When to shutdown light to avoid battery depletion?

Discussion in 'General Electronics Chat' started by spinnaker, Dec 26, 2012.

  1. spinnaker

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    This is in reference to my solar light controller here

    http://forum.allaboutcircuits.com/showthread.php?t=77021

    I have the ability to shut off the light once the battery level reaches a voltage that is selectable.

    I am currently shutting the light off when the battery reaches 12V. But of course when I remove the load of the battery the level rises above 12V. But the design of the controller keeps the light off as it should. The process is reset once charging starts.

    The other night I noticed that the light was off so I checked on it. It shutdown at 11.8V (the lcd shows the shutoff voltage) but the battery voltage was 12.1V (because there was no load).

    I understand that lead acid batteries should bot go below 12V? If so, is that loaded or unloaded? In other words could I safely set my level to say 11.5V?
     
  2. nsaspook

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    I normally drain my solar battery bank to 80% to 50% SOC before recharging.

    This is a pretty good article about using only voltage for SOC measurements with known loads. http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

    So depending on the load 11.5V could be anywhere from 0% SOC to 70% depending on the load. With a very light load 11.5V means a flat battery.
     
  3. spinnaker

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    The light consumes <500ma. I would assume this is a light load? So can I safely go to say 11.8?
     
  4. nsaspook

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    What the load is depends on the Ah capacity of the battery. Look at the discharge chart curves for rates C/3 - C/100 of the battery. C would be the battery capacity.
     
  5. spinnaker

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    Sorry but I am confused as to how to find the AH of the battery in those charts.

    The article says:

    The data presented here on the graphs was generated
    from our set of Trojan L-16W deep cycle lead-acid
    batteries. Each Trojan L-16 battery is composed of
    three series connected, 350 Ampere-hour, lead-acid


    My battery is a 12V 22AH SLA. I have only one.
     
  6. nsaspook

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    You have 22Ah battery for the C, your load is .5Ah

    22/.5 =C/44

    That's a pretty light load so 11.8V would be a battery at about 20%, 11.5 would be just about dead.
     
  7. spinnaker

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    So if I understand correctly, I should actually be shutting my light off at a much higher voltage (if I were to stick with your 80% rule)? Maybe 12.6V?

    If that is true the light would never be on for long.

    And if I understand correctly, the lighter my load the more battery capacity I need to allow lower voltage levels?
     
  8. spinnaker

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    No wait to allow the voltage to drop down further I need a lower C/X number so a smaller battery?
     
  9. THE_RB

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    A lead acid battery is damaged if the voltage gets below 12.5v or so, and is left there for any length of time.

    It's better to use a large battery and kepe it >12.6v at all times if you can.
     
  10. spinnaker

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    Wow 12.5? So I am really pushing it at 12V then. OK I will up the voltage. Thanks.
    I am going to have to look at it at the end of a nice sunny day to see what the charged voltage gets too.

    But I guess I have to do the math on the panel. It is a 60W panel.

    Maybe the replacement battery should be even larger than 22ah? It is only a 500 ma load.

    The original light that we purchased had a 6AH battery and the light would be on till the battery ran out or the sun came up. Sort of explains why we went through so many batteries.
     
  11. nsaspook

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    As long as your solar panel can fully recharge the battery every few days you can take the SOC down to 40% without any problems with hard sulfation. SLA batteries are limited in the number of charge cycles, the deeper the discharge the fewer recharge cycles the battery will have. My 80% SOC rule is mainly to maximize battery cycle life and still have a energy reserve for emergencies in larger banks.

    Lets say your panel can provide 4 amps for 4 hours daily on a weekly average. Being conservative and using a 50% efficiency factor you can use 8Ah daily for the load at night and still keep the battery fully charged.

    Your battery capacity needs to be matched to the amount of recharge capacity you have and the load. A 60W panel, a 22Ah battery and .5Ah load seems to be a pretty good balance for long term battery life.

    I would set the low voltage point at 12.0V with your load to limit discharges to about 25% SOC. That will maximize lighting duration at the expense of some battery life.
     
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