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Utter Newbie Needs Help Understanding Battery Bank

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  • Utter Newbie Needs Help Understanding Battery Bank

    We inherited a small off-grid solar system. 4 x full-size panels (unsure of wattage), 8 x Trojan T-105 6V batteries (2 strings of 4), 12V bank, small charge controller, Whistler 2000 watt inverter. The batteries are brand new. We also have a 6000W propane generator to charge the batteries and provide cabin power when required.

    How long should it take to recharge the bank from the generator, assuming the batteries are at 50% discharge?

    Also, how do I know the maximum wattage device I can run from the bank? We tried plugging in a 1300W instant pot but it wouldn't work and the inverter yelled at us with an alarm.

  • #2
    I am not sure what your battery system is. You said you had 8 x 6V batteries wired in 2 strings of 4. That will yield a 24v system not a 12v bank.

    Based on a typical T 105 rated at about 225Ah your system is approximately a 24V 450Ah which will require about 40 to 45 amps to charge.

    A MPPT type CC would need to be rated at least 45amps and you would require 1000 to 1100 watts of panels wired for a 24v battery bank.

    It is hard to say how long a generator will take to charge the batteries without knowing the type of charger and it's amp rating.

    As for the inverter not liking the 1300w load. My guess is the battery system gets pulled down below the low voltage limit which will cause an alarm and the inverter to shut off.

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    • #3
      Thanks for the feedback. It's definitely a 12-volt bank, so I must be wrong about it being two strings of 4. What I know is that there are two rows of batteries, 4 on the left and 4 on the right. On the left-side batteries, the positives are connected together. On the right-side batteries, the negs are connected together. And each right-side battery is connected to a left-side battery, pos to neg.

      We have a voltmeter on the wall of the cabin which reads 12.6 when the bank is fully charged. When the generator is running, it reads up to 14.5. During the day when the panels are gathering light, it might read 12.8 or higher. At night we don't use much battery power, just enough to keep the cell phone signal booster running, the satellite receiver, and charge our phones. By morning, the voltmeter reads 12.1 or 12.0. It seems like it should last a lot longer, and it should support a 1300W crock pot.

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      • #4
        14.5 volts is a charging voltage
        12.5 is a full battery

        12.8v indicates a problem with the solar, either not enough to charge the batteries to 14.5, or bad controller or panels
        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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        • #5
          4 strings is not really too cool. My ? is what size are the cables on the batteries and between batteries and inverter? They could be too small to carry surge current to the inverter. With all those connections you could have one or more lose and/or bad connections.
          2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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