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12 Volt Solar Charger Outputs 20 Volts, need help understanding this.

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  • 12 Volt Solar Charger Outputs 20 Volts, need help understanding this.

    Hello everyone, great forum.

    I have recently set up a solar system on my RV. It works great. It utilizes a 30 watt solar panel that feeds a solar charger. Here is where I'm stuck. The solar charger gets an input of about 22 volts on a sunny day. Furthermore, in a open circuit test the output is also about 22 volts. My question is, why would you charge a 12 volt battery with 22 volts? Could somebody give me a hand and explain how this works and why it is done this way? Also, does this mean I could charge the battery with a 22 volt power supply? I've always thought a 12 volt battery must be charged with 12 volts.

    Any help would be greatly appreciated.

  • #2
    Well you are under the impression a 12 volt battery is 12 volts.

    That is only part of the equation. A 12 volt battery is only 12 volts when there is no charge applied to it. A fully charged 12 volt battery depending on the exact chemistry at rest will measure 12.5 to 13 volts...

    However to charge a battery using a 2 or 3 stage charging algorithm can require up to 16 volts in order to push the current into the battery.

    All 12 volt solar panels are actually 16 to 18 volts at Vmp, and around 20 to 22 Voc. So if you are measuring 22 volts at the input, that means you are not charging as there is no current flowing.

    Depending on the type of controller you have, mainly PWM or any series type controller, when the battery is discharged you can measure as low as 11 volts at the input of the charge controller which means you are wasting a lot in heat loss which is the nature of any series type controller. Typical loss is a minimum 30% and up to 50% loss.
    MSEE, PE

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    • #3
      Not to worry too much. A 30W panel is so small, it likely can't damage a 100A RV battery.
      It's likely just barely enough to keep it charged while the RV is parked.

      It is NOT large enough to recharge the RV battery when you use the RV, you will have to rely on something else to recharge after your use.

      More worrysome is the possibility that the charge controller is broken, and will eventually drain your battery.

      Connect everything up in the sunlight, and use a digital volt meter, and see if the solar is able to raise the battery voltage after an hour.
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

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