Solar Panel As Battery Charger

Discussion in 'The Projects Forum' started by NM2008, Sep 5, 2011.

  1. NM2008

    Thread Starter Senior Member

    Feb 9, 2008

    I have built a circuit based upon an oscillator and a decade counter to flash six LED's in sequence. This circuit requires 5.5 -6v to operate and draws no more than 15mA.
    The circuit when in operation has to run for several years (day and night), it needs to be isolated from the mains supply and also maintenance free.

    At the moment I have a 6v 200mA solar panel and five 1.2v 3000mAh Ni-MH "AA" batteries. These batteries I had purchased for another project and had them to spare.

    If I were to make a 6v battery pack from these five batteries would it be possible to keep it at 6v throughout its lifetime, by connecting the solar panel to them.

    If I were to connect the solar panel direct to the 6v pack, would it be advisable to fit choke or current limiter to aviod overcharger the batteries?

    Also in the night time situation will the solar panel have a reverse effect and become a load on the battery pack, (are solar panels resistive without light?)

    At night will it need to be disconnected from the batteries?

    Any help appreciated,
    Regards NM
    Last edited: Sep 5, 2011
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
    It would likely work OK for a while but ... connecting the solar panel directly to the batteries would not provide optimum charging without an appropriate charge controller designed for Ni-MH batteries. Proper charging and dis-charging affects the usable life of the battery. And a properly designed solar charge controller would isolate the panel from the battery at night. You should read up on the battery charge and discharge characteristics so you understand what a properly designed charging circuit needs to do then build a circuit to do it properly.
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    What would help us help you is a circuit diagram, preferable as an image attachment in .png format. One reason why this is important is to determine if we need to regulate the voltage from the solar panel or not. The particular IC's you are using probably can handle five 1.2V NiCAD or NiMH batteries(fully charges at 1.45V x 5 = 7.25V, but the solar panel even it the brightest sun will not produce 7.25V(or can it - did you measure it.)

    You would have to have them on permanent trickle-charge so as not to over-charge and sufficient to last the night and dark days when the solar cell is no able to provide the necessary charge voltage. Not sure the panel you have is enough.

    Yes, you will need to limit the current.

    A blocking diode will fix this issue and stop the batteries from discharging into the solar panel at night or when the panel's output voltage is less than the battery voltage.

  4. #12


    Nov 30, 2010
    Another way to say it is, NiMh batteries are very picky about their charging current. Do it right or the batteries will mutiny.
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    I think it can work. Panel output / year, about 243 Ah, load about 131 Ah / year. Panel will marginally charge only 3 batteries, so either panel ottput V needs to be boosted , or LEDs drive designed to operate on lower V. Charge control will be dificult with limited panel current output.
    Last edited: Sep 6, 2011
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Consider Ni-Cd "C" cells @ 1.2V & 3-3.5 Ah. for simple charging.
    What color LEDs? Operating on what duty cycle, or can a little off time be allowed?
  7. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
    There are six Red 5mm leds operating within a chaser circuit, the leds will not be on constant.

  8. k7elp60

    AAC Fanatic!

    Nov 4, 2008
    I have built many Ni-cad and NMih chargers. If they are charged at less than .09xMAH they will never over charge, but they will get a full charge. This is at nominal room temperate about 72 degrees F. I use a LM317 connected in the constant current method. The output from the charger needs to be able to supply 1.5V per cell connected in series.
    If the batteries for your project are going to be in a hot environment, I would use a 6V SLA battery as they are not as sensitive to the temperature extremes.
    I just finished a project to illuminate my house numbers at night with LED's. The whole thin is solar powered. A solar panel, a battery, the lights, and the control board that has a charger, voltage sense to only turn on the lights if the battery has a charge, and a flasher circuit to flash the lights in an emergancy so the emergancy personnel can locate the house should the need arrise.
    John Berry likes this.