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  • Voltage changes in battery array question

    HI all -

    Newbie here. I purchased an off grid summer place two years ago that had a full solar system installed and new batteries. It's worked well but I still have a lack of understanding of what's actually happening in regards to storage and usage from day to day. The setup is:

    12 Trojan 105RE 6v batteries - The house has both 12V and 120V fixtures so I'm assuming it's wired in series to make six 12V batteries (the batteries are rated as 225ah).
    Trace Engineering DR2412 Inverter
    Blue Sky Solar Boost 3024i charge controller
    Solar Panels but I have no idea what their specs are.. I think it's 4 panels based on the dimensions but honestly I'm not sure and there was no documentation about the panels that I could find.

    So my question is around battery voltage and knowing when they're getting low. I've had a general rule of trying not to let the charge controller go below 12.2 volts and if it starts getting around that low I"ll run the generator to charge the batteries. The house had a 240V line from the generator to the system so it's simple to charge things back up safely.

    I notice as dusk is approaching (and/or the panels start going into the shade of late afternoon) that the charge controller will say be in the 13.x volts all day (sometimes all the way up to 14.4 if it's perfect sunny day) and then as soon as even approaches it drops to 12.7 or 12.6. What I don't understand is why there's that almost immediate drop but then I can be on the computer, have lights running, TV going, etc and the voltage never goes below 12.4. I mean there's never a heavy load in the evening but it seems like the voltage holds a lot longer as it approaches 12 volts vs when it's up high from the solar panels it drops as soon as the sun goes away.

    One other question somewhat related is can I just leave the inverter on all the time? The previous owner had suggested that he turned it off every night as he didn't need it for anything but I wasn't sure if that is increasing or decreasing the life of the inverter. Either way I'm contemplating doing an upgrade in the next few years in order to get an electric refrigerator as the propane refrigerator is pretty ancient and getting propane tanks to my location is non-trivial.

    Thanks for any help!
    Mike
    Last edited by earthtrip; 07-01-2017, 08:53 AM.

  • #2
    12.7 Volts is a typical "Resting Voltage" for a battery at 100% State-Of-Charge.
    When the Charge Controller is ON then the battery voltage will be above 12.7 Volts
    When the Charge Controller is OFF then the battery voltage will drop to 12.7 Volts and continue to decrease as discharged.

    13.5 Volts is Float mode
    14.8 Volts is Absorb Mode

    Typical resting voltages ...
    12.7 v = 100%
    12.5 v = 90%
    12.4 v = 80%
    12.3 v = 70%
    12.2 v = 60%
    12.1 v = 50%
    Last edited by NEOH; 07-01-2017, 10:25 AM.

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    • #3
      Stateofchargepercentagechart.jpg Voltage will always be higher when charging
      2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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      • #4

        Thanks guys - this is exactly what I was looking for! So based on the color chart I'm assuming green is OK in terms of not potentially damaging the batteries?

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        • #5
          As long as you can recharge them right away MOST of the green is safe. The thing is, the deeper you discharge them the shorter their life will be. Under brief heavy loads your voltage will sag, but should recover. The only way to trust voltage as a state of charge indicator is to allow the batteries to "rest" for hours, with no load or charging at all.
          2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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