Plug-in solar panel option

Discussion in 'OnBoard Electronics & Controls' started by Stuft, Nov 30, 2008.

  1. Stuft
    Joined: Apr 2008
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    Stuft Junior Member

    Our little wooden boat could use a solar panel connection to keep the battery in good condition while on the swing mooring.

    With a lot of good advice garnered from this forum, I have recently re-wired the electrics so I now have the flexibility to add more circuits easily. One circuit bypasses the battery switch to run a little auto bilge pump. I'm thinking to hook a 12v lighter socket (what are they called otherwise?) into that circuit so I can plug in the old BP solar panel without disturbing the battery cover.

    This would give a direct electrical path from panel to battery, with the pump on the same circuit. I think the panel puts out about 5w on a good day, the pump is the smallest Rule auto bilge, nothing's very big. Is this level of simplicity safe and workable or should I have some isolation in there?
     
  2. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Personally I wouldn't hook it up like this, since I prefer to have in and out seperated. But particularly here, since the likelyhood of the bilge pump getting a short or burning out is pretty good over time. So having it hooked directly to a charger (the solar panel) is even more dangerous.
     
  3. marshmat
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    marshmat Senior Member

    Hi Stuft,

    Stumble makes a couple of good points: firstly, that it's nice to separate the power sources from the loads, and secondly, that bilge pumps tend to fail shorted.

    Your bilge pump needs to have its own fuse, with nothing else sharing that fuse. Said fuse should be sized per the pump supplier's specs, so that it won't set the boat on fire if it shorts out.

    If you've set it up so that you have an "always connected" bus and a switched bus, hooking both the pump and the solar panel to the always-on bus could work, provided they're fused separately.

    Still, I really don't like the idea of having a lot of things connected directly to the battery, upstream of the master switch and/or fuse/breaker.....
     
  4. Stuft
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    Stuft Junior Member

    Thanks Marshmat & Stumble, poor explanation on my part in the first post.

    Yep, it's set up pretty much as you suggest, with all but the bilge pump going through the main battery switch first, then onto their own fused, switched circuits etc. The bilge pump has it's own circuit and fuse coming off the active run before the battery switch, inside the panel.

    For the solar panel, I'd like to install another fused circuit running back into the unswitched side of the active, but with separation, for the reasons you mentioned.

    What's a good way to provide this separation? Should I have a little reg in there somewhere? If so, what sort of reg? I'd prefer to use the active and earth inside the panel, just so it's all in one place.

    Thanks again..
     
  5. Ratch
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    Ratch Junior Member

    Hi Stuft,

    Did some quick calculations and it looks like if you run the pump for around 5 minutes a day you should be ok with that solar panel. I have not factored in battery discharge rate and that could be a concern. Discharge rates can vary from 3-20% per month and that's 1.2w- 8w per day on a 100ah battery

    Load running pump for 5 mins a day.
    2 amp x 12v = 24w
    (24W/60min)x5 = 2w/h

    Unless you bought the panel from California as they make the suppliers quote real life ratings I am going to rate your panel as 3w. I will not go into this here as it will run into pages but if you buy a panel look in the fine print for minimum guaranteed rating and note that this rating is under lab conditions.

    On a nice day in winter you should get 4 solar hours in Sydney.
    3w x 4h - 20%(loss when charging lead acid) = 9.6w per day

    So providing your discharge rate is not bad on a smaller size battery this should work. Don't worry about a regulator that panel will not overcharge a battery just hook it up direct.

    9.6w in, 2w discharge out, 2w pump out, 4w lost for rainy days....looks good providing your boat don't leak too bad :)
     
    Last edited: Dec 1, 2008
  6. Stuft
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    Stuft Junior Member

    Thanks for the sums Ratch. When we're at a mooring it runs for under a minute every couple of hours. Doubling your numbers to allow for the inherent leakiness of a wooden boat in it's late 30's, I'm still safe.

    I read last night on a battery site that 1% per day is a typical drop in capacity for lead-acids sitting idle.

    The panel is an original, now ancient BP Solar unit, surface area approx. 400x200, 3w is probably generous after all these years.

    I would be very interested in why panel ratings are (or are not) as they are printed. Perhaps another post for general info one day, or feel free to send a link.
     
  7. Ratch
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    Ratch Junior Member

    Solar cells are not produced to a uniform quality they are sorted by measuring the output, the really good cells end up in expensive high wattage panels.

    To get a rating a sample out of a batch of panels is sent for independent testing. This sample would the best of the batch in order get the highest rating possible. The samples are put in a room at an exact Temp, air pressure, Mj of a set spectrum of light per m2 etc etc. where in the real world you get changes in air pressure, temp, clouds, dust, air pollution and differences in light intensity due to geographic location and season.......

    Say this batch gets rated at 100w and they are the best, the worst or wherever they draw the QC line would be 'minimum guaranteed' this minimum is up to the manufacturer, lets just say it is 90w so the panel you are buying is somewhere between 100w and 90w. Marketing people turn this into 100w big capital letters first page and repeated as many times as they can throughout and minimum guaranteed in very fine print last page down the bottom.

    With your project I would suggest before the battery can go flat and taking note of the recent weather. Cover or unplug the panel and take the surface charge off the battery by turning on something that draws a bit of power for a short time, spot light is good. Measure the volts to see how full the battery is (12.8v full charge and 12v flat). This will tell you if it is going to work at least in that season figuring as you get half the power in winter you do summer.
     
  8. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Stuft, do not use a lighter socket. It is poorly made from cheap materials and will let you down because of corrosion. Use a light alloy marine socket instead with a spring loaded cap.
    Put a Schottky diode in series with the panel. The old-technology solar panels have a high leakage current at dusk and dawn and may draw more current than they put in when the sun shines. You can also use an ordinary silicon diode, but the Schottky type has a lower voltage drop so is more efficient.
     
  9. Stuft
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    Stuft Junior Member

    OK, the Schottky sounds like the weapon of choice. Easy solution. Thanks.
     
  10. Stuft
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    Stuft Junior Member

    Marshmat, what are options then? Put a switch in the circuit as well? No problem there. So, working up from the always-active terminal to the socket: isolating switch, fuse, diode, then into the socket.

    By the way, it's a very small circuit involved, no master fuse, just a two pole 350A battery switch onto a bus.
     
  11. BillyDoc
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    BillyDoc Senior Member

    Stuft, I think Ratch is right, you are unlikely to overcharge your battery with such a small solar panel. If you want to increase the panel size though, you will need a regulator or you run the risk of boiling off the fluid in the battery. There is a very simple (and cheap) regulator here: http://poiesisresearch.com/assembly.php. It was designed for use with windmill generators, but unless you go out of your way to tell the regulator, it won't care.

    The regulator in the link works by operating a relay which much be used to switch a load on to "use up" any extra current when the battery is charged. This load can be anything you want that doesn't mind being switched on and off, like a ventilation fan for example.

    BillyDoc
     
  12. Stuft
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    Stuft Junior Member

    Hi BillyDoc, appreciate the feedback. I'll knock it up on a breadboard and test it all on the balcony first. A new panel is next on the list for weekending anyway so the timing is good.

    Got a brother on the land who makes his own power, we've been experimenting with wind turbines, this is just what he needs too. Thanks..
     
  13. Ratch
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    Ratch Junior Member

    Billydoc,

    That's an interesting idea making use of excess power, i wonder how the surface charge on the battery will effect it?. I am not too keen on that circuit through.... I am concerned there is no hysteresis on the op amp also unless you need a big relay there is no need for the darlington pair on the output, no power supply filter circuit and the unused inputs not tied down.

    From a practical standpoint I am thinking when the batt voltage is high enough it turns on the load....ok...but that load could cause the voltage at the battery to drop enough to switch the relay back off again, with the load now gone the voltage at the battery goes back up and that switches the relay back on...click ...click...click...click............
     
  14. BillyDoc
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    BillyDoc Senior Member

    Hi Ratch,

    All good comments, especially the one about hysteresis, and all easy fixes if so desired. In this case, however, I don't think any change will usually be necessary.

    First the issue of hysteresis. I am assuming here that the battery being charged will be large and have a surface charge sufficient to make the time constant of that ...click...click...click fairly long. Or at least long enough that the relay used will not sound like a buzzer. This, in turn will also depend on the load being switched. If, on the other hand the time constant is short, then the load will see something akin to a pulse-width modulated input --- which depending on the nature of the load is usually a good thing. If some hysteresis is desired for whatever reason, a low value resistor between the zener diode and the positive input (pin 9) plus a high-value resistor between the comparator output (pin 14) and the positive input (pin 9) should do the trick.

    The relay is of course optional. The darlington pair shown could be used to directly drive many loads without further ado. The problem here is that many people will not have the knowledge base to be able to determine this and will either directly overload the darlington pair or thermally overload it. So, I included it as a sort of fail-safe to drive the relay and hopefully accommodate more people with little or no electronics experience. I would also argue that the ability of the LM339 to dissipate thermal loads is minimal, and the external darlington pair takes care of this problem, or rather moves it to a better place for treatment.

    Finally, big batteries make pretty good power supply filters all by themselves; and you are quite right the LM339's unused inputs and outputs should all be "tied down" to ground. That is, pins 2, 4, 5, 6, 7, 10, 11, and 13 should all be tied to ground. That was sloppy of me. Sorry! The only problem with not tying unused inputs and outputs to ground, however, is the current used from occasional transitions caused by possible capacitive coupling to unused inputs, should this happen.

    BillyDoc
     

  15. Ratch
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    Ratch Junior Member

    Hi Billydoc,

    The voltage will increase slowly as the battery charges and will sit on the threshold for a while so I think the hysteresis will be needed, but as you say it is no big deal adding it.

    I did have a thought on how to save power on the pumping side. There are losses putting power into the battery around 20%, also losses getting it back out again, Peukerts law.

    My idea is that you use one of the other gates to detect if the panel has sunlight either via the panel or an LDR so the pump only works during the day drawing some or all the power from the panel avoiding losses if you took all of it from the battery. Add another float switch higher up so it can pump at night if the water level gets too high to wait for day.
     
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