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Dollar Store Pumpkin Light Solar Conversion

  1. Oct 1, 2011 #1
    Hello. First time poster, many-a-year browser.

    I will describe my current project in the fullest detail I can and follow it with my attempted solution. I will point out off the bat that while I have a BSc in Mathematics, I had a terrible time learning physics in high school and get stage-fright when trying to recall/work with the necessary formulas.

    So, I have a 13W solar panel (with controller integrated) hooked up to an old car battery. Testing with load it shows good amps and steady 12.5 volts. My guess is it could actually start a car so long as it wasn't below -30. From this car battery I have a fuse block set up which allows me to tap into the battery for various things around my yard like rain barrel pumps and lighting.

    Few days ago, I bought 10 of those dollar store stakes with the pumpkin face on them. Essentially, it is a plastic decoration with a very simple inside. An incandescent light bulb marked with 2.5V 0.3A that connects directly to 2 AA batteries. So, I figured, if I wire 5 together in series then connect this to my solar power set up, I should be pulling 12.5 Volts at about 1.5A. (I Have NO idea if this is correct because my physics skills are so weak)

    But, I have fused the power source with a 5 Amp fuse currently and it isn't popping, so clearly it is under 5 amps. Oh, btw. they light up beautifully. I don't know the resistance of the light bulbs nor how to find it....... So I am lost. What I want to know are two things.

    1) Can I hook up another identical setup in parallel to this one? And if so, what will the current draw be? 3A? (Also, will the resistance be reduced by 1/2??)
    2) And perhaps of more interest to me right now, How much power are these drawing?

    For solving part 2) I thought since P=IV, I am pulling 18.75 Watts. I don't really understand where to go from here. I guess that per second I use 18.75 Joules? What I am concerned about is: If I leave the lights plugged in (either the single strand or double strand) for 24 hours a day, am I going to deep discharge my battery? Will a solar panel with 13W rating manage to recharge the battery every night enough that it wont matter?

    SO! Basically. Milk and cookies for anyone who can walk me through this. I would love someone with the "Teach a man to fish" attitude. My skills in science are pretty basic but my math skills are basically all I have going for me, so I am damn good at it. I would love to learn more about electricity and how all the formulas inter-relate and the like.
  2. jcsd
  3. Oct 1, 2011 #2


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    Your amperage is incorrect. The amperage will still be .3A. Think of it this way, if you were to just hook two lights in series to a 2.5V source, you would be doubling the resistance and thus halving the current. 5 in series will increase the resistance by 5, but you are also increasing the source voltage by the same factor.
    the resistance of the bulbs can be found from their rating of 2.5V 0.3A. Ohms law says I=V/R, so R= V/I = 2.5/0.3 = 8.33 ohms.
    Right equation but wrong answer (due to the above error in amperage). you are actually only using 3.75 watts
    Unless your battery has a really low amp-hr rating, this should not be a problem. Even two strands running 24 hrs uses only 14.4 amp-hrs. Amp-hr ratings for a car battery are figured for 20 hrs (a battery rated at 60 amp-hrs would provide 3 amps for twenty hrs before dropping to 10.5V)
    Last edited by a moderator: Oct 2, 2011
  4. Oct 1, 2011 #3

    That makes sense to me as you put it. V=IR implies current stays constant if resistance and voltage are proportionally changed. This makes me wonder though, What degree could I have pushed the amps to the lights before causing failures? That is, If I used 4 lamps and would thus have increased resistance but not voltage as much.... so higher current? yes? I assume this is device specific and may just shorten lifespan rather than blow them?

    3.75 Watts! okay. way more reasonable sounding.

    you lost me. but I googled it and understand it to be a rating of current drawn over time.

    -Battery specific stuff-
    "Amp-hr ratings for a car battery are figured for 20 hrs (a battery rated at 60 amp-hrs would provide 3 amps for twenty hrs before dropping to 10.5V)" Is 60 amp-hours a typical rating for car batteries? is 10.5 a magic number for batteries regarding health?

    I get that you are saying I shouldn't be concerned about discharge. But I would like to know how close I am. Mainly because I want to be able to run other things off this battery. For instance, some homemade LED decorations and maybe a small bilge pump fountain in the summer. How 'close' am I to exhausting my power supply. They way I see it in my head is that the battery is a reserve and the solar panel is the true source of power. So if I am using more than what my panel can recharge each day, I will damage the battery eventually. Is that at least a good assumption?

    According to this http://www.wavemaker.co.uk/posts/calculate-your-solar-power-needs.php 2 strands would need 180Wh per day. It also says I need 15Ah of battery power. Which jives with what you were saying. But the part that loses me is... How many Ah can I use each day? Is this back to my idea of the battery as a reserve? My solar charger will have (13 W x 6 hrs = 78 Wh of power each day) Which if I divide by 12 is 6.5 Ah of charge added to the battery each day.... but this makes me think I am taking out too much from the battery if I need 15 Ah....

    See how mixed up I am? Thanks for your reply so much. I already feel I am gaining understanding, but I still feel I need more help.
  5. Oct 2, 2011 #4


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    No, The current through a series DC circuit is the the same. You get a voltage drop across each bulb ( each resistance).

    We often see questions along similar lines ..... my radio has a plugpack thats rated at 9V 500mA. If I plug a 9V 2A plugpack into it will I damage the radio? .... NO, the radio will only "draw" the current it needs ( Because of its fixed resistance across the power supply)
    it wouldnt matter if you connected it to a 2A, a 10A or a 20A PSU, it would still only draw 500mA (0.5A) at that 9V

    The more lamps you add the dimmer they will all light up.

  6. Oct 2, 2011 #5


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    This is correct. Unless those lights are switched so that they only run a few hours a day then they will gradually deplete your battery as the daily discharge will exceed the daily recharge. Of course adding a second string of those lights will only make the situation worse.

    Depending on the conditions under which that "13 Watts" is rated your estimates of the daily charging Amp-hrs may be an over-estimate, particularly in winter.

    You would probably be best off sticking to LED light solutions unless you want to upgrade your solar panel.
  7. Oct 2, 2011 #6
    You guys rock!
    Thanks for your feedback.

    Interestingly (and concerningly) I woke up to very dim bulbs indeed. So dim I had to check to see they were even on by opening the cover. I checked the battery and it is reading 4.06 Volts.

    This makes me think there is three possibilities:
    1) The math we did is wrong and it was pulling way more (I think this is unlikely)
    2) I have a leaky wiring system (Also unlikely as I am pretty good at putting things together)
    3) The battery is actually pretty garbagety and for some reason doesn't hold its Ah like its rated to.

    How likely is option 3). And if it requires that I replace the battery to get improved performance, where and what type of battery can I get on the cheap?

    Great comment. And almost always I would only use LEDs. But these pumpkin lights were cute and only a buck each. I was debating replacing the bulbs with LEDs. but I wasn't sure I Was up to the task. Plus I am pretty sure I would need to go buy some resistors and stuff.

    Regarding the amps only drawing. Then what is the cause of failures when using too many volts? I chose to use 5 lamps because it made sense to match the volts up with the lights requirement. But I am guessing If I had used fewer lights, they would have burned brighter and at some point even popped. What is the math that causes this?
  8. Oct 2, 2011 #7
    You need more than 12.5 volts to keep a 12 volt lead acid battery charged. The actual float charge value varies slightly depending on temperature. I think it's somewhere between 13.2 and 13.8 volts.
  9. Oct 2, 2011 #8


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    Well, you did say;
    Lead acid batteries will lose capacity over time.
    And a reading of 4.06 volts is about the worst thing in the world for a lead acid battery.
    From http://batteryuniversity.com/learn/article/lead_based_batteries" [Broken]:
    10.4 volts is effectively fully discharged.
    So if you drop your voltage to 4.06 volts each day during the night, your battery will be destroyed in about 2 weeks.
    I use deep cycle marine batteries. They are about $70 for a 100 ah model. They are designed for such things. Though if your output is greater than your input, you'll destroy those also:

    It will just take 6 months instead of 2 weeks.

    I may get some argument here, but "power" ratings are generally based on a devices ability to dissipate heat. Your bulbs have an implied power rating of 3.75 watts. Since the resistance is effectively fixed, the only way you can exceed this limit is by raising the voltage. The math involved with this would be a little complicated, as it would involve the melting point of the components used in the bulbs, and the ability of the bulb to dissipate heat.
    Last edited by a moderator: May 5, 2017
  10. Oct 2, 2011 #9
    I have worked with 12 volt systems while in vehicles. And then yes, they were 13.8 V. But, with this battery sitting hooked up to trickle solar charge for probably a month unused, it was reading 12.5 volts. So I was assuming that was the charge of this system.
  11. Oct 2, 2011 #10

    jim hardy

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    "it was reading 12.5 volts. So I was assuming that was the charge of this system."

    fully charged zero current probably should be closer to 12.6
    but now we're pushing measurement limit of a 1% meter...
    and need to know the chemistry of those cells.

    page 2 of this pdf has a handy graph

    I'd charge them fully maybe with a plug in charger
    then let them cool down to room temp and settle into their 'happily charged and resting' state,
    write down voltage and temperature for future reference.

    old jim
  12. Oct 2, 2011 #11
    Thanks for the tips.

    I have lots of reading to do about batteries and cycles. Because while I can accept that it is a bad battery. I don't undrstand why this small load would drain it to 4 volts. Is this battery just losing power on it's own? If I left it unplugged from the solar source would it rapidly decharge?

    Then if the answer is new battery, since I could probably get my hands on many used batteries for free, could I hook them up in parallel to make a 12 volt system with a much higher ampere hour rating. For example. Three batteries might give me 120 Ah (that would be idealistic I guess, but you see what I am saying)
  13. Oct 2, 2011 #12


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    Your battery is actually made of 6 individual cells, each running at around 2 volts. Each of those cells has a certain capacity which changes with the life of the battery. Near the end of life, one or more of those cells will have much less capacity than the rest. Near full discharge, the good cells will be "the battery", and the dead cells will then become loads. So a voltage of 4 volts indicates that 2 of your cells reversed polarity. (2v*4 good cells -2v*2 bad cells)=4 volts.
    Yes. That would work. But batteries, like the cells in each battery, are not created equal. If one of the batteries were to develop an internal short, it would, over time, destroy the rest of your batteries.
  14. Oct 2, 2011 #13
    Whoa. I had no idea that was possible. That's freaking cool! But if those cells polarity have been reversed, I guess the battery is permanently damage and will never charge up to 12.5+
  15. Oct 2, 2011 #14
    Just checked voltage after lunch here and it is up to 10.99 V ...

    I am on the prowl for a new battery though. Brother works at a Mercedes dealership so I am betting they recycle some good enough batteries pretty frequently.
  16. Oct 2, 2011 #15


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    If you are charging it with your solar panel, then that is a good sign. If and when it gets to 12.5 volts, we can describe load/capacity checking methods.
  17. Oct 2, 2011 #16


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    What is the voltage of the solar panel system when it is not hooked to the battery?
  18. Oct 2, 2011 #17
    Since you're going to get a new battery anyway, you should probably get one that's more suited to your application. Car batteries are not designed for what you are doing. A deep cycle battery or an SLA battery would be better. Also, a battery condition monitor would be better than a cheap voltmeter.
  19. Oct 2, 2011 #18


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    Please read, and comprehend, everything I say below before doing anything else:

    Of course, if you are using something more powerful to charge your battery at the moment, there is the possibility that it will explode, if it hasn't already.

    If this is the case, do not run out to your battery and disconnect it. The battery is most likely generating large amounts of hydrogen gas, and that gas will ignite when you disconnect the leads and get a spark.

    Even if you are only using your solar panel to charge the battery, I would not disconnect the leads at the battery until after you've turned off the solar panel, and properly ventilated the area.​

    There are only so many assumptions we can make about your system without seeing it.
    For instance, if you keep your battery in some type of semi-weatherproof container(box, closet, old fish tank), it may accumulate hydrogen gas exterior to the battery.
  20. Oct 2, 2011 #19
    -to omcheeto-
    JESUS MAN! Scary.
    The battery is inside the garage, i am running the cables for everything through a crack i found in the cinder block foundation. (Old house. don't ask).
    >I have measured the solar panels voltage to be ( 17.6-18V)
    >When I test power output of battery I am still getting the 10.99V (this is with the solar panel hooked up)
    >When I test battery without solar panel hooked up I get (10.8 V)

    I am only charging via solar because i do not own a trickle charger. I may buy one at some point because it would be useful for other things as well.

    So the battery seems to max out its charge at 11V. which would mean one of the cells is still reversed? Bah. I will be replacing battery for sure. but I should ask: Can I leave it in use until I find a new one? I am hoping to let it charge up and I will just run the lights 4 hours a night until halloween.

    -To turtle-
    Agreed. But this project is meant to be free and fun. If I wanted to buy a battery, I would just buy batteries to stick in the lights. And I also have used a battery tester and before this incident, it was checking out quite well as being a healthy battery. I am hoping it still is, but my test this morning definitely showed it was weak. It thought the battery was a 6 volt system and under load it showed all red.

    =TO PF=
    This is an amazing community. So helpful. Milk and cookies to all!!!!
  21. Oct 2, 2011 #20


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    Since your battery was pretty close to zero capacity and your charger only puts out an amp, it would take ~60 hours of sunlight to fully charge it.

    The funny thing about a really badly damaged battery is that the same thing will occur when you try and charge it. The same bad cell will reverse polarity and your charging voltage will go down, when it should be going up.

    As I said, we can walk you through load/capacity checking when(if ever) your battery is ready.

    As far as I can tell, if you were right now to unhook your panel and hook up your lights, the voltage would probably drop below 10.5 volts within minutes, if not seconds.

    Do not let your voltage drop below 10.5 volts. Ever.

    ps. It's been 30 years since I've actually studied lead acid batteries, so you will have to forgive me if it takes time for some of these old memories to come unhinged from the cobwebs.
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