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Capacitor for high current discharge

  1. Apr 6, 2012 #1
    Dear Experts

    i have a solar panel but the current is not high enough .

    I have about 5 volts to charge giving about max 100mA (specs).

    I am thinking of using a Capacitor to store all the low currents such that when the capacitor is fully charged, I need a circuit to discharge all the high current in the capacitor to the battery.

    I am not sure if this is the same as hooking up the panel to the battery without capacitors.

    I also not sure is there a minimum current (at 5 volts) the lithium ion battery would accept current? Am I right to say any small amount of current will charge the battery at the right voltage and the only difference is how long it takes to charge?

    By the way, I will add a rectifier to maintain a steady voltage.
    But does this not mean that if voltage is not sufficient, there is no charging at all vis-a-vis without a recitifier ?

    Eg. If I use a 7 volt rated solar panel, connecting to a rectifier taking about 2 volts, the remaining 5 volts will charge the battery. But if the sun is not strong enough, producing only 6 volts or less, the rectifier may not have 5 volts output . What happens? No charging?

    Thank you for reading my question.

    Have a nice day.

    Best regards
  2. jcsd
  3. Apr 7, 2012 #2


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    Staff: Mentor

    It usually is no different.
    Li-I batteries are very fastidious and fussy, and are not safe to experiment with. They are easily damaged and can catch fire if conditions are not to their liking. https://www.physicsforums.com/images/icons/icon9.gif [Broken] Can you use NiMH cells instead?
    If the voltage is lower than the regulator needs, then there will be no charging.

    A 7v panel delivering 100mA should be about right for 5 x NiMH cells, each 1.2v. Place a current meter in series to verify that charging is proceeding okay. No regulator required. :smile:
    Last edited by a moderator: May 5, 2017
  4. Apr 7, 2012 #3


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    Gold Member

    Uh ... I thought solar cells produced DC voltage, no? You have one that produces AC voltage?
  5. Apr 14, 2012 #4
    Dear NascentOxygen and Phinds

    Sorry, my bad, I incorrectly used the word 'rectifier' for regulator.

    NascentOxygen has given me great advice. I really appreciate your kind help.

    By the way, I could not find a 7V solar panel at 100mA. The store only has 6V at 100mA.
    So, I bought another 4 x 5v panels rated at 100mA and connected them as 10v pairs.
    Because of the potential difference, I drop the voltage down using a regulator at 5.5v and connected a 1 farad capacitor at 5.5v before regulator whose output goes to charge the battery.

    Based on the voltmeter, it seems that adding a cap stabilized the voltages a bit especially when the sky is cloudy. Is this observation making sense?

    If I add a same cap after the regulator, would that make it even more stable? But what happens when the sky is too cloudy? would that not make the capacitor discharge to the solar panel? So, I added a small zener diode to block the discharge.

    Still my phone, which has a larger lithium battery than iphone, is DIScharging rather than charging my phone.

    That is why I wondered is it due to the position I placed the diode ? I could not understand why the phone is not being charged. For example, after 5 minutes, the charged % actually dropped rather than increased.

    Best regards
  6. Apr 14, 2012 #5


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    Staff: Mentor

    I didn't suggest you would. It was you who nominated that figure of 7V. :smile:
    One farad?!
    It should make no difference, unless the regulator is not stable and is oscillating or something. Maybe you are using a switching regulator? What is the type number on the IC?
    Data sheets for some regulators may specify where you can add a small capacitor to give it greater stability.
    Capacitors draw practically no steady current, so you are not losing any power in the capacitor. Your inclusion of a zener diode is wrong. Zeners conduct in both directions, it can not help you. Leave it out. Possibly your battery may discharge into the regulator when the panels are producing no voltage; can you put a current meter in series there to measure any current in the wrong direction?
    You are using the solar panel to charge a lithium cell? As I indicated, lithium cells are dangerous and are not sold for experimenters. I recommend that you charge only NiMH cells from your homebrew solar charger. You could charge 6 or 7 NiMH cells from your 10V panels and as NiMH cells are tolerant and relatively safe, there would be no need for a wasteful regulator. There may be a safer way to charge Li cells from the fully-charged NiMH cells.
  7. Apr 14, 2012 #6


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    A switching regulator could be very efficient and you could charge a lower voltage battery than your solar cell without losing appreciable energy. The regulator could be made 'intelligent' so that it charged your battery as fast as possible, too.
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