Tech help needed on homemade solar system

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    I have just recently built a solar system for my cabin. 4 brand new golf cart batteries 6 volt 225 amp-Hr@ 20 hour batteries wired 2 in series, 2 in parrallel to provide (hopefully) 12 volts 450 amp-Hr holding capacity. I have 2 15 watt solar panels on the roof and a 7w charge controller.

    I hoped to power a couple of CFL bulbs only at night( 13 watt), a stereo 12 hours per day (35 watt) and a few LED nightlites (0.5 watt each)thru a cheap 400 Watt (car lighter plug in type) inverter. A fairly lite load i think.

    Problem: Batteries went flat on me after about three weekends. After installing the solar panels, i checked and found 20 volts comming from the panels so I think they are working. Charge controller has a yellow light telling me i am charging. Cabin sits idle all week and is used only on week-ends. What could be wrong?

    Also, I now meed to charge up these batteries using a generator. Does anyone know how long it would take with a 12 Amp battery charger?


    seen it, products look great, however this does not help my problem.

    Nick Rosen

    my freinds and i have designed an electric generation systim which runs on fresh air and have found only brick walls in our attempts to make it feasable for general use . if you or any member has any ideas for us to move on i would be gratefull , all corespondance to my email …. ianhirst@hotmail.com


    For starters you say you have a 7 watt charge controller, and 30Watts of panel, this will not work.

    you may have a 7Amp charge controller which is fine.


    where is the system located (zip code or state)?

    your load is 732 amp hours and you do not have that much charge capacity or that much battery capacity.

    if you have 4 peak sun hours ( deep south US ) you are generating about 7 amp hours of power a day. on a good day.

    so if you are charging for 5 days you will get 35AH of charge back in your batteries, you need a lot more panel. This all assumes that your loads are what you think they are and that the inverter is 100% efficient which it is not.

    Hope this helps


    blazie’s right… you’re never gonna charge up 450Ah with 30w of panel. You need a rightly sized system and it seems (maybe im wrong) your battery capacity is a little over the top for what you want to do. Unless of course you bought them as an investment and looking to do more. You need to keep your batteries from discharging below 20% (worst worst case) and they should be given regular trickle charges when you aren’t using them…


    This is a nice guide. I found a site that gives step by step video instructions on how to build home solar panels. It gives some interesting information.



    Sixsoxs I’m assuming you still check this forum from time to time.

    Blazie made some good suggestions. How about responding so we know any advice is actually received.

    I see several reasons why your system may have drained the battery bank. This advice is applicable to any person building their own system. Location by latitude is important to know. At higher latitudes you get much less solar input compared to the theoretical rating listed for the panel. If you are above latitude 45 the actual output of the panel could be as little as 50% during winter months. Secondly does the panel have isolating diodes. Several of the lower wattage panel do not because of the inherent voltage drop created by the diode itself. This means the solar panel actually drains the battery at night .

    Thirdly that 35 watts for the music system amounts to nearly 2.9 amps X 12 hours or roughly 34.8 amp hours of consumption. In other words your music system is using all of the potential power the solar panel might be able to deliver but drain the battery bank of the charge in a couple of days. One week end in other words.

    Not to mention which you are not going to get full output from that solar panel for the full period of daylight time. At most you can expect 4-5 hours of maximum output on a good day. Early morning and late afternoon is not going to deliver as much power no matter how much the sun is shining.

    You mention using a 400 watt capacity inverter to run some night lights but do you shut off the inverter when not in use? The parasitic load will kill your fully charged battery in less than a week; even if the inverter is not driving any load.

    The short answer is you need much bigger panel capacity. Secondly you need to understand where the hidden current drains can be found.

    Forget the 7 amp charger it is not sufficient. It will barely handle the self discharge of the battery bank, let alone deliver 425 amp hours of real charging power.

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