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Matching Panels and Li-ion Batteries

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  • Matching Panels and Li-ion Batteries

    Hello,

    I am currently trying to design a small solar system. I was planning to use a 18 volt, 5 watt panel to charge two 12 volt, 5aH LiFePo4 batteries. However, I recently read that a 12 volt panel is actually capable of putting out 18 volts under more ideal conditions. I'm in Toronto so I won't always be getting ideal conditions, but this got me wondering if I need such a high voltage panel. I'll be using two of these batteries: http://shop.relionbattery.com/collec...oducts/12v-5ah. I haven't settled on a panel yet.

    So my questions are:

    Do I need such a high voltage panel to charge these batteries and what may be the lowest I could safely get away with?
    What are some considerations to take when choosing a charge controller?

    Thanks everyone, looking forward to hearing from you.

    Matt

  • #2
    Matt

    Please do not continue to post the same question multiple times. New posts with links need to be approved by the Mod so please have patience.

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    • #3

      The 5 watt panel seems a bit small to charge these batteries. It is only under ideal conditions that you will get 5 watts from the panel, around 3.5 watts is a more reasonable figure. Each of those batteries stores around 13*5=65Wh (battery voltage*charge capacity) of energy. The 5 watt panel will take 65/3.5=18.6 hours (battery energy stored/power from panel) to charge each battery from 0% to 100%. This doesn't take into account power losses in the charge controller and battery which could be up to 25% if you use a PWM charge controller with a solar panel that has a maximum power output at around 18 volts or around 5% if you use an MPPT charge controller.

      You need at least 14 volts from the solar panel to charge the battery plus whatever extra voltage the charge controller needs which should be stated in the charge controllers specifications.

      Simon
      Off-Grid LFP(LiFePO4) system since April 2013

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      • #4
        Originally posted by MattDal View Post
        Hello,

        I am currently trying to design a small solar system. I was planning to use a 18 volt, 5 watt panel to charge two 12 volt, 5aH LiFePo4 batteries. However, I recently read that a 12 volt panel is actually capable of putting out 18 volts under more ideal conditions. I'm in Toronto so I won't always be getting ideal conditions, but this got me wondering if I need such a high voltage panel.
        Quit thinking, it is getting you into trouble. You can use a 600 volt panel to charge a 12 volt battery no problem at all. You just need the right Charge Controller.

        For a 12 volt 5 AH battery the minimum panel wattage you should be looking at is 10 watts and even that is a bit low. Ideally 20 to 100 watts for LFP. At 5 watts if you discharged the battery could take a over a full week or two to recharge. At 100 watts a day. At 20 watts 3 to 4 days. Take your pick.
        MSEE, PE

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