i didnt have before.Could power my AAA mini-torch all night again. EVEN IF IT ISNT FULLY CHARGED - SOME Charge + POWER WOULD STILL BE USEFULL

    (12 hours of sunlight is normal here in Cape Town ,Up to almost 14 Hours in Summer - Sun up= 4:30/5:30am to sundown which ranges from 6pm(in winter) to almost 10 pm - with very little clouds-it only really rains in Winter.

    i guess the next step is just gonna be buildin 1 to test - come hell or high water...regardless of my expectations..which are now low..thnx
    Please can you recommend a decent resistor size - if i was to try this?
    the calculation is R=V/I right - so to get 70 mA with an input voltage of 3.6 V - I would need a 51.428 Ohm resistor - rounded up to 52 Ohm is gives me the 70mA i need then.

    As its actuall Vsolar strip = 3.6 V i would need to put a resistor to deal with any exess Wattage right?

  4. Audioguru

    New Member

    Dec 20, 2007
    A solar garden light in Canada costs 2 dollars which is only 18 South African Rands.
    Its solar panel is about 50mm x 50mm.
    Its solar panel points straight up to the sun at noon.

    I have never measured the solar panel in a calculator. Some are just a photo of a solar panel. If it works then you will probably need a lot of them in parallel.
  5. Audioguru

    New Member

    Dec 20, 2007
    I said in my post #2, "The current from the solar panel on a solar garden light is only about 60mA so it doesn't need a series resistor. The charging battery cell limits the voltage."

    But the 60mA is only at noon when the sun is directly above the solar panel.
    In the morning and in the afternon the current is less so a 700mAh AAA Ni-MH cell will not be fully charged in one day.

    You must connect a diode between the solar panel and the battery cell to prevent the battery charge from going into the solar panel when it gets dark.
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Have a google for "solar AAA battery charger". There is a lot of stuff already out there.

    Even if you don't want to buy one it will show you the rough size and style of solar panel they use.
  7. PsiCo

    Thread Starter New Member

    Jan 17, 2013
    hi guys - thanx for the info.
    i have googled solar battery charger and nothing really catches my eye here.very costly...We work hard for our Rands....
    And trust me - if i could find Solar garden lights here that cheap - id buy them in a heartbeat. I dont have creditcard/paypal so i cant buy online from China + stuff - found a 10 pack for 360 rand.I only need 1 + dont even have garden-just a cement walkway - so the other 9 be useless - unless it actually works well...

    i will look around more - but i think ill just build the silly little thing anyways
    cant take more than 10 minutes. + cost should be negligible...the gasoline used to go buy the calculators strips + a diode will end up being way more than the bits themselves....ill try 3 strips in parralel - hopefully get some power like that.

    Will try do some tests with a multi-meter once its going.will give results when done.
    for what is worth.....even if its just so u can c me fail + say "i told ya so..."

    thanx for the suggestions
    Last edited: Jan 18, 2013
  8. PsiCo

    Thread Starter New Member

    Jan 17, 2013
    so i found a 4.8 V 60mA solar cell from communica.
    Its 60x60 mm (definately small enuf - bigger than i wanted but viable?)
    They dont have the price on their site - (or a pic) i will phone them later in the day.
    its 6am here
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    A solar cell is a current device so all you need do to make a basic charger is put a diode between it and the battery. That's all solar yard lights do.

    If you use more then one solar panel they must all be matched, and then they can be added in parallel (for more current) or series (for more voltage). Combine equal ammounts of cells: if you have 3 cells you can't put 2 in parallel with one in series: it will work like 2 in series (but nothing bad will happen).

    Don't put the series resistor as your friend suggested: it doesn't help and will stop it from charging.
  10. Audioguru

    New Member

    Dec 20, 2007
    Solar CELLS are about 0.5V each with a load. A 4.6V (no load) solar PANEL probably has 7 CELLS in series.
    Then the size and voltage are larger than you need.

    You need a 3.0V (no load) solar panel.
  11. PsiCo

    Thread Starter New Member

    Jan 17, 2013

    thanx for all the advice guys.