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  • 3kw battery bank.

    I'm looking at getting a pv system capable of generating 3kw a day (power needed for a stand alone system). With what I've researched I've discoved that battery wiring is preferable via series instead of parallel. With this in mind I've been looking at purchasing 6x2v/1000ah batteries. Here is my reasoning.

    -3kw per day needed.
    -2 days of autonomy preferred.
    -6kw for 2 days (x2) working on 50% discharge.
    -12kw for 2 days (divided by number of volts)
    -12kw divided by 12v = 1000 (min a/h battery required)

    keeping in mind that the 2 days of autonomy will not be consecutive (e.g.: 2 days use/charge/2 days use/charge etc).

    i will also be having a 2kva gen for those times of need.

    This system would also be fitted to a 30' houseboat so space is limited. I am aware that the battery bank will be extremely heavy (>300kg) and pv array may take up as much space as 20m sq (lucky I have a top deck for the pv array.

    would i be correct in assuming that the 6x2v 1000 a/h battery bank wound be sufficient for the storage of the 3kw/2 day autonomy?

    any feedback would be appreciated.

    cheers.

  • #2
    Originally posted by Floating down here View Post
    3kw a day
    That is a meaningless term; do you mean 3kw HOURs a day? Bruce Roe

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    • #3
      Ahh yes I do. 3kwh per day. ??

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      • #4
        Ignore the ??. My "thumbs up" icon is obviously not compatible.

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        • #5
          We have 4400 watts of Solar panels and a 48VDC / 600AH battery bank [twenty-four EXIDE marine deep cycle batteries [27MDCST] in 6 strings with 4 batteries in each string].
          4400w, Midnite Classic 150 charge-controller.

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          • #6
            It is not just days of autonomy that is important. Another important factor is the maximum load that you will need to supply. We went for a 4kW inverter with the reasoning that it will supply one Australian standard 16A 240V circuit which means that I can run any device that will run on a 16A circuit, like a welder, electric oven etc. etc. What size inverter were you planning to use?

            I am no expert on FLA batteries but my understanding is that to run a 4kW inverter you would need a standard deep cycle FLA battery with a capacity of around 30kWh-40kWh (~3,000Ah@12V)

            I have similar power requirements to you and can get by with a much smaller LFP (Lithium Iron Phosphate) battery of around 10kWh. This battery gives me three days of autonomy as I can use 90% of the capacity of the battery. There are a number of suppliers of full turnkey lithium ion based systems in Australia or depending on your level of technical knowledge you can purchase the various parts and put together a system yourself.

            What are the reasons for choosing 12V? It maybe easier and cheaper to go for a higher voltage like 24V or even 48V.

            Simon

            Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
            BMS - Homemade Battery logger github.com/simat/BatteryMonitor/wiki
            Latronics 4kW Inverter, homemade MPPT controller
            Off-Grid LFP(LiFePO4) system since April 2013

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            • #7
              Thanks karrak for your prompt reply. I'm not going to even start to claim that I have a great deal of knowledge regarding pv systems. I'm kind of doing Ad-hoc learning. The greatest single power demand will be a coffee machine at approx 1.2kw followed closely by a small microwave at say 800w. I think the biggest inverter I would need would be a 2kw or maybe even a 1.5kw.

              as for the battery bank I guess I am a bit fixated on 12v for no real reason (guess it's to do with the unknown territory of 24/48v). I think I may have to do some more research into this and LFP batteries.

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              • #8
                DO NOT try to run a 4KW inverter off a 12V battery. It's not a battery issue, it's a safety, wiring & connection issue ( 350A @ 12V ).
                If you need 4KW, you need to look at a 48V system, not 12V
                Even 24v would be a better, lower current density solution. (only 500 ah batteries needed)

                Here's another thought. Build the bank out of 6V, 200ah golf cart batteries (24 or 48v bank) - only as a 1 year test. You will learn how the system works, and then when the bugs are ironed out, look at replacing the lead acid with Lithium batteries. BUT DON'T jump directly to Li batteries as a first step.
                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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                • #9
                  If you must use an electric coffee maker look for one with an insulated carafe or buy a separate one Once the coffee maker is done brewing you don't need a hot plate draining your batteries.
                  If you have some 12 volt lighting, water pumps and electronics aboard and you do have space available you can run a separate small 12 volt system. Works great for me.
                  If not done already replace all incandescent lighting with LED lighting. Stay away from anything in the "cool white" range.
                  Consider AGM batteries. Fullriver makes some quality deep cycle AGM batteries. Careful with telecom float service batteries, they aren't built for deep cycling.
                  2.2kw Suntech mono, Classic 200, NEW Trace SW4024

                  Comment


                  • #10
                    Originally posted by Floating down here View Post
                    As for the battery bank I guess I am a bit fixated on 12v for no real reason (guess it's to do with the unknown territory of 24/48v). I think I may have to do some more research into this and LFP batteries.
                    It means you are stuck inside a 12 volt box and do not know you are asking for fire and death. To run a 4 KW Inverter is going to be a 48 volt batt minimum voltage. Last thing you want to do is listen to Karrak about anything. He will get you killed or injured.

                    When you ask for a 4 Kw Inverter there are some minimum requirement that have to be met.

                    1. 48 volt minimum battery voltage. If you use a hybrid golf cart batteries, you would need 8 x 6-Volt 225 AH batteries and that is pushing it. At full 4000 watts you would only be able to operate a few minutes befor voltage sag becomes a problem. We are talking about a $1200, 500 pound battery. You cannot go smaller than that with a 4 Kw Inverter.

                    2. Minimum Panel Wattage to charge those batteries is going to be 1500 watts with a 30 amp MPTT controller. Another $2000 down the drain.

                    3. This part you will really like. You get to replace those expensive batteries every few years.

                    4. Do not complain about the price or anything. You have more money than you know what to do with. You get what you ask for. You want solar power, then money is no object.
                    MSEE, PE

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                    • #11
                      Originally posted by Sunking View Post

                      ... you are asking for fire and death... You want solar power, then money is no object.
                      I haven't been here long, but when I start reading a post like this, even before my eyes dart to the left to confirm it, I already know who wrote it!

                      Comment


                      • #12
                        Originally posted by kb58 View Post
                        I haven't been here long, but when I start reading a post like this, even before my eyes dart to the left to confirm it, I already know who wrote it!
                        And confirmed by the information that followed in the rest of the post that might be useful and/or something to think about or scrap as readers think fit.

                        Comment


                        • #13
                          Originally posted by kb58 View Post
                          I haven't been here long, but when I start reading a post like this, even before my eyes dart to the left to confirm it, I already know who wrote it!
                          I do not pull punches or lie to you like many here will do. I am telling you a 12 volt 4Kw inverter will cause a fire, not if, but when. You are talking 400 amps. You do not have the tooling, skills, or experience to work with 400 amps.

                          Secondly is cost. If you want to take something off the grid means money is no object.

                          That is just two simple facts that cannot be denied. Most knowledgeable uses here do not want you to know that, and the rest are just clueless about the risk and cost. I actually care more about your money and safety than you do. I could care less if I hurt your feelings speaking truth.

                          MSEE, PE

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                          • #14
                            I don't know why you all think I'm after a 4Kw inverter, I never stated that. I did state that my requirements were 3kwh a day and my single highest drain in around 1.2kw in the form of a coffee machine so as I mentioned before, a 2kw inverter should be sufficient.

                            because this is all for a houseboat the few electronics are 12v (lights, toilet, pumps, windlass etc). Fridge, water heater and oven are LPG.

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                            • #15
                              sorry, I co-mingled Simon's 4K inverter & 16A outlet to your requirements.

                              2Kw inverter, that helps quite a bit. i'd ditch the coffee maker and use the stove top. My wife's vice is electric toaster.
                              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

                              Comment

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