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  • Lead Acid SOC

    Hello. Can someone please assist me?

    I am testing a small setup and will hopefully one day scale it up but i seem to have an issue.

    I have one 20 watt panel and one 12v 12ah sealed lead acid battery. At full charge pulling 13 watts ac via a 160 watt inverter i barely get 2 hrs from my battery. (should get aleast double that @50%)

    I only take the battery down to 12.10 volts closed circuit for fear of discharging too much, should it go further than this? I have been getting conflicting State of Charge data for SLA batteries online. Some site say 50% is 12.06v some say 12.30. Can someone please help?

  • #2
    The only time SOC is applicable is on a battery that is disconnected and rested for a few hours. Under load the battery SOC will read low. As for what voltage depends on if the batteries are Flooded, AGM, or Gel and what kind of plates they have of being either lead calcium or lead antimony. However it is a moot point because SOC is pretty much worthless on a working system.

    Not sure of your location but sun hours are low right now. If you are using a PWM controller the charge efficiency is only 50%. So if all you are getting is 3 Sun Hours in a day with a 20 watt panel, all the panels can generate is 30 watt watt hours usable in a day. Secondly you claim you are only drawing 13 watts. Is that 13 watts AC from the inverter, or 13 watts from the battery. If I had to guess I would say 13 watts AC through the inverter. Depending on the efficiency of the inverter means you could be drawing as much as 50 watts DC from the battery which means what you are seeing is normal.
    MSEE, PE

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    • #3
      Lead Acid SOC

      Thanks for your reply. The battery is an AGM SLA. I am using a cheapo charge controller, dont think its PWM, it doesnt say. The 13 watt ac light is connect to the 160w inverter. I will get a clamp amp meter and test the real amp pull when its on. Im in Toronto, Canada we dont have much sun right now but in the summer we will get more than a few hrs/day.

      Thanks,

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      • #4
        Originally posted by Greenplasticbag View Post
        Thanks for your reply. The battery is an AGM SLA. I am using a cheapo charge controller, dont think its PWM, it doesnt say. The 13 watt ac light is connect to the 160w inverter. I will get a clamp amp meter and test the real amp pull when its on. Im in Toronto, Canada we dont have much sun right now but in the summer we will get more than a few hrs/day.

        Thanks,
        Your CFL is pulling 13 watts of true power, but it is probably pulling 20 or more Volt-Amps (VA). That should not require more power from the battery, but some inverters may be wasting power trying to deliver the VA needed.
        Also add in the idle current (zero load) of the inverter, which is unknown.
        A DC clamp ammeter will tell you, but be sure that what you get does DC clamp measurements rather than just DC amps via leads and AC-only via clamp. A DC clamp is going to cost you at least $50. Search the forum ("DC clamp site:www.solarpaneltalk.com" on Google) for some brand suggestions.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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        • #5
          Originally posted by Greenplasticbag View Post
          I am using a cheapo charge controller, dont think its PWM, it doesnt say.
          Then that confirms it is a PWM controller. MPPT is expensive, and none are made for that low of a wattage panel.
          MSEE, PE

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