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Series Vs Parallel Power

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  • Series Vs Parallel Power

    I feel this question should have an obviously simple answer however my brain isn’t working well this week!! Say I have 2 x 100 Ah 12 volt batteries. I know the Watt hours contained in them are the same whether I series or parallel them but the voltage and Amperage changes, so far so good.. The thing that is confusing me is the Ah CAPACITY. To run something longer we need more capacity? But if I series the batteries I only get 100 Ah and parallel 200?

    But does voltage come into play as Well? If I have two identical 100 Watt loads but one is 24 volt and the other 12 volts would they both run for the same time if hooked up to the correct corresponding voltages using those two batteries I mentioned earlier?

    The 24 volt load has half the capacity but more voltage, the 12 volt load has double the capacity but less volts, they both have the same Watt hours available though.

    Im leaning towards they both will run the same?

    Thanks,

    Steve

  • #2
    Look at some of the stickies that Sunking has posted. He is a fountain of knowledge.

    Simply stated, voltage adds when in series and current stays the same. In parallel, current adds and voltage stays the same. I am a visual learner so I often draw a picture to help me understand a concept. Have a cup of coffee and figure out what learning process works for you. It can be impowering.

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    • #3
      Hi Steve, another Steve here.

      Originally posted by Tired sparky View Post
      But if I series the batteries I only get 100 Ah and parallel 200?
      That's right, in series you'd have 100 AH at 24V. In Parallel it would be 200 AH at 12V. Either way, it's 2400 Watt Hours, the same amount of energy.


      Originally posted by Tired sparky View Post
      But does voltage come into play as Well? If I have two identical 100 Watt loads but one is 24 volt and the other 12 volts would they both run for the same time if hooked up to the correct corresponding voltages using those two batteries I mentioned earlier?
      Yes. If the loads are resistors, the one designed for 24V would be 5.76 ohms and the one for 12V would be 1.44 ohms. Ohm's law tells you that both would see 100W and run for the same amount of time in both cases and put out the same amount of heat.

      Originally posted by Tired sparky View Post
      The 24 volt load has half the capacity but more voltage, the 12 volt load has double the capacity but less volts, they both have the same Watt hours available though.
      If by "capacity" you mean wattage, they are both the same in your example. One has more resistance, but a higher voltage across it, so it heats up the same as the one with lower resistance and lower voltage. If by "capacity" you mean something else, that's a misunderstanding, resistive loads don't have "capacity" other than the ability to dissipate heat.
      Last edited by sdold; 02-03-2020, 01:24 PM.

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      • #4
        My concern would be if you try to run both a 24V and a 12V load from the same 24V battery bank at the same time. The 12V load will drain half of your battery system making it unbalanced.

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        • #5
          Thanks sdold, that explains things. So most loads that run off of solar batteries are resistive and wattage is what counts? What about inductive loads, fans etc?

          Steve

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          • #6
            Originally posted by Tired sparky View Post
            Thanks sdold, that explains things. So most loads that run off of solar batteries are resistive and wattage is what counts? What about inductive loads, fans etc?

            Steve
            Batteries supply DC, inductive loads could be a factor if the batteries power an inverter that supplies AC to inductive loads. But here's an example with an inverter: If you have two 60W 120VAC light bulbs, one running from an inverter connected to a 12V battery, and another on an inverter connected to a 24V battery, they would both supply the same 120VAC at the same current, and both would consume 60W DC from the battery (plus a little extra for inefficiency, losses etc.). The 24V battery would supply 2.5A and its 100AH battery would last about 40 hours, the 12V 200AH battery would supply 5A and also last about 40 hours (again, that's a simplified calculation, not taking the efficiency of the inverters into account.)
            Last edited by sdold; 02-04-2020, 01:31 AM.

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            • #7
              The advantage of the higher voltage, is the amps consumed at the same wattage, is lower amps, which means less loss in the wire, and you can use smaller (cheaper) wire. Price out a 20 foot roll of #8 wire and then #4 wire. Quite the savings !!
              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

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              • #8
                200Ah battery instead of 2 x 100Ah in parallel is wiser if you use a 12v system.

                Let's say for a small cabine with 12v loads and , like charging laptop, some needs higher than 12v.

                ​​​What is wiser?

                * a 24v system and stepdown dc-dc or
                * a 12v system and stepup dc-dc or
                * a small inverter dc-ac and adaptors ac-dc.

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