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12V system w/ 2000W inverter and 1200AH battery bank. Why is this bad?

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  • 12V system w/ 2000W inverter and 1200AH battery bank. Why is this bad?

    My first thread probably should have been my second. So I'm making my second my first.

    After reading suggestions here & elsewhere, I'm considering revamping my planned 12V system in favor of 24. But no matter what angle I view the change from, it seems to only make things much more difficult. I don't want to be 'that guy' who ignores good advice, but I'd really like to understand the 'why' behind the advice to go 24 over 1000W, particularly as it pertains to my original plan and my particular use-case (RV 'from scratch' conversion with lots of 12V needs).

    What I had originally envisioned was:

    6 x 1200AH, 2V AGM batteries (single string in series)
    Magnum MS2012 12V 2000W inverter / charger
    4 x 300W, 36V panels (2 on each controller, each pair paralleled to minimize shading issues)
    2 x 50W MPPT controllers
    4/0 battery interconnects, 4/0 cabling from bank to inverter (5' max), 250A DC breaker between the two, plus a 300A fallback fuse.
    2 parallel-capable 2000-ish watt generators (for cloudy-day charging, power tools, short-term AC use, etc)
    An upgraded alternator + battery isolator to provide a third battery-bank charging source.

    In order to meet my storage requirement w/ 24 volt, I'd end up using 8 x 400AH 6V batteries (2 strings of 4 each), and while I'd then only need one controller, the 2 extra batteries cost way more than the savings of a single controller (and weigh ~200lbs more, which is a major consideration for me). With 24V, I'd be using #1 AWG vs #4/0 (neither of which I could terminate myself anyway), and while there would be some cost diff, it still wouldn't be enough to make up for the extra battery cost. Same with the DC breaker/fusing. 24V w/ 8 batteries is still more costly than 12V w/ 6. And then there's the cost (in money & efficiency) of a 24/12 converter, and/or hard(er) to find 24V lighting/appliances. I'm just not seeing 24V being more economical. For me, it's less.

    95% of the time, I will not be drawing anywhere close to 2000W. The peak load with pretty much everything I've planned on running, all operating at once, is 80A. The microwave is the one thing I designed for that would temporarily push this system from 1000W to 2000 territory. And we won't be using that much if at all.


    Assuming I use only quality cabling built by businesses with the proper equipment to do so (not me), and I have an electrician either build and/or inspect my load center / wiring (which I would also inspect on a regular basis), and I keep my wire runs code-compliant (more than code compliant, actually), can my case be an exception to the rule? If so, what should I do to make my 12V system as safe as a comparable 24V equivalent. And if not - why not?

  • #2
    The single greatest drawback to a 2000W 12V system is that the 200A current required through the battery bank is just too high and dangerous for the typical casual user. It is hard to get fuses or CBs for that current that will handle DC. It is hard to get and work with large enough wiring for 200A DC, given that the effects of voltage drop will be very large as a fraction of the 12V nominal voltage.
    If you say that is OK because I will not ever draw more than 80A DC, then you need to design your wiring and overcurrent protection for 80-100A and the 2000W inverter will then draw more idling current than a 1000W inverter would. As long as you can handle any turn-on surge from the microwave (probably small at worst) you would still be OK. The microwave is a relatively efficient way to cook from a limited power system.
    Your determination to use appropriately manufactured 200A cables will certainly help increase system safety. You need to have suitable 200A overcurrent protection right at the battery terminals, or you will be building a DC welder and fire starter.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.


    • #3
      Hey magicbus,

      This is a little off topic, but FWIW - I have a very good condition, 3 year old Xantrex 3012 3000 Watt Inverter/Charger with the SCP remote panel I can make you a good deal on if you go the 12 volt route.
      I also have a Xantrex TRUECHARGE 2 60 AMP Battery Charger with remote panel.


      • #4
        It's because at some point, the wiring infrastructure is too costly and inefficient and yep - unsafe when you take 12v too far. They learned this the hard way at the turn of the last century.

        Voltage drop - means you have to have HUGE gauge cable to pull that much current without dropping voltage over even short runs. Not to mention that if the infrastructure switching melts together, your wiring is the "fuse" as it melts into the RV.

        Even if that weren't bad enough, ever see how inefficient a 2Kw ac inverter is when you just use it to charge cellphones? Just idling, it is gobbling power running in the totally inefficient range - most ac inverters like to run well into their power band. Much like taking your car onto the freeway and never leaving first gear.

        Leave the 12v stuff to the "camping" or totally frugal emergency use. For real-life, one needs to go higher voltage.

        Basically I think you need to rethink your microwave. That could be the tipping point for this project. If you don't absolutely need microwave popcorn or instant coffee, you might get away with 12v from other cooking ideas.
        Last edited by PNjunction; 05-14-2019, 02:09 AM.


        • #5
          Thanks so much for your input, folks.

          I have rethought the microwave. It was the tipping point, and taking it out pretty much got me into 'safe' territory. And saved me $$$. And saved me weight. I can power it and other high-draw items (like a blender for margaritas... a must have says the wife) off a main panel circuit on generator/shore power.

          However, all the integrated inverter/chargers in the 1000W range have what I believe to be (please correct me if I'm wrong) anemic chargers for my battery bank (6 x 1200AH 2V AGM). What would be the best solution to seamlessly charge the bank while on shore/generator power using the inverter's AC pass-through?

          Finally, now that I've got the inverter sizing in the 'good' range, is there any problem I'm unaware of regarding my choice of 2 x 50A charge controllers and 4 x 300W panels?

          Thanks so much for all your help and advice, everyone!


          • #6
            As a motorcoach owner with a small 100w of solar, 4-6v FLA batteries at 215ahr each, and a Magnum ME2012 Inverter with built-in battery Charger, with AGS and a Onan 6kw 50amp generator, there is also another element to the Inverter's 'silent usage' which may can assist you - the SEARCH WATTS feature.

            Magnum's Search Watts feature in the Inverter allows you to provide a setting between 5watts and 50watts to 'wake up' the Inverter whenever a 120v device, item, or appliance 'needs' the power, staying 'asleep' when not. This has worked really well for us and our 120v Refrigerator when we want to have the fridge stay cool during temporary storage, between trips. Rather than having to empty the fridge, and if several days until the next trip, I turn on the Search Watts and let it handle the fridge only when the fridge's compressor requires to be run, typically on the 45-50watts setting. This saves some amps during the period when no solar is available, and overnight, of course.

            The Search Watts feature is basically providing a very small amount of power thru the system, just enough to 'sense' when a device is asking for power, and then wakes up to full 120v output. Your microwave may show a very 'faint' sign of life, if it has the typical digital clock type interface... just enough so that if you hit the 'add 30 seconds' button it should light up and start the cooking, etc. If you have other smaller draw devices, such as phone chargers, they may not require enough wattage to set off the Inverter, therefore it will continue to 'search', until enough watts have been reached to trigger it fully on.

            just a thought : )


            • #7
              Charging a 1200AH 12V AGM bank
              A desired charge rate would be 120A but 120A @ 14V is going to be hard to come by, that would be pulling 1680w from an outlet, with perfect efficiency.
              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



              • #8
                Okay I'm going to stick my neck out here and hope I don't get my head completely bitten off...
                Magicbus - In my mind, the following questions are related to your topic, but I'm not trying to hijack your thread just tell me to jump off if you want.

                So here it goes - Our company does custom work on a lot of large vehicles that are very similar to RVs. There are clinical buses, dental buses, Mammography buses, entertainer RVs, experiential marketing trailers, etc. Most all of them run Xantrex 2000 and 3000 watt 12 volt inverter systems. The majority run stacking setups. All the ones that I'm aware of will produce 240 volts AC at 60hz.
                The x-ray equipment is crazy sensitive and they require these large pure sinewave inverters to operate. They won't even do screenings on shore power. It all HAS to run through these inverters... and they are all 12 volt.
                These 12 volt inverter systems run x-ray equipment, lighting, slide-outs, microwaves, multiple computers, air conditioners, etc.
                They all have large battery banks of more than 900ahs. Some approaching 2000ahs.
                They do have generators and use AGS. But the generators are not always running. Most of these are designed to run 4 to 6 hours on battery power. The generators fire up when and if the batteries get low or to assist in large equipment startup assist modes.
                A lot of this equipment is over 10 years old. One that was in here a while back was a 2003 model with a large 12 volt system on it. We never see issues of melted wires or melted fuses or any of the scary stuff that I've been reading about.

                I get the HIGH amperage created when trying to run large appliances on low voltage vs higher voltage systems. I understand the necessary precautions for breakers/fuses etc. and the resistance created from improperly crimped terminals and/or loose terminals and the potential dangers. I've read the stickies and they have been very informative and eye opening...

                Most of these 12 volt inverters, as I'm learning, are crazy sophisticated. As an example, NCmountainsOffgrid in the post above refered to "search mode" that will run one of those inverters at a little more than what a LED light bulb uses. when the inverter is on, it uses less than 30 watts. In "search mode", I believe it uses like only 7 or 8 watts. Theses inverters also have built in battery chargers that can be programmed to charge almost any kind of battery chemistry. These Inverters are expensive - $1600 to $1900. But not crazy prohibitive from what I've seen when it comes to 3000 watt off-grid inverters.

                I've explored the costs of the cables and if the inverter is within 48" or less of the battery bank, it's not like the cost is hundreds of dollars more... A lot of these vehicles have inverters and batteries in the same compartment. They are sealed batteries of course.
                So, assuming proper cable size, professionally terminated cables and proper fuses and breakers - Does the concern for 12 volt systems [B]primarily revolve around safety and the DIY'er and their lack of knowledge?[/B] ( I totally fall into that category if you couldn't tell...)
                Is the RV industry just behind? With newer tech, should they be looking at higher voltage systems? None of these large 12 volt systems are connected in anyway to the engine charging system (other than with the flip of a switch, you can start the engine on the "coach" batteries if the engine batteries were to die...)

                For what it's worth, I don't work for Xantrex or any of these companies. That just happens to be the 12 volt equipment that we see the most.
                Also, I want to make it clear that I don't doubt any of the knowledgeable people on here. I'm sure there are several of you that have forgotten more about this topic than I'll ever know... I'm just trying to learn...
                Last edited by Ky Speedracer; 05-14-2019, 03:21 PM.


                • #9
                  The issues revolve around cheap gear and naive users that can't comprehend why fuses are needed, 5KW inverters with #2ga wire internally, and no easy way for neophyte users to terminate cables with a solid hydraulic crimp good for 100's of amps.

                  It's just not easy to accomplish if you are not a professional in the field. Just because you can bolt a starter motor into a car and not have it burn up, does not translate to a 2Kw 12V system.
                  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



                  • #10
                    When there are good reasons to force fit a high power system into a 12 Volt world, such as maintaining compatibility with a vehicle alternator for charging, and avoiding exposing the vehicle techs who live in a 12V world to higher voltages, it certainly can be done. And from your description the appropriate engineering and construction to make it work was done by professionals.
                    That does not invalidate our advice that for a stationary system, not tied to compatibility with a vehicle, both the economic and the safety aspects favor going to a higher voltage system when that power level is needed.
                    A commercial UPS system for a full computer facility or other large load is likely to use a battery voltage close to or higher than the nominal AC voltage being supplied. It just makes sense to do it that way. For the typical "home" user, however, there are significant advantages to keeping the nominal DC voltage no higher than 50V.
                    And finally, it may seem that there is a much greater choice of equipment (inverters, charge controllers, etc.) available for 12V systems than for 24V or higher. But that appearance can be misleading. I would say, rather, that a company which produces well designed and engineered, reliable, equipment for 12V will likely also offer higher voltage options.
                    SunnyBoy 3000 US, 18 BP Solar 175B panels.


                    • #11
                      Thanks to everyone again for their input. Based on your comments, I've given up on the 12V idea for good. Going with 24V and a 2000W inverter/charger.

                      I understand the safety issues, and the rationale in turning newbies away from high-current shenanigans. However, not everyone who's new to solar is new to electrical work. An electrician I am not, but I understand how to make and maintain solid clean connections, how to determine the correct wire gauge, how length/temperature/conduit impact component selection, how to terminate cables (as well as knowing when you don't have the capability to do so), how to run cabling safely, etc. I also know how to research things I don't know (in an OCD sort of way), and I did plan on enlisting the assistance of a licensed electrician to - at a bare minimum - look over everything I had done and make sure it was code compliant. Hence the reason for this post.

                      Regardless, I realize that even if I did do everything 'right' (and though I'm sure I would it's still an 'if'), all the remaining arguments you've presented in favor of 24V remain viable. I really wanted to be able to charge off the alternator, but now that I've spent some time pondering the subject, I don't think it's necessary, and doing so would likely present another set of problems, particularly with the size bank I planned.

                      Now that I've looked harder at what's available (and what's not), I was wrong regarding the economics. My new 'plan' w/ 24V has saved me $$, is more capable (microwave is back in the game! Plus a margarita blender!), and has the ability to grow if I find either my new battery bank (now 600-800Ah with 2 strings of 6V AGMs) insufficient or end up wanting to run more than 4 panels.

                      So - THANK YOU ALL for saving me from myself I learned something valuable from every post in this thread.