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Want to increase the Output current from soalr panel

  1. Mar 31, 2005 #1
    Hello everyone,

    I am trying to increase the output current of a solar cell by using two AA rechargeable batteries (1.2 volts/750mAh), any suggestions how may I need to connect them. I am a retired man and have a hobby of doing things. So please be patient. I would greatly appreciate any help. I am trying to charge my grand son car batteries which have the capacity of 900mAh. The solar panel i am using provides an output voltage of 4.5volts/100mA current. Please help!

  2. jcsd
  3. Mar 31, 2005 #2


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    supperman - what you need is the voltage rating of your grandson's car batteries.

    Then you need a source that can supply more voltage than those batteries you wish to charge.

    After you have a source of charging voltage higher than the batteries voltage you can determine how long it will take to charge with the current you have.

    If you hook up a lower voltage you will only deplete the battery further, higher voltage always wins regardless of current (which flows as a byproduct of voltage).

    Your solar panel provides less than 1/2 watt of power, pretty small amount since each AA battery has twice the power output (but will be dead in less than an hour at that rate). So it would be more appropriate to use the solar panel to charge the two AA batteries connected in series! :smile:

    If you can find the voltage on the car battery and the type (lead-acid,ni-cad, lithum-ion) that will help too in determining the maxium amount of current you want to use to charge it.
  4. Mar 31, 2005 #3
    Want to increase the Output charging current

    Hello Cliff_J, many thanks for your reply!

    Oh so sorry that i forget to mention the voltage rating on the car battery and their type. The voltage rating on the car battery is 4.2 volts and they are Li-Ion type.
    So you are suggesting that , i should use the solar panel to charge these 2 AA batteries. But now i think that 2 batteries will not be enough to charge the car batteries, i need 4 of them. is that right? as you mention before the charging voltage should be higer if i understood right?

    If i put 4 batteries in series, i will get 4.8 volts with 750mAh current. But how to connect now solar panel so it can charge these batteries, just connect the wire coming out of the solar panel to these batteries, connected in series. or do i need to put more electronics stuff inbetween? and how do i control the charging? I mean when the 4 AA batteries are charging the car battery, do i have to disconnect the solar panel, or just leave it on while charging the car battery?
    I really appreciate your answer!
  5. Mar 31, 2005 #4
    since your solar panel is 4.5 V ,and your gr-sons battery is 4.2 V then why not connect the panel directly to the battery ..?
    btw the panel might have a built in diode , to prevent depleating the battery if there is no sun..
  6. Mar 31, 2005 #5
    Increasing the Ouput Current

    Hello willib, thanks for your answer.

    Yes I tried to do that, but it takes forever to charge the battery. So I did some research on the net, found that increasing the current might help boost the charging time. So now i am trying to build that one.

    So i guess, i could use a switch to control the charging of car battery, and the AA batteries if i do not need anything else to connect inbetween the solar panel and my 4 AA batteries.

    thanks for your help. I really appreicate that!

  7. Mar 31, 2005 #6


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    Ok, first thing of note is nominal values, as in some sort of meaningful average-type value that is useful. Lets use a full sized car battery, we call it a 12V battery. Brand new and fully charged and it will likely be around 12.8V. Discharged its more like 12V and if you run it way down closer to 11.5V or even 10.5V when dead. (this kills a lot of capacity too to run a lead-acid battery down that far) When the alternator is cold it will likely charge at something close to 14.4V and then settle down to something like 13.3V when hot. If the alternator stayed at 14.4V it would reduce the life of the battery faster by over-charging it (and heating it up). And those are a typical GM or Ford value, it varies even more than that and is computer controlled on some cars too!

    A basic multimeter will give you at least a little better idea of what the actual voltages are for each piece in the chain.

    If the pieces have voltages as advertized, the 4.5V solar panel connected directly to the 4.2V toy car battery makes for a workable charging solution. The trick is figuring out how long it will take to charge the battery, might be more than a full day of charging under sunlight since solar panels drop in output tremendously when the sun isn't optimal.

    Using batteries to charge other batteries is an odd problem. As the battery discharges its resting voltage generally drops (ceteris paribus, heat it up by working it and the volts can go up). So at first the low battery is at a low voltage and the good batteries are at a high voltage. As the charge is transferred the voltage difference reduces and the current slows.

    Now there are very elegant methods to charge the Li-Ion batteries with constant current (and its cheap too with ICs designed for it) but a simple voltage difference would require little other than knowing what the fully charged voltage of the battery is and then hooking the solar panel up to the discharged battery. After regular time periods (like 1/2 hour) you could disconnect the solar panel and measure the voltage once the battery cools (if it even heats up) and figure out how long it takes to return to its fully charged voltage.

    It would only be approximate and the sunlight on the panel affects things, but hey, its cheap and easy! Plus the Li-Ion batteries can be charged at like 50% or more of their rated current, with this setup you're only 10% of the rated current so you're at much lower risk of over charging damage. So it should be an easy experiment and at the low end of expectations the panel won't charge the battery much.
  8. Apr 1, 2005 #7
    Thank you Cliff_J!

    Now what i did is using the solar panel to charge my 4 AA batteries, and than i am using a manual switch to start charging the car battery, and turned off the solar charging of the AA batteries. But i have only one concern, is the solar panel going to effect my AA batteries, since the voltage goes up and down, depending on the sun? or that is alright for the AA batteries. And one more question , is there any electric switch there, which can replace my manual switch? I mean electronic switch? it will be help full if we do not have to paly with the manual switch.
    your response is greatly appreciated!

    thanks for your great help!

  9. Apr 4, 2005 #8


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    Adam - its difficult to say for 100% if its ok to be hooked up all the time but the chances of a problem should be fairly low. If you get dead batteries you can figure out the setup will not work.

    The voltage going up/down shouldn't matter - when its higher the current will flow into the batteries and when its low no current should flow at all. This might mean it only charges for 30 min at high noon when the sunlight is just right. Its a challenge with solar, some setups follow the sunlight like a sunflower plant where it pivots and changes angle to track the sun across the sky.

    A solid state switch like a transistor has the same problem a diode would if used here (that would offer a one-way check valve for current) and that is the voltage drop that happens as the current flows across a semiconductor.

    A very low power relay may work as an automatic switch, a simple potentiometer could be used to adjust the voltage the coil trips the contacts. Not very elegant, but simple enough to tinker with.
  10. Apr 5, 2005 #9
    But i have only one concern, is the solar panel going to effect my AA batteries, since the voltage goes up and down, depending on the sun? or that is alright for the AA batteries.

    Isn't the voltage constant and the current affected by the sun?
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