Solar battery charger w/ protection

Discussion in 'The Projects Forum' started by Kefka666, Aug 13, 2008.

  1. Kefka666

    Thread Starter Active Member

    Mar 4, 2008
    I'm trying to design a solar power battery charger that won't overcharge batteries. I've worked out how to make a basic solar charger (solar panel, blocking diode, voltage regular, battery), but I can't figure out how to prevent the batteries from overcharging if I leave them out. I was thinking that an NPN transistor might be useful as a switch, but I don't know where to start.

    Does anyone know how to protect batteries from overcharging in a solar power charger?
  2. HarveyH42

    Active Member

    Jul 22, 2007
    A good start would be some specifics... Battery type/size, solar panel... Might also consider most batteries don't like to be completely drained either. I bought a commercial charge controller for $30, 12w solar panel + 2 x 12v 5Ah SLA in parallel (plan on a bigger battery later).
    Might take a look here, huge sight, but this part deals with charging.
  3. humblelearner

    New Member

    Oct 19, 2008
    I'm currently working on a solar battery charger. I'm going to charge a 12v battery with 2 solar panels which nominal Vmpp=15.4V, Pmpp=54W. I wonder whether a blocking diode(with fuse) is necessary to connect between the panels and battery.

    As i read from some websites that blocking diode is essential to prevent reverse current flow back through a solar panel at night time (dark current). But, at the same time, some stated that a blocking diode would cause a substantial lose of power in a 12v system due to its drop of voltage. It's quite a dilemma..:confused:

    And if a blocking diode is needed, what is the rating that i should look for?
    All the helps and guides are greatly appreciated..:)
  4. kara7

    New Member

    Jan 24, 2009
    You know I also interested in it, so thanks all of you for any recommendation.
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
    I wish I could stand by the circuit referenced in the link, but I built it and had no real luck with it, just disassembled it yesterday to recycle some parts.

    But maybe I did something wrong, so for a few bucks and very minimal parts, it might be worth a try to test it for yourself.

    I am now leaning towards using a solar panel to supply the power necessary to power a small AA battery charger that runs off or 6 or 9VDC. But then again, Kefka666, I don't think you specified the type of batteries you were wanting to charge.
  6. Solar_Panel_Sasch

    New Member

    May 7, 2009
    Yeah, in any serious solar charge controller the reverse current diode is a fundamental feature to its function. You have got it right - it prevents the battery bank from slowly draining through the panels at night.

    There is some power loss but it's not really significant (esp compared to complete potential loss of your batteries' charge through the solar panels at night). The power loss would be the voltage drop across a diode (standard 0.7V I believe) times the current flow. So yer 54 Watt solar panel produces roughly 54W/12V = 4.5A (this is actually higher than reality but close enough). So power loss would be 0.7V x 4.5A = 3.2W, or about 6% of the solar panels rating. This is in the right range of expected power loss for a solar charge controller. To see some examples of commercial solar charge controllers and there efficiencies check here.

    Depending on how complicated you want to make your circuit, you might consider adding logic that detects the voltage across the solar panels (input to the charge controller). Once the logic detects that the voltage has dropped beyond a certain point (presumably because there is insufficient light on the photovoltaic panels) the circuit switches the current flow to use a reverse current diode, otherwise it goes straight through the rest of the charge control circuitry.

    Another thing you probably want to allow your solar charge controller to do is support adjustable battery voltages - so that it can work optimally with slightly different manufacturer's solar battery specs. For some detailed specs on both flooded lead acid deep cycle batteries and sealed deep cycle batteries you can check out here. As you probably already know, sealed deep cycle batteries used for solar charging applications are much more sensitive to overcharging (i.e. delicate) and have a lower voltage that the less expensive flooded deep cycle batteries (which can take more of beating - and at worse just need more distilled water added to them).

    Heh... well, ok that may have been more info than you were looking for! :) But hopefully there was some useful data there that you may not have yet encountered.
    Last edited: May 7, 2009