Lead Acid Battery Ah and resistance question

Discussion in 'The Projects Forum' started by russpatterson, Sep 8, 2011.

  1. russpatterson

    Thread Starter Member

    Feb 1, 2010
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    Hello, I recently ran a test charging a 35aH AGM battery with two 12 watt solar panels connected in parallel. While the panels put out 750 - 800 mA at 20V open circuit (each) I only got about 700 mA charge current at about 12.8V on the battery (with both connected). Also the battery did not seem to charge very well. After two days (one very cloudy, the other very sunny) the battery only ran for about 20 minutes (at a 1.3 Amp load) before voltage dropped below 12V.

    So my question: is the resistance of the large battery too much for the small panels to overcome and charge sufficiently? Would I be better off with a smaller battery?

    Also, I assume that when the battery drops below 12V under load I should disconnect? (even though the voltage pops back to 12.8 once the load is disconnected).

    This is going to run some security lights that I want to run for 5 hours every night, then for 5 minutes at a time after triggered by a motion sensor. I've read the articles at Battery University http://batteryuniversity.com/ but did not find info about battery size and resistance.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    How old is the battery, and how well has it been maintained?

    You should not allow the battery to fall below 12v, or it will have a short life.

    If your battery has been in service for awhile, and you've been discharging it below 12v, it may be sulfated. I've seen charge rates for AGMs being up to C/3, so that would be up to ~11.67A in your battery's case. Batteries start sulfating when they drop below ~12.5v. As always, you should consult your manufacturer's datasheet for your specific battery model for their recommendations. Failure to follow their recommendations will result in poor performance and short lifespan.

    To operate your lights for 5 hours with 80% charge remaining requires a 32.5AH battery, so yours seems to be sized conservatively. Are you monitoring the battery voltage at the terminals, or is there a length of wiring between it and the monitoring point? If so, what gauge wire are you using?
     
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  3. russpatterson

    Thread Starter Member

    Feb 1, 2010
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    The battery is brand newhttp://tinyurl.com/power-sonic-35Ah. So sulfation should not be an issue.

    My meter is connected directly to the terminals, the probe leads are decent quality. I'm using 16 gauge wire for all the connections to the panels, controller, and battery.

    I expected better performance with the two 12 watt panels. I tried connecting the panels directly to the battery and got the same charge current so it's not the controller circuit.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Do you mean the panel puts out 750-800mA when shorted? There's no current with an open circuit. Overcoming the emf of the battery reduces current a LOT compared to a short, so your results - getting roughly half of shorted current at more than half of open-circuit voltage - are not surprising. You're probably on the expected current vs. voltage curve.

    You would not be better off with a smaller battery. Bigger is always better except when buying it.
     
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  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Low power panels usually have bogus ratings on power output. The real output under normal conditions might be 50% lower like you have. You need panels that can bulk charge at about 5-15% of the total battery capacity to do a proper recharge cycle with panels on a daily basis. A 60W to 80W (STC) panel is needed to keep that battery recharged if the daily depth of discharge is greater than a few percent of the total capacity.

    http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf
     
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  6. russpatterson

    Thread Starter Member

    Feb 1, 2010
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    Excellent link to that paper. Thanks! So this brings up an idea that occurred to me when reading on the Battery University site about how the batteries worked best being used for short periods (e.g. running a sump pump, on for 30-60 seconds, sit idle, voltage comes back up, etc.). For a lighting system this isn't an option, unless you had multiple batteries and could pass the load between them.

    What if I bought two smaller batteries (instead of one 35Ah, two 20Ah batteries). They would be able to be charged by a smaller panel and then probably be happier about the 5 hour discharge if I switched between them (say every 10 seconds or 10 minutes, whatever turns out to be best). I think that the extra expense on two batteries would be about a break even on the savings for the smaller panel.

    I would just charge battery 1 fully, then let it sit at float charge while I charge the second battery. It's just a slightly more complicated circuit and code to support the multiple batteries. Thoughts?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    You'd be better off to power the lights directly, leaving the battery out of it. You only get back 75%-80% of the power you put into charging the battery. Why add an unnecessary overhead?
     
  8. russpatterson

    Thread Starter Member

    Feb 1, 2010
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    The lights need to run at night, so I need a battery(s) as part of the system. My first project, the solar garden fountain, was much easier as the battery was only there as a big capacitor, to smooth out the changes in power and get the pump motor started.
     
  9. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    A (realistic) PV design rule of thumb is you only get about 50% of the power from the panels out from the battery on a daily basis. 10Ah of usage from a 35Ah battery should be ok but you need to put back 20Ah daily. With an avg of 5 solar hours per day that's about a 4A charging current. People underestimate the amount of panel it takes to generate reliable power off-grid. Investing in a larger panel (100W+) with the current market oversupply might be a good idea if you plan on other solar powered projects that can be run from a central battery bank.

    Solar is 'Green' because it eats money.
     
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