Need help on charging supercapacitor

Discussion in 'General Electronics Chat' started by tattee, Jul 18, 2008.

  1. tattee

    Thread Starter Member

    Apr 24, 2008
    16
    0
    Hello! I've been doing experiments with supercapacitor. I'm trying to integrate it to help prolong battery runtime.

    I tried some scenarios like connecting the Supercap directly in parallel with the battery with no added components/circuit. The thing is it didn't help the battery at all rather it degrades it even more. After reading some articles i've found out that the in-rush current may have been the cause of my problem.

    Now, im planning to add some components that would somehow limit the current draw from the battery using only simple components like resistor/transistor/mosfet/etc.. and at the same time charge the supercap fast. Ive seen some chips for supercap chargers but my budget is limited so I opt to use simple components if its possible.

    From my experiments, (correct me if im wrong) i have noticed that the higher the current supplied to the supercap the faster it charges. So im planning to use darlington pair so that i can input small amount of current(therefore preserving the battery) and provide an ampified current at the output(thus, charge the Supercap fast).

    Is my idea somehow effective or im not making any sense at all? I do appreciate any suggestions you want to share. This is my first time handling supercap and i dont have that much experience with electronics either.

    Im using two AA rechargeable batteries(1.2V, 2100mAH) and 1 SuperCap (2.5V, 33F). My Load will take about 200mA-500mA. Im also planning to replace the SUperCAp with smaller value (0.1F, 2.5V) because it's taking too much time to charge this 33F supercap.

    Thank you in advance!:)
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    A typical supercap based design involves using the supercap to supplement the primary DC power source during intervals when the DC power source is turned off or has failed.

    When the primary DC power source is operating, the charge on the supercap is replenished.

    What is your primary DC power source? Have you made provisions to replenish the supercap when your primary DC power source is present?

    hgmjr
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    It will be important to remember that power out = power in. If current out > current in, then voltage out < voltage in by the same proportion.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Try charging the supercap through a flashlight bulb that's designed for two AA batteries.

    The nice thing about using an incandescent light bulb is that it has a high resistance when it's hot, and a much lower resistance when it's cold. When you first start charging the cap, the light will burn brightly, gradually start dimming, and as the cap rapidly nears full charge, the bulb will go out.

    This will charge your capacitor at a rate that won't cause your batteries duress. It's sort of a "poor man's constant current" circuit. Cheap and effective.
     
  5. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    You need to determine the maximum current you want to draw from the AA
    cells and calculate the charge time I = C dv/dt. For a AA Alkaline (non-rechargeable cell) you get 2900mAh based on 25mA continuous current at 21degC. If you draw continuous currents of 500mA you will get a lot less.

    What is the duty-cycle of your system? If it is 100% the supercap is not going to
    help -- it will give you less battery life since you will drop voltage across the ESR of
    the cap (which can be high for supercaps).

    If you are running a low duty-cycle system the supercap could work. It depends on
    the duty cycle. In one of my designs I use a supercap and coincell. I use a consant
    current sink connected to the coincell to continuously charge the supercap.
    The current sink uses a single 3V supply (coincell), an op-amp and P-channel
    MOSFET.

    (* jcl *)

    www.wiblocks.com
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    Adding a capacitor will not prolong battery runtime.
     
  7. tattee

    Thread Starter Member

    Apr 24, 2008
    16
    0
    Thank you so much for ur replies! i'l try to consider all your suggestions and let you know about the changes i will be making later on.

    For now, I have already change my supercap to a lower capacitance (0.047F, 5.5V). I will also try to use Loads which draws a lot more current like motors and see if supercap improves battery life.

    Thanks again!:)
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you're trying to improve your battery life by charging a supercapacitor from those very same batteries, it's not going to happen.

    If you charge the supercapacitor from an external source, then it may help.

    You can get better efficiency from batteries if they are run at relatively lower current levels. At higher current levels, more power is dissipated in the battery internal resistance. The more quickly they are discharged, the more power is wasted in the internal resistance.
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I don't know what to say :confused:

    Ask yourself where the capacitor gets its energy from.
     
  10. tattee

    Thread Starter Member

    Apr 24, 2008
    16
    0
    Hi Guys,

    Ive been working on a solar panel right now found on common calculators. It gives up (2.5V, 0.02mA) indoors. Im wondering if i could use this for charging the supercapacitor. But with 0.02mA of current from the solar panel i dont think its going to help at all. Is there any way i could amplify this current coming out from the solar panel??
     
  11. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    At 20uA you have nothing to work with. Current up = voltage down and since there is only 2.5V to start with this is never going to work.
     
  12. tattee

    Thread Starter Member

    Apr 24, 2008
    16
    0
    really?:( what am i gonna do now? im really running out of options here. i guess il just have to buy solar panels with the right specs i need.

    Question: From what ive seen from the solar panel manufacturers(ex. 2.5v, 150mA). Will i be able to achieve these number indoors? using only flourescent lamps?
     
  13. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Short answer, no. Solar panel outputs are usually specified under optimum conditions which normally means in full, direct sunlight. Even large solar panels produce virtually no usable power under domestic fluorescent light.

    Full sunlight is roughly 1kW/ sqm on the earths surface. Obviously a flourescent lamp which might produce 20W of light energy is at a considerable disadvantage from the start.
     
    Last edited: Jul 23, 2008
  14. tattee

    Thread Starter Member

    Apr 24, 2008
    16
    0
    :( oh i guess il just have to forget about the solar panel. any suggestion on how am i going to charge my supercap? am really having a hard time on this thing.all i want to do is prove that this will enhance or prolong battery life but i havnt benn succesful so far.. too bad!

    @blocco
    I really appreciate the way you reply to my questions. its very straight forward and fast. it saves my time quite a lot! thanks!!:)
     
  15. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Unless you have some other source of energy, other than the battery in the circuit, a supercap will not prolong battery life.
     
  16. tattee

    Thread Starter Member

    Apr 24, 2008
    16
    0
    any suggestion on what il charge my supercap with? addtional batteries will not help my cause.
     
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