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  • Using a UPS as part of an RV solar system

    I have a small RV. 21 foot, Toyota Sunrader. I also have a supply of used APC Smart-UPS 750 battery backups sitting around. My company has decided when one "goes bad", we replace the entire unit rather than the replaceable battery. A terrible waste of money, IMO, but that decision is above my paygrade.

    I did a little research on line to see if anyone had repurposed these things as inverters and of course there are hundreds of youtube videos showing it has been done! Is there anything that hasn't been documented on yt yet? So, my plan is to use this UPS as an onboard inverter/charger. I would also like to incorporate a modest solar setup. 100-200 watts maybe?

    I think this will work, but there is one thing that complicates it a bit. This UPS uses a 24V battery supply. No problem, string together a pair of group 27 RV batteries, right? That should supply modest AC output. My desire is to be able to run a small 600 watt microwave for a few minutes at a time and charge electronic devices. The tricky part is how do you gain the ability to charge these from the truck 12V charging system. You can get step up DC converters pretty cheaply, but I wonder how efficient they are? And how about the solar side? Can a small PV array charge a 24V bank? My last concern is powering the coach 12V devices. This isn't much of a demand, lights, water pump, fridge (when not on lp). Am I going to have any battery issues if I power this stuff from one of these batteries? I considered splitting it up between the two, but then you have the issue of grounds at 2 different potentials, not a good thing.

    Of course, before actually deploying this, I plan on doing fairly extensive test runs with it all sitting on a bench in my basement. No point in burning down my RV. I kind of like it.

  • #2
    Originally posted by pete c View Post
    I have a small RV. 21 foot, Toyota Sunrader. I also have a supply of used APC Smart-UPS 750 battery backups sitting around. My company has decided when one "goes bad", we replace the entire unit rather than the replaceable battery. A terrible waste of money, IMO, but that decision is above my paygrade.
    No waste of money at all, you cannot mix old and new batteries together. If you do you turn your new battery into an old battery that needs replaced with the rest of the old ones.

    I am from the telecom sector and we figured out a way to save a ton of money. The Employment Prevention Agency make companies track their batteries from cradle to grave. It cost them a lot of money to dispose of batteries properly. So they figured out a way around that. They give them away or sell them cheap to someone and makes it their problem.

    One last point, those batteries are what we call Station Batteries. They are designed to be used in Emergency only. They only have 50 to 150 cycles in them. They are worthless for Deep Cycle purposes. A good deep cycle battery should have at least 1000 cycles. UPS batteries are designed to be discharged in 15 minutes and only have a short cycle life.

    Originally posted by pete c View Post
    I did a little research on line to see if anyone had repurposed these things as inverters and of course there are hundreds of youtube videos showing it has been done! Is there anything that hasn't been documented on yt yet? So, my plan is to use this UPS as an onboard inverter/charger. I would also like to incorporate a modest solar setup. 100-200 watts maybe?

    I think this will work, but there is one thing that complicates it a bit. This UPS uses a 24V battery supply. No problem, string together a pair of group 27 RV batteries, right? That should supply modest AC output. My desire is to be able to run a small 600 watt microwave for a few minutes at a time and charge electronic devices. The tricky part is how do you gain the ability to charge these from the truck 12V charging system. You can get step up DC converters pretty cheaply, but I wonder how efficient they are? And how about the solar side? Can a small PV array charge a 24V bank? My last concern is powering the coach 12V devices. This isn't much of a demand, lights, water pump, fridge (when not on lp). Am I going to have any battery issues if I power this stuff from one of these batteries? I considered splitting it up between the two, but then you have the issue of grounds at 2 different potentials, not a good thing.

    Of course, before actually deploying this, I plan on doing fairly extensive test runs with it all sitting on a bench in my basement. No point in burning down my RV. I kind of like it.
    As for the rest you are going about it wrong. If you want to use the vehicle alternator to do the charging is standard practice in an RV, and special purpose vehicles. Real simple you use 12 volt battery and an Electronic Battery Isolator. The Isolator will charge both your vehicle SLI battery, and House batteries.

    UPS Inverters are not designed to be efficient, as that is not their purpose. They are not made to be ran at low power levels efficiently. They are made to run hard and fast, just long enough to get the generators up and running. Efficiency has no priority in the design.

    You can certainly do what you want. To run a 1000 watt Inverter you are going to need a pair hybrid batteries aka 6-volt Golf Cart battery of 225 to 250 AH. You can use solar to provide some charging. A good fit is a Grid Tied 200 watt panel with a 15 amp MPPT charge controller. The real muscle is the battery Isolator. A 50 amp Isolator will generate more power in 1 hour than a 200 watt panel will generate in 2 to 5 days.

    If you size the battery correctly, say 3 to 5 days run time, and only use the system for short outings for a couple of days, you can do away with the solar panels. All they really do is extend your run time, and keep the batteries charged up while parked at home ready to go. The isolator will recharge the battery quickly between stops, on the way to camp, and on the trip home. And if you have cloudy days or on an extended stay, the isolator saves your but, just start the engine and recharge.

    If all you do is run a 600 watt MW oven say 15 minutes per day, and charge up cell phones and an occasional laptop (200 to 250 Wh/day), a pair of 225 AH golf cart batteries will last you 5 to 7 days without needing to be recharged. The only reason the batteries need to be that large is to run a 1000 watt Inverter so you can run the MW oven without the Inverter tripping off-line from under voltage from the batteries sagging.
    Last edited by Sunking; 06-06-2017, 05:18 PM.
    MSEE, PE

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    • #3
      UPS systems have small internal trickle chargers, little endurance capacity (no big heat sink or cooling fan) and the batteries are generally much too small to last more than an hour. If you can make that all work, you have a system. Adding larger batteries means they take longer to recharge
      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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