Has anyone thought of using capacitors between the inverter and battery? Would this work

MikeH
MikeH Registered Users Posts: 3
in Solar Beginners Corner #1
Hello folks,

For the record I have no Idea what I'm doing:

I am setting up a tiny system for my oxygen concentrator to run on. It requires 940watts of 120volt AC operation to run properly 16 hours a day.

I have the inverter and half of the batteries purchased so far. (As far as I know)

How many amp hours would be required for my total battery capacity for up to 7 days of no input to batteries?

What size solar panels in watts would be the correct amount for Upper Michigan to keep the system charged and running?

I also realize that up here the sun alone would not do it.

What size in watts would you recommend of a wind generator to go with this solar panels power this system for viability.

Has anyone ever tried using a car audio style multi farad capacitor in line between the battery supply and the inverter for a more constant flow to the inverter?

Would this help or hurt and why?

Thanks for any help you can give me, it is much more windy than sunny up here.

12 volt system. In June purchasing solar then, in July purchasing wind generator not made of money and what wire guage from battery to battery and battery to

inverter should I use. ( most efficient)
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #2
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    MikeH wrote: »
    Hello folks,
    I am setting up a tiny system for my oxygen concentrator to run on. It requires 940watts of 120volt AC operation to run properly 16 hours a day, for up to 7 days of no input to batteries?
    12 volt system.

    WOW! Sorry, but the news is not good. "Tiny" it will not be! 940 watts for 16 hours a day with 7 day backup? That's roughly 16 Kwh per day, a huge and very expensive system for which 12 volts will be out of the question. Even building a 48 volt system to supply this much power will not be easy. This is just to prepare you for the more detailed responses to come. :cry:
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    Lots of people have thought of using capacitors on inverter DC input. It doesn't do any good because that's not how capacitors work. They don't produce power, they just 'borrow' it. There already are all the capacitors the inverter needs built in to the inverter.

    Unlike a car audio system there's no alternator running to make up the 'borrowed' power to the capacitor. In effect adding such to an inverter system simply adds more load on the batteries.

    Batteries have much, much higher capacitance than capacitors do. If you size them right for the expected load there is no problem. if you don't, no amount of jerry-rigging will correct the deficit.

    Wayne's right about that power demand: 940 Watts * 16 hours is 15kW hours. That's as much as my on-grid house uses in a day, including electric hot water.

    I think the first thing you want to do is get a Kill-A-Watt meter and plug the oxygen concentrator into it and see how much power it really consumes. I know they are power hogs, but I expect that 940 Watts is a peak rating and not a continuous draw.

    To actually supply 16kW hours per day you'd need a minimum 760 Amp hours @ 48 Volts, and that would be at 50% DOD with no reserve for cloudy days. You'd be looking at a minimum 4.8 kW array too. That's like $5,000 just for the array.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #4
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    Is this to "save money", emergency backup, or to live off the grid?

    In general for emergency backup, you would be better off getting a Honda eu2000i genset for ~$1,000 and burn ~2-3 gallons of gasoline per day for anything but full time off grid system.

    If you are trying to save money--Unless your utility bill is in the $1-$2+ per kWH range (approximately 10x higher than most utility billing), the only way you can possibly save money is with a Grid Tied system (solar panels + GT inverter)--And even then it depends on your utility and state/local laws (not all utilities allow GT solar, not all billing plans save money for the end customer).

    For California, usually there are some price breaks for medical power usage.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #5
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    In cars capacitors are used to make up a shortfall in amp to battery cabling, and to provide punch for surges the amp requires. It isnt that the battery cant provide that punch, just that its too far away, on too small cables.

    The rule for inverters is very near to bank, and very fat cables, so... problem solved.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #6
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    This is going to be expensive and difficult. And because its for medical life support, you cannot "jury rig" it, which might work. You can size the batteries for 2 days of reserve, and start your generator on the beginning of day 2. That will let you down size a lot of it into a 5 gallon gas can.
    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 ,

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  • verdigo
    verdigo Solar Expert Posts: 428 ✭✭
    #7
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    I plugged my Dad's oxygen concentrator to a Kill-O-Watt for a 12 hour period. It used 4.42 kwh. The thing runs 24/7 so 8.82 kwh for a 24 hour period. I'll let the experts sort that out for you. Pretty big draw, but not as much as you are describing. Get a Kill O Watt meter and measure how much power it actually uses. If it is really using 16 KWH a day I would shop for another one. There are battery operated portable units that I am sure will do better.
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #8
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    MikeH wrote: »
    How many amp hours would be required for my total battery capacity for up to 7 days of no input to batteries?
    About 175 kilowatt-hours of storage. In terms of batteries, about 250 GC2 golf cart batteries.
    What size solar panels in watts would be the correct amount for Upper Michigan to keep the system charged and running?
    About 15,000 watts to run through the winter.
    What size in watts would you recommend of a wind generator to go with this solar panels power this system for viability.
    Don't bother, unless you are willing to spend a lot of money for big machine and a very tall tower.
    Has anyone ever tried using a car audio style multi farad capacitor in line between the battery supply and the inverter for a more constant flow to the inverter?
    Sure, but that only helps with surge loads, not steady state operation.
    12 volt system. In June purchasing solar then, in July purchasing wind generator not made of money and what wire guage from battery to battery and battery to
    To be realistic you'd be talking about a 10kW turbine with a ~120 foot tower. Figure about $50,000 if you have a good site for it. Add another $20,000 for the panels and say $35,000 for the batteries.
    inverter should I use. ( most efficient)
    Outback line of inverters have a great reputation.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #9
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    MikeH wrote: »
    How many amp hours would be required for my total battery capacity for up to 7 days of no input to batteries?

    How often do you want to replace batteries ?
    Since this looks to be a backup usage only install, you will likely have the batteries die from old age, before they reach the max # of cycles for extra deep (-80%) discharges. But if you plan to take them this low for a week, after day 2 or 3, the sulfate crystals start to harden, and the cells loose capacity as more sulfur gets locked up in the hardened crystals (the 2nd way a battery dies).
    After a couple of these long, deep cycles, you will likely see some capacity loss
    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 ,

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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #10
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    bill von novak wrote: »

    To be realistic you'd be talking about a 10kW turbine with a ~120 watt tower. .

    I totally agree with every other comment "bill" has made in his detailed post, except this one which has got to be a misprint. I'm sure Bill meant a ~120 FOOT tower. (not a ~120 watt tower) Those of us familiar with such things instantly recognize the error, others new to things off grid could become rather confused.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #11
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    Thank you Wayne... I fixed the post.

    -Bill B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • MikeH
    MikeH Registered Users Posts: 3
    #12
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    Thanks I appreciate your resopnse. I will look into Outback Inverter pricing. I guess two days would be better than none. This Oxygen concentrator cools the O2 down to liquid and Phillips told me it consumes 940 watts, due to my heavy liter flowrate liquid is the only way to go. How many farads of surge capacitors should I put in line between the battery and inverter to make a difference? Just trying to offset the electrical costs a little,@ $50,000.00 I don't think I'll be around for any ROI. What wire size would you use between the batteries, and the inverter. Of the smaller (non-120 foot tower) wind generators who makes a quality unit that won't preform a walletectomy on me? Thanks for your help
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  • MikeH
    MikeH Registered Users Posts: 3
    #13
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    thanks! I will get a killowatt and see exactly
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    MikeH wrote: »
    How many farads of surge capacitors should I put in line between the battery and inverter to make a difference?

    None, because it won't make any difference.

    If you want to off-set the electric costs look into grid-tie. Off-grid electric invariably costs much more than utility power. But there is the chance that a GT system may be viable.
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #15
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    MikeH wrote: »
    How many farads of surge capacitors should I put in line between the battery and inverter to make a difference?

    Energy (in watt-seconds) is equal to 1/2 C V^2. So to figure it out:

    Figure how much voltage sag you are OK with. Let's assume 13 to 11 volts is acceptable during startup.
    Figure out the surge. Let's say 2000 watts startup surge.
    Figure out how long the surge lasts. Let's say 1 second. So you need 2000 watt-seconds.

    Now let's run the math. A common Maxwell ultracap pack is 58 farads (about $140 from Digi-Key.) From 13 to 11 volts you get (4901-3509)=1392 watt-seconds. So you would need two capacitor packs for a total of 116 farads.

    So that would certainly help with surge and will last almost forever. However, you also might want to put that same money towards more battery, which will help with surge AND total energy,
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #16
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    bill von novak wrote: »
    So that would certainly help with surge and will last almost forever. However, you also might want to put that same money towards more battery, which will help with surge AND total energy,

    Yup.
    It's called "doing it right" instead of "doing a cob-job".
    Unlike an automotive system with a running alternator the capacitors in this case have no place to borrow the power from except the battery. Result = zero actual improvement.
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  • rice81
    rice81 Registered Users Posts: 25 ✭
    #17
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    How about a low-tech, low cost, no maintenance solution? A big ol' O2 cylinder as backup? An M cylinder is the 'big one' that is used on welding rigs and holds over 7000 liters, pressurized. Depending on the typical O2 demand, one cylinder will last 0.5 to one week. Never goes bad or needs replacement and needs no electricity. What if the O2 concentrator broke? This would cover that as well.

    And you can buy welding O2 to save $. Same oxygen source, just less rigorous liability and chain of custody of tanks. Buy your own tank and label it and ask for it to be refilled and tell them you want your own tank back. Then you will have a 'medical oxygen' tank. Medical tank rental is around $0.25 per day plus all sorts of delivery, hazmat, fuel charges, shop fees and more other nonsense charges than you can imagine. Buy your own and take and pick up your own tank.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #18
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    Bill Von Novac. There is slight problem with your cap calculations.. Super caps lose about 30% of their capacity in first 6 months. When you attempt to use them to stop sag they are no where near as useful as using a bigger battery/ies.. And far more expensive..
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #19
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    rice81 wrote: »
    And you can buy welding O2 to save $. Same oxygen source, just less rigorous liability and chain of custody of tanks. Buy your own tank and label it and ask for it to be refilled and tell them you want your own tank back. Then you will have a 'medical oxygen' tank. Medical tank rental is around $0.25 per day plus all sorts of delivery, hazmat, fuel charges, shop fees and more other nonsense charges than you can imagine. Buy your own and take and pick up your own tank.

    They may sound like nonsense to you but they are there for a reason. Cylinders used in industrial applications frequently come back from the field contaminated because they are allowed to run completely out of gas and get backfilled with other gases. I used to work in the specialty gas industry; once an O2 cylinder came back to us loaded up with phosgene. Very nasty stuff. That cylinder could never be used for human consumption O2 no matter how many times it was filled and purged. Industrial O2, no problem. PPM level contaminants can get into cylinders in the fill process as well if they are not handled appropriately for human consumption use.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #20
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    Cariboocoot wrote: »
    Yup.
    It's called "doing it right" instead of "doing a cob-job".
    Unlike an automotive system with a running alternator the capacitors in this case have no place to borrow the power from except the battery. Result = zero actual improvement.

    However, if you have a relatively high internal resistance in the battery AND the load is pulsed rather than continuous, adding a modest amount of capacitance can reduce I2R losses and reduce voltage sag to the inverter during the current pulse.

    Sort of like GTI with net metering can "improve" the output of a PV panel array even though it has nowhere else to get the power from except what you pay for.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #21
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    Battery resistance goes up when battery is charged.
    No gain from adding caps.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #22
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    Cariboocoot wrote: »
    Battery resistance goes up when battery is charged.
    No gain from adding caps.


    Work out the numbers for a pulsed DC load (at a 120Hz rate or higher) and any particular battery resistance and you will see that you can get lower losses to battery resistance by spreading out the current flow. The 2 in I2R is important.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #23
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    There are of course no capacitors inside your inverter. :p
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #24
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    Cariboocoot wrote: »
    There are of course no capacitors inside your inverter. :p

    NONE?? NOT EVEN ONE LITTLE TINY INSIGNIFICANT MINISCULE ONE? WAAA. that not good. it woulde be an in capacitated inverter without at least one...
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #25
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    Cariboocoot wrote: »
    There are of course no capacitors inside your inverter. :p

    The XW line of inverters has a "capacitor overheat" fault code, so I know they have at least one!
    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 ,

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #26
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    john p wrote: »
    NONE?? NOT EVEN ONE LITTLE TINY INSIGNIFICANT MINISCULE ONE? WAAA. that not good. it woulde be an in capacitated inverter without at least one...

    Terrible, isn't it? You spend $2,000 on an inverter and they don't even put any capacitors inside! That's why you have to go buy many Farads worth of super-ultra-audio caps so that when your VFX3648 is rapidly fluctuating between 3600 Watts and 7200 Watts you can reduce the current variation on the 000 gauge wires to the 450 Amp hour battery bank to zero.

    Gee, boB; why don't you guys design these inverters right? :p
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #27
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    inetdog wrote: »
    However, if you have a relatively high internal resistance in the battery AND the load is pulsed rather than continuous, adding a modest amount of capacitance can reduce I2R losses and reduce voltage sag to the inverter during the current pulse.

    Agreed. However you can do the same with a larger battery bank, likely for cheaper. There are applications where more capacitance could make sense (small battery pack, loads with large/short starting surges) but in general it will be more expensive than just adding batteries.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #28
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    bill von novak wrote: »
    Agreed. However you can do the same with a larger battery bank, likely for cheaper.

    Definitely cheaper.
    There are applications where more capacitance could make sense (small battery pack, loads with large/short starting surges)

    Yes, like car audio where the battery size and wiring is limited by other constraints.
    but in general it will be more expensive than just adding batteries.

    Having the right batteries and wires is cheaper and works better too. ;)
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #29
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    I just realized why Cariboocoot cant find any capacitors on the inverters . !!! He has been looking for "condensors"
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #30
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this
    john p wrote: »
    I just realized why Cariboocoot cant find any capacitors on the inverters . !!! He has been looking for "condensors"

    I've been searching my distributors for them too! :p
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  • madflower
    madflower Registered Users Posts: 2
    #31
    Re: Has anyone thought of using capacitors between the inverter and battery? Would this

    The new car audio stiffeners, are actually hybrid supercaps. http://absoluteusa.com

    They are different then the normal supercaps in that they have higher voltage ranges per cap and more storage because they essentially have a battery and a supercap tied together. LIC supercaps or lithium ion capacitors (there are others as well) are starting to creep into the energy density of batteries. They last a million cycles without degradation, they eliminate most leakage, and while more expensive initially, the lifetime is longer. They still have a high charge/discharge rate.

    While most of what was said in this thread is true it was in relation to supercaps, the storage of the hybrid supercaps is really eliminating most of the problems pointed out in the discussion. You may only have to replace the supercaps once in the 30 year lifetime of the solar panels, and you will have to replace the lead acid batteries 5-6 times. Essentially because you are limited by the charge controller and the inverter, you can overspec generation, so the bursts or high times get stored in the capacitor, rather then be limited by the capacity of the inverter and the charge controller. which can extend your max input before draining. it helps especially if the input fluctuates. IE the input fluxes between 4k-6k, with a 5k inverter, you can achieve the steady 5k.

    Supercaps are quite a bit harder to spec and wire. It isn't as simple as a battery.
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