UPS as an inverter / charger for off grid system

arghhharghhh Registered Users Posts: 18
I am curious to the viability to using a 240V split phase UPS as the meat and potatoes of an off grid system.

I have an Eaton 9155 15kVA UPS (http://powerquality.eaton.com/K41511000000000.aspx?CX=3&TAASPEC=1) that can do 240 in / out and is 7.5kVA for each 120V leg. The DC side of it is 192V nominal or 16 12 volt batteries in series. I have 16 of the UPS batteries, they don't list the AH but I think it's around 90AH at 192V. I found a data sheet (http://www.industronic.com.mx/fichas%20tecnicas/Eaton%2012V%20390W.pdf) but it is mostly rated for high rate discharges.

So what I am curious about is is there a charge controller than can do upwards of 216V (96 cells * 2.25 VPC charge)?

I know the UPS can run forever on batteries as long as you don't hit the 1.67 VPC cutoff and has about a 50A battery charger on it. It can also take generator input which will feed the load and recharge.

Am I crazy here or do you guys think this might work?

I might need more battery, I think the math only works out to 37kWH of batteries with the one string. 50% DoD gives me about 18.5kHW and I figure 7KW per day of use x inverter efficiency of .92 = 7.6kHW/day so about 2.5 days without charging.


Some other info. I have a 10KW duetz diesel genset I can use to feed the UPS and recharge the batteries.

Comments

  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: UPS as an inverter / charger for off grid system

    What you describe is almost exactly what I have - genset, converted the battery bank to flooded lead acid, UPS. Two downsides to using it 24/7 off-grid (I only use mine as a whole-house backup). First, the built-in charger is designed for a smaller battery bank so it charges very slowly, way below the recommended 5-10%. You would have no choice but to add an external charger, I live with mine since it usually only gets partially discharged. Second, you may get a higher efficiency from another inverter. UPSs are designed to just hold the load until a genset can power up so ultra efficiency isn't usually their strong point.

    Safety is another thing - 192 volts DC nominal, which is really 220+ on the charger, can be quite dangerous. To work around this safely I designed my battery bank to be separated into 4 battery boxes with 8 batteries going in one direction, they make a U-turn in the 4th box, then they go back to the start. This way the largest voltage I'm exposed to is 48 volts when I open up the last box to disconnect the farthest cable. Then I only work with one box open at a time. Once that cable is disconnected I'm only exposed to 24 volts in any box, at most 48 if I were to accidentally drop something that would connect all 4 batteries in that box in series. If I didn't do this my first battery box would have the full 192 (really 220) volts on the end 2 terminals.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: UPS as an inverter / charger for off grid system

    I remembered another issue, UPSs are designed to charge sealed lead-acid batteries, which charge and float at a lower voltage than flooded batteries. I compensate for this by charging each battery with a good 3-stage charger a couple of times a year. Not perfect, but I live with it. Another argument for getting a good external charger.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • arghhharghhh Registered Users Posts: 18
    Re: UPS as an inverter / charger for off grid system

    Interesting to see someone has already done exactly what I was planning. The UPS I have is rated for up to 14 strings of 3.4AH batteries with user selectable settings for number of strings. When you set the number of strings it uses that number times 3.4A per string for the charger. So my max charge rate for my batteries is 39A so if I set the UPS for 10 strings I should max out at 34ish A at 220V. The batteries I currently have are actual UPS batteries and are sealed.

    I really like your idea to segment the string into separate boxes, although these batteries don't have the maintenance but when they age and I replace them I would probably go with wet cells if I can't get a good deal on more UPS batteries.

    This UPS is the type where the load is always on inverter and the specs claim about 92%.

    How did you hook up the AC side? Does the UPS feed the whole house panel or do you have a subpanel for it's loads?
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: UPS as an inverter / charger for off grid system

    Mine is an online (sometimes called "dual-conversion") UPS like yours. We get frequent brownouts as well as blackouts so while I hate the hit I take by running it all the time its the price I pay to keep most of my stuff on a clean supply. Keep in mind that yours will peak at 92% but probably is lower through most of its power curve. If you can get a copy of the curve from the manufacturer it will help you decide to keep it or not.

    Also your UPS is awfully large to run 24/7 off-grid. The internal useage by the system will be far higher than keeping something like a 500 or 1000 watt inverter powered up. Bottom line, you'll be spending a lot more on PV just to keep your inverter running (not even powering anything) when you would do better spending that money on a smaller inverter for day-to-day use. Most of the time a house needs less than 500 watts, the readout on mine usually says I'm using 200-300 watts 75% of the time. Keep the big guy for occasional big loads. One problem with this is you'll have to reconfigure your battery bank to run each one... 192 volts vs. 24 or 48 volts.

    Another possible problem - many of those mid-level server room UPSs can not start from DC alone. They need the AC source to power up and do system checks. Often its a BIG load during that startup as it tests the inverter and power factor correction. You might luck out and be able to start it up using your smaller inverter, the guy on http://www.priups.com was able to do that with his. But yours is big enough you may need to power up your genset temporarily just to power up the UPS.

    I guarantee when you price out new UPS batteries vs. flooded lead-acid you won't have to think twice. My bank cost me around $1300 and more than trippled my capacity compared with the relatively large bank that came with it (128 "bricks"). Buying those same 128 sealed batts again would have cost 3 times as much.

    I added a subpanel. My UPS came with a wall-mounted bypass switch in a small cabinet, with all the big 50-amp twist-locks needed. I added a 50 amp twist-lock outlet below my genset panel which the bypass plugs into, then the subpanel and UPS plug in to the bypass. If I had to I could take the UPS and bypass away and plug the subpanel directly into the outlet coming from the genset panel, not that I will need to since I have the bypass. I moved some of the loads from the genset panel and some of the loads that were still in my service panels to the subpanel. I've ended up putting about 90% of my 120 volt loads on the subpanel and could go 100%, I just don't have a need to power non-essential things like my attic lights or garage opener through the UPS (or the genset) so they are still on the service panel. There are a few 120 volt loads only on the genset, like my microwave and some of my outside lighting. Stuff I'll need at some point when the utility is down but not all the time so no reason to put it on the UPS even though I could. My well pump is on the genset panel, and I have yet another subpanel which is really a small manual transfer switch + panel which holds the breakers for my electric water heater and one of my two A/C units. That gets fed either from the utility panel or the genset panel. This does 2 things. First, if I'm away and the power goes out, I'm not wasting fuel keeping water warm and air cool. When I get home I can manually switch one or the other over to the genset (it can't support both at the same time along with the well pump). Second reason is this keeps these big loads off of the circuit that powers most of the rest of my stuff. So they don't cause a voltage sag when they come on, causing problems. Obviously during an outage when I move them over to the genset I just live with that possibility.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: UPS as an inverter / charger for off grid system

    And yes, I have a lot of panels. Two service panels, one 60 amp manual transfer switch/panel, the big automatic transfer switch/panel for the genset, and the subpanel for the UPS.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • RobertMfromLIRobertMfromLI Solar Expert Posts: 34
    Re: UPS as an inverter / charger for off grid system

    We were considering the same thing with a FERRUPS FE10 for this application.

    There are other UPS units that will do far better, if you can find one. We sadly missed out on getting one that was being disposed of ("if you can cart it off, you can have it") - no place at the time to store an inverter that large. Someplace I have the model... they were designed for a slew of battery cabinets for large telco/NOC/building backups, but they are NOT cost effective unless you can get them for "cart it away and it's yours". They are easily high 5 figures or well into the 6 figure range.

    The advantage is they will do all the line filtering, handle power surges, dips, frequency stabilization and so on - and many (like the high end APC or FERRUPS lines) will do so without a hit on the battery except for massive drops in current. Comes in handy when powered off a generator that might not produce the best voltage/frequency output when loads change.
  • arghhharghhh Registered Users Posts: 18
    Re: UPS as an inverter / charger for off grid system
    techntrek wrote: »

    Also your UPS is awfully large to run 24/7 off-grid. The internal useage by the system will be far higher than keeping something like a 500 or 1000 watt inverter powered up. Bottom line, you'll be spending a lot more on PV just to keep your inverter running (not even powering anything) when you would do better spending that money on a smaller inverter for day-to-day use. Most of the time a house needs less than 500 watts, the readout on mine usually says I'm using 200-300 watts 75% of the time.

    Another possible problem - many of those mid-level server room UPSs can not start from DC alone.

    I guarantee when you price out new UPS batteries vs. flooded lead-acid you won't have to think twice. My bank cost me around $1300 and more than trippled my capacity compared with the relatively large bank that came with it (128 "bricks"). Buying those same 128 sealed batts again would have cost 3 times as much.

    I did think of the inverter losses, on this unit you are right they are about 700 watts just to run the thing. It would be more suited to a whole house UPS.

    This one does have DC start feature, although I have not tested it.

    Oh I know the cost of UPS batteries are through the roof, I got mine 6 months used from a data center that upgraded to bigger ones for scrap price, some 26 batteries for $100. It is good to have a friend who works with industrial batteries. The price was right so I figured I could scrap something together as the inverter and batteries are both big ticket items.
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: UPS as an inverter / charger for off grid system
    ...are NOT cost effective unless you can get them for "cart it away and it's yours". They are easily high 5 figures or well into the 6 figure range.

    The advantage is they will do all the line filtering, handle power surges, dips, frequency stabilization and so on - and many (like the high end APC or FERRUPS lines) will do so without a hit on the battery except for massive drops in current. Comes in handy when powered off a generator that might not produce the best voltage/frequency output when loads change.

    I got ours for $200, one of those rare true Ebay deals. Not bad for a unit that cost around $10,000 new. Its 11 years old now and has around 65000 hours on its timer so I don't expect it to last forever, but when it dies I can resell the bypass and the 50 amp twist-locks built into the unit and break even. If I tear out the multi-tap transformer I might even get ahead. 8)

    Ours does very well on the dirty output from my Generac. I had to adjust the slew rate on the UPS so it could keep up with the frequency swings w/o throwing a constant alarm. In the future when I switch to a mainstream inverter I'll re-create a similar "online" system using an AC charger that will accept a wide voltage range plus has power factor correction, and run the inverter 24/7. The only thing it won't do that my current setup will is supplement from battery in very low voltage situations, but I've never seen voltage that low so I'm not going to worry about it.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • RobertMfromLIRobertMfromLI Solar Expert Posts: 34
    Re: UPS as an inverter / charger for off grid system
    arghhh wrote: »
    This one does have DC start feature, although I have not tested it.

    Very interestingly, I've found quite a few where AC power is supposedly needed to start them - but not really. Various APC units I've had simply require hitting the "Test" button, which when starting the test and noticing a load, turn the UPS online. I had 3 medium sized BestUPS units (about the output of my SmartUPS 1400RM2 units, but with MUCH larger batteries and 10 times the runtime)... they supposedly would either turn on with AC current, or (without AC hooked up) when a certain load was exceeded and one attempted to turn them on - with no load (and no AC power), they wouldn't do anything at all.
  • arghhharghhh Registered Users Posts: 18
    Re: UPS as an inverter / charger for off grid system
    Very interestingly, I've found quite a few where AC power is supposedly needed to start them - but not really. Various APC units I've had simply require hitting the "Test" button, which when starting the test and noticing a load, turn the UPS online. I had 3 medium sized BestUPS units (about the output of my SmartUPS 1400RM2 units, but with MUCH larger batteries and 10 times the runtime)... they supposedly would either turn on with AC current, or (without AC hooked up) when a certain load was exceeded and one attempted to turn them on - with no load (and no AC power), they wouldn't do anything at all.

    The biggest issue of battery start is if you can get the internal DC up from lower battery voltage to start the inverter. Apparently for this one you just have to provide DC power by turning on the breaker and hitting start UPS from the screen. The caveat is that the units last start has to be from AC so if you do a DC start and shutdown you can't restart without AC. I think it has to do with the autosense of phase angle and voltage on the input to match the output.


    100/200, 110/220, 120/240 VAC 180 degree phase displacement
    120/208, 127/220 VAC 120 degree phase displacement

    Although you don't have to have the same input as the output but you can't go to bypass if you have them different.
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