Two inverters on one set of batteries??

the_uglydogthe_uglydog Registered Users Posts: 19 ✭✭
Hi, We have a 10kw, 48 volt inverter that uses external solar charge controllers but an internal utility charge controller. Our normal day load  on this system is between about 5kw and 7.5kw and our night load between 2kw and 3kw. We have 4 solar strings, with a charge controller each, providing about 12900kw total input midday in full sun. The setup allows us full operating ability from about 7AM till 4PM as it is now and the system just clips the extra power from about 9AM till 3PM. We will be replacing the existing batteries with 24, 2v 3000ah AGM batteries so the system will be able to run most or all of the night without switching back to the utility.
I don't want to push our 10k inverter to it's limit in case some extra appliances happen to be turned on at the same time, so I was thinking of adding a 4kw inverter to the battery system to power an extra 2hp a/c unit during the peak of the day which would be controlled by a water heater timer so it would use the extra power instead of wasting it. We already use the water heater timers on another system that is just for the pool pump and a/c units and it works great, no power bill on that meter at all.
Would I be able to add an inverter that just has a utility charge controller, or would it be better to get one that has no charge controller or is the whole idea nuts? 
Your input is greatly appreciated. the dog

Comments

  • mike95490mike95490 Solar Expert Posts: 9,040 ✭✭✭✭✭
    >  .... I was thinking of adding a 4kw inverter to the battery system to power an extra 2hp a/c unit
    >  ....during the peak of the day which would be controlled by a water heater timer so it would
    >  .....use the extra power instead of wasting it.

    Have you tested that a 4kw inverter can power your 2hp AC ?   I think it's borderline, when you add in the PF losses.
    Powerfab top of pole PV mount | 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 ,

  • Dave AngeliniDave Angelini Solar Expert Posts: 5,508 ✭✭✭✭✭
    edited July 27 #3
    What make of equipment? Typical to use a master / slave scenario for multiple inverters for them to sync. You wrote 12900kw ? Its that a typo?
     24 x 3000ah, is that a typo? I install/spec a similar battery 24 x 2V x 1100 ah @20 hour rate. Very few people need that much energy storage in a home. The number of people and the type of loads drive this. You numbers for storage are not bad 2 to 3 kwh, but sound low to me for what you have written.about your loads.

    This might be in the 'nuts" category but anything is doable with enough money. Give it another stab.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • the_uglydogthe_uglydog Registered Users Posts: 19 ✭✭
    Hi,  Yes our systems are large. We have our house and our corporate offices next to each other and are powering both. We don't use central a/c, so everything is either a mini split or a wall unit. That's why at night, we can use between 2-3kw and in the day the load will go up to about 7. The split inverter type a/c units save so much over the central a/c or even the standard split a/c it's amazing. The two 1hp inverter a/cs and the two inverter bar fridges use only about 1.2kw combined at night and our 1.5hp split inverter a/c uses only about .9kw sometimes as low as .6kw.
    We have 2 systems in operation now, a 4kw one and a 10kw one. We have been using the 4kw inverter to run two 1hp inverter style split a/c units, two small bar style fridges, 2 large flat TVs, a few lights and a microwave and it works fine. When everything comes on together it reaches 3.8 kw and the solar isn't powerful enough so it will switch back to utility power, but it doesn't overload. I have also tried one of our 2 hp inverter a/c units on that inverter and it works fine as well. It doesn't like the standard a/c units, but the inverters seem to use much less power and have a lower draw on startup as well. One 11/2hp standard a/c also works OK on the 4kw inverter. We will be moving the pool pump to a new system we are building and at that time we will move all loads from the 4kw inverter to the 10kw inverter so it should be able to run OK and put one of the 2hp inverter a/cs onto the 4kw system.
    Our primary inverter is an Ames Global LF Series 10kw inverter. We have 36, 360 watt panels on 4 combiner boxes with 3 strings of 3 on each box. These run to four 48 volt 60 amp charge controllers. I know this is way over powered for what we need midday but it works terrific allowing full power almost all day only clipping the power midday. We will be installing 24, 2 volt, 3000ah AGM batteries so we can make one 48 volt battery and not have imbalance issues and hopefully run the critical loads all or most of the night. We intend to reuse our present batteries on the other systems. The system I'd like to add another inverter to is the system with the 10kw Ames inverter. I know that everything should be compatible like our charge controllers, they work great, but I already have this Ames inverter and was thinking of just getting a little more out of all the extra power we have midday. I was thinking of having the extra inverter just more or less go along for the ride since the charge controllers are independent of the main inverter and are hooked direct to the batteries. I'd run that a/c on a timer as well and set the inverter to power saver and set the utility charge controller in it lower than the setting on the Ames inverter so it will mostly shut down when it's not in use,. I did that with the Ames inverter and the charge controllers, now they make a lot more power and the utility uses a lot less, that made me think about this. Maybe I'm thinking too much??
    I hope that answered the questions, if I'm not really clear, just ask.

  • the_uglydogthe_uglydog Registered Users Posts: 19 ✭✭
    12900 watts not kw! Pardon me..... My brain has stopped but my hand is still typing.............The older you get the more it happens!
  • mike95490mike95490 Solar Expert Posts: 9,040 ✭✭✭✭✭
    > We will be installing 24, 2 volt, 3000ah AGM batteries so we can make one 48 volt battery and not have imbalance issues

    suggestion.  If your layout will accommodate it, and the inverter is OK with it, install 25 batteries.  Yes, now it's a 50V bank, and you have to tweak everything up 2v, but if you ever have a cell go bad, you have the option to jumper around it, and adjust settings and keep on going while the replacement is in route.  23 batteries in a 48V system, is very close to many inverter shutdown setpoints.
    Powerfab top of pole PV mount | 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 ,

  • Dave AngeliniDave Angelini Solar Expert Posts: 5,508 ✭✭✭✭✭
    Makes more sense except why you need storage of 150 kwh? 50v x 3000ah?  75KWH @ 50% discharge?

    From your usage, 8 of the 400 AH AGM's would supply close to 10 kwh of storage. With the money you save on batteries you could buy equipment that could all be networked in an easy to see, operate, and configure system anywhere in the world. Maybe the anywhere in the world is not a big deal but that is up to you.

    The easy thing to do would be add a grid inverter/rewire some of the arrays, you really do not need the storage as your usage is low. 

    It might be good for you to list all of the requirements, current grid use monthly, the storage used now, and then list what you want to do. It might be easier to "see the light" 
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 31,080 admin
    Just for clarification... The difference between WH (kWH) and Watts(kWatts)...

    Watts is a "rate" of electrical flow. Sort of like Gallons per Hour...

    Watt*Hours (WH, kWH) is an amount... Like pumping 5 gallons per hour * 2 hours = 10 Gallons pumped

    So, your refrigerator uses something like 100-120 Watts (typical 120 VAC compressor based refrigerator) and can even use upwards of 600 Watts for a short period of time (frost free/defrost heater). That is the rate of energy usage (power).

    And there is the total amount of energy used in (for example) one day... For your refrigerator it may be:
    • 120 Watts * 0.3 duty cycle (on 18 minutes an hour, off for 42 minutes an hour) * 24 hours = 864 Watt*Hour per 24 hour period = 0.864 kWH per 24 hours
    So you size the inverter to run (and start) your refrigerator (typically around 1,200 to 1,500 Watt minimum to run a single energy star rated fridge and a few other smaller loads). i.e., 600 Watts for heating, around 600-1,000 Watts (VA) for starting.

    And you size the battery bank for energy storage. We typically start with 2 days of storage and 50% planned discharged (for flooded cell lead acid batteries). Or ~4x the daily energy usage:
    • 864 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/48 volt battery bank = 85 AH @ 48 volt battery bank capacity suggested
    There is also the ability of the battery bank to "surge" enough current to start the fridge (second check on battery bank sizing). For a 48 volt battery bank (Flooded Cell Lead Acid), that is a minimum of 100 AH to support a 1,000 Watt continuous load (both inverter and battery bank should support ~2x rated output for a few seconds to start motors/other "surging" loads)...:
    • 1,200 Watt suggested inverter (for one fridge, running inverter at capacity) * 1/1,000 ah per 100 AH capacity = 120 AH @ 48 volt battery bank (to run 1,200 Watt inverter at rated capacity/2x surge ability)
    In the above "mythical" example--Energy usage (WH/kWH) would suggest an 85 AH @ 48 volt battery bank minimum would work... However, if you use a 1,200 Watt inverter to its 1,200 Watt rating, then a ~120 AH @ 48 volts minimum battery bank capacity would be suggested to support the higher continuos current inverter rating (assuming other loads are sometimes applied besides just the fridge).

    In general, it is almost always cheaper to conserve energy than to generate it. Adding lots of insulation, LED lighting, Energy Star rated appliances, changing from (for example) a 300 Watt desktop computer left on 24 hours per day to a 30 Watt laptop that goes to sleep for 16 hours per day (when not being used)... Not only saves you the energy of running a computer, it also reduces waste heat to the home and reduces A/C cooling load.

    Of course, if you are running servers--A laptop is probably not useful for that application.

    Anyway, that is why we ask for a bit more detail... Try to address the totality of energy requirements--Not just how to store and generate XXX amount of energy/power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • the_uglydogthe_uglydog Registered Users Posts: 19 ✭✭
    Hi, Thanks for the suggestions. The electricity usage numbers I got from the charts listed on the internet and the meters on the inverters seem to match, maybe I'm reading it wrong. On our main system, the 10kw Ames inverter, 24 hours a day we use two security camera systems, two 1 hp inverter a/c units, one 11/2 hp inverter a/c unit (sometimes 2) and two full size refrigerators and two bar size inverter refrigerators. In the daytime we use three 70 inch flat screen TVs,  3 small 30 inch flat screen TVs, 4 computers, 6 ceiling fans, about 1 hour of microwave use, 2 hours of a coffee maker and about 3 hours of a 1hp well booster pump and about 5 hours of a pool pump.
    The extra battery to make 50 volts would work, I never thought of it, the system runs at about 56 volts all day anyway. I have 8 AGM 400ah batteries coming for another project, so I'll see how they work out on this first. We are in The Philippines, the electrical system here is the same as the US with two legs at 110 volts each 60hz, but the costs from the utility is crazy. Our electric bill before I got any of the solar working was over $1500. per month, now with this working the bill is down to just under $500. and that was just for the refrigerators, lights, computers, pool pump running 6 hours and a/c in the bedrooms. Except for the pool pump which I'm moving to another system I want to get these things to be able to run for 48 hours off grid. For emergencies we have a 30kw propane powered standby generator that can run everything, but we can't get bulk delivery so we have a relatively short run time, maybe 6 hours. If there's a bad typhoon or earthquake power could be out for weeks.
    So far we have not been running the a/c units in the offices or in the living areas. We have all LED lights both in house, offices, parking lot and our house security lights. Construction here is different than the US, there are no separate inside walls for the exterior, just the finished concrete and the only insulation available is that metallic sheet stuff, we put it under the roof, it helps a lot, but insulating walls isn't really possible. The daytime temperature of the exterior walls is about 90* when its about 100* outside the interior walls average about 80* at night they even out to about 73* for all. We try to have only 1 computer on all the time, it does go to sleep after 10 minutes inactivity, the others are switched off so we save a bit there and we doubled up on the windows, rather like the old fashioned storm windows in the northern US.
    We are installing a 10kw grid tie system now to run the pool pump, the a/c units in the living areas with those units controlled by water heater timers so they will only operate 9-5 or less. Grid tie is not so good here for 24 hours since they only give small credits and they run you crazy with paperwork and counterproductive regulations, so it's better to run it at zero export and only use it for daytime power. We are also installing a 12.5kw heavy duty inverter to run a 4hp well pump, two 2 hp well pumps and a dryer with priority relays to prevent overloading since most of the use is only short and only during the day, I planned to put those AGM batteries here. 
    We bought everything direct from the manufacturer and all the construction workers are already on staff anyway so we will only have about $35k invested when all is finished and we figure at the current electric rates we'll be payed back in less than 4 years.
    I was just wondering about the extra inverter added to the Ames system to add another office a/c, maybe it's not a good idea?
  • RCinFLARCinFLA Solar Expert Posts: 1,327 ✭✭✭
    edited August 23 #10
    I have four inverters tied on one battery bank.  The only thing you need is a charger control that has a maximum bulk voltage timer limit.  With multiple battery chargers tied in parallel, one or more of the chargers can drop out of bulk voltage first preventing the other chargers from ever reaching their bulk charge current termination level. This is where the time limit takes over and causes them to drop out of bulk charging mode.  Most chargers have this timeout setting for safety reasons.

    For running extras based on available PV, in addition to timers you might want to consider monitoring batteries state of charge to decide running the extras. 

    You can use a PV charge controller with load dump output control that represents battery state of charge and excess PV output availability.  You want to have a retriggable minimum ON timer triggered by PV load dump control as you don't want to immediately pull the plug on an air cond everytime a cloud goes by.

    A 4kVA inverter might run 2hp motor with a softstart/cap boost starter.  I put a MicroAir EasyStart softstarter to run my 4 ton central air cond on 11 kVA inverter.  It took the startup surge from 200 amps at 240vac to 73 amps. 

    A water pump likely already has a centrifugal switch controlled start capacitor so it would have to be modified by turning over the start capacitor's control switching to the softstarter control.  The way these softstarters work is coordinating the start capacitor boost with a soft start triac voltage ramp up.  There is a relay that bypasses the triac after startup.  The softstarter unit determines when to disconnect the start capacitor and bypass the triac.  They self train to a given motor, adjusting the ramp rate on first five startup trials then pick the best setting which is saved in flash memory of its controller.
  • the_uglydogthe_uglydog Registered Users Posts: 19 ✭✭
    Thanks for the suggestion....
  • Marc KurthMarc Kurth Solar Expert Posts: 826 ✭✭✭✭
    RCinFLA said:
    ............................................................ The only thing you need is a charger control that has a maximum bulk voltage timer limit.  With multiple battery chargers tied in parallel, one or more of the chargers can drop out of bulk voltage first preventing the other chargers from ever reaching their bulk charge current termination level. This is where the time limit takes over and causes them to drop out of bulk charging mode.  Most chargers have this timeout setting for safety reasons........................................................................



    I am unclear what you mean by "bulk charge current termination level"  and  "Most chargers have this timeout setting for safety."




    I always have more questions than answers. That's the nature of life.
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,508 ✭✭✭✭✭
    I think he is talking of the old days when devices in a system were not networked and not of the same manufacturer.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • Marc KurthMarc Kurth Solar Expert Posts: 826 ✭✭✭✭

    Well, as a service tool, I often need to charge at very high current, so I use 2-3 charging sources together in parallel to get there.  Regular, standalone 3-stage chargers. I do this for 12v, 24v, and 48v on my AGM battery banks. My approach will NOT work if you need more current than one source produces during the Absorb phase. In that case, you NEED to be networked.

    This is obviously not for you, Dave – but some folks do not know this. Here is an example of what happens:

    Chargers are set for the same Bulk and Absorb voltage but the battery bank is low on charge, so it presents a load that exceeds the combined charging capacity. Because the charger can’t get to the setpoint yet, they simply put out what voltage they can at their maximum amps – hence constant current with a gradually increasing voltage. That’s what Bulk is all about.

    The voltage keeps increasing as the battery internal resistance climbs along with SOC while the chargers are still maxed out on current. Eventually getting to a point where the voltage setpoint is reached, Generally around 80% to 85% SOC) As the charge continues, the current will begin to fall while the voltage remains at setpoint. This defines the absorption phase.

    From this point on, one of the chargers will take the lead because it hit the design voltage first, causing the others to unload. Note that they do not work together at all once the Absorb voltage is reached. But I don’t care because the bank will not accept full current anyway, so it very minimally impacts the total charging time.

    I have seen some chargers that can develop a swing because they hunt for a stable operating point due to the action of the others. So I simply make the setpoint slightly higher on ONE and slightly lower on the others. This has no impact prior to reaching the Absorption phase because the charging voltages are still way lower than the setpoint.



    I always have more questions than answers. That's the nature of life.
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,508 ✭✭✭✭✭
    XW charges 48v at 140adc. Never needed more for typical offgrid.

     I know you need to get accurate absorb V,  but for offgrid, they just stop charging at bulk V and worry about it tomorrow.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

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