DC generator to boost batteries

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fooddiva
fooddiva Registered Users Posts: 15
We have been off the grid for 20 years. Have been using AC generators for backup all this time. We now want to try and do a dc generator to directly boost our batteries (16 Trojan T105's) and bypass our Magnum Inverter.

Anyway, just wondering if anyone knows if there is a way to charge the battery bank with a high output dc alternator and low rpms? We may have to build something ourselves but is there anything already available out there?
We have a 24 volt system. In researching we found a kit that would work but won't put out the 30 amps we need at a low rpm rate.

We plan on adding some more panels to compensate for less generator usage but as you know, we need it as a backup. Any ideas?

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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: DC generator to boost batteries

    Spin an Axial Flux generator.
    Free downloads: http://www.scoraigwind.com/download/index.htm
    Forccefield sells parts & kits, you don't need the wind if you have another way (Listeroid) to turn the generator.
    Article: http://www.fieldlines.com/story/2007/12/30/15943/946
    http://www.fieldlines.com/story/2007/12/30/21440/810
    http://www.fieldlines.com/story/2008/1/2/231538/0408
    in 3 parts

    This is on my plate for future
    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 ,

  • Quabillion
    Quabillion Registered Users Posts: 22
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    Re: DC generator to boost batteries

    I have installed several of these http://www.shawneepower.com/dccharger.html with great success for customers. They produce full rated power very reliably, and are not a glorified car alternator attached to a lawnmower :)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    Re: DC generator to boost batteries

    Looking at the "portable" 1.2 model:
    • DC Generator
    • Honda GX 160 gasoline engine
    • Recoil starter
    • On/off switch for easy operation
    • Low oil sensor with automatic shutoff
    • Electronic ignition for easy starting and dependable service
    • Large fuel tank 6.5 gallons
    • Run time at full-rated output 20+ hours
    • Overall dimensions 30" x 18" x 24" (LxWxH)
    • Weight 100lbs
    • Quiet operation only 70dB
    • Automatic overcurrent foldback protection
    • Operates with or without attached battery system
    • Anderson style output connector
    • Adjustable output voltage
    Output Voltage Range Amps 12 13.0-14.4 100 24 26.0-28.5 50 36 39.0-42.9 33 48 52.0-57.2 25

    12v * 100 amps * 20 hours / 6.5 gallons = 3,692 Watt*Hours per Gallon

    The Honda eu2000i running at 1/4 load (less efficient) is 400 watts * 15 hours for 1.1 gallons of fuel:

    400 watts * 15 hours / 1.1 gallons = 5,455 Watt*Hours per Gallon

    The data sheet for the 2.4 kW unit shows that it scales fuel usage down, roughly to only 50% output level (PDF File):
    Propane (2.4 kW peak):

    25% load -.47 gallons per hour
    50% load - 0.50 gallons per hour
    75% load - 0.75 gallons per hour
    100% load - 1.0 gallons per hour

    91,500 btu per gallon of propane
    114,000 btu per gallon of gasoline

    1 gallon propane per hour * 91.5/114 = 0.8 gallons per hour of gasoline (rough conversion based on btu content)

    2,400 Watts * 1 hour / 0.8 gph = 3,000 Watt*hours per gallon of gasoline

    Unless you have a need for a pure DC genset--these do not seem to be terribly fuel efficient, and as they roll back to lower output (say 25% of rated load--assuming 0.47 fuel flow)--for the 1.2 kW DC Genset version:

    3,692 Watt*hours per gallon * 0.25 electrical / 0.47 fuel flow = 1,848 Watt*Hours per gallon at 25% load...

    The little Honda eu2000i, even with the losses of a good battery charger (assume 80% efficient), is still way more fuel efficient as the batteries are being topped off (less than maximum current flow):

    (5,455 WH/g honda / 1,848 WH/g DC gen set) * 0,8 charge controller eff = 2.4 times more efficient for the Honda (or only 42% of the fuel for same output power).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fooddiva
    fooddiva Registered Users Posts: 15
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    Re: DC generator to boost batteries

    Thanks for the feedback. There is a lot to digest and we are going to consider everything. Someone also recommended a diesel generator. Wouldn't it be better to just increase the amount of panels and generate more electricity? But I guess we would have to increase our battery bank also. Is it true that you can't just add more batteries to an existing system? That you have to swap out the entire bank at one time? We just got a new bank of 16 last year.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    Re: DC generator to boost batteries

    Regarding adding new batteries to a bank... Generally, the assumption is that the new batteries will fail, very roughly, at the same time as the old/existing bank. Since your bank is still relatively new--perhaps that is less of an issue.

    Also, the internal resistance of batteries changes with age/cycles. New batteries have there resistance drop (and capacity increase) as they cycle a bit. So--your older bank may carry more of the load (assuming new and old banks are connected in parallel) until the new bank cycles some.

    The question is why are you adding more batteries? Are you discharging them more/deeper than you should? Want more "quiet time" (less generator usage)?

    Adding solar panel vs adding a genset. If you have kept track of your power usage and solar panel generation--Do you need more panels? To a degree, adding more batteries may increase the need for more panels for proper charging (rule of thumb is that you need 5%-13% of your bank's Amp*Hour rating in solar panels / charger capacity). Too little charging current, batteries don't mix electrolyte during equalization. Too much current is a waste (and may overheat the battery bank with too much charge current).

    If you don't have one yet--a Battery Monitor would be a good investment. You can see how well you are charging your bank, how much load you are using, and how everything is performing over time.

    Running your genset in the morning to "bulk charge" the batteries (run genset at 50% capacity or greater) is most fuel efficient. Let the solar array finish the absorb and into float charging the rest of the day.

    And, if your array is small--you can use the generator once or twice per month for a couple hours to equalize the bank (combined with solar power to reduce fuel use/keep genset+charger charger on the smaller side).

    Regarding diesel or other fuels (gasoline, propane)--which is easier for you to get (at a good price). If you already have propane--it sure is a nice fuel for storage and clean burning. Diesel is a bit more of a pain to store, engines are pretty fuel efficient--but is "dirty" burning (smelly) and if the genset is operated below (roughly) 50% of capacity--tends to carbon up.

    Are you running the gensets more than you want (say more than 3 winter/low sun months per year) and want more solar panels + batteries to reduce genset use?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: DC generator to boost batteries

    Conserve more ? 16 T105's give you how much storage ?

    Does it last all night ? only 2 cloudy days ? Do you have some phantom load, maybe something is dieing, and sucking power when it shoulden't ?

    The fieldlines article is about as simple as you can get, but you need some work space and tools.
    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 ,

  • fooddiva
    fooddiva Registered Users Posts: 15
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    Re: DC generator to boost batteries

    We have been here about 21 years and got about 10 years out of our first bank of batteries and about 11 from the second. Our original panels from 20 years ago were Arco M75's and I doubt they are putting out maximum power anymore. We run our AC generator way more than we would like. I thought if we added more panels that might help compensate for less generator usage. We do our "bulk charging" in the morning and let the sun do the work throughout the day. We find at night if we are using numerous things, the water pump goes on, etc. we don't want to drain the batteries so we run the generator. Our batteries do not get drained overnight. We just want to keep them healthy by making sure they are always charged to what they should be.

    We don't want to add more batteries and do use a monitor. We will have to do the math about adding 2 more 125 watt panels to help our system. I would love to add more but can't afford it at this time.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    Re: DC generator to boost batteries

    It sounds like you are getting very good life out of your battery bank (you know what you are doing)--so unless you added more loads (and everyone does)--adding more batteries does not sound like a priority (unless you are running the genset all year).

    There was a thread where we discussed battery bank size vs costs... And, at least for me, it was not always obvious that a large bank with very little loading was much more cost effect vs a smaller bank with deeper cycling. From the math (battery life vs depth of discharge)--it was looking like Bank "A" with 8 batteries that lasted 10 years vs Bank "B" with 4 batteries that would last only 4-5 years (actual battery costs over 10 years are only slightly more with Bank "B" configuration).

    Time to Question the 3 day Rule ?

    Then it comes down to adding solar panels (and possibly another charge controller) to reduce genset usage...

    Perhaps finding a local solar installer and buying from him when he makes his next large buy can reduce your purchase costs some (save on shipping costs for just a couple panels).

    You can compare your fuel costs vs loss in battery life (if less genset use) vs more solar panels and estimate what would be your best investment.

    Remember, that if your system reaches "float" early in the day (like before 10am) instead of after 3pm--you are already using less than your full charging capacity of your solar panels (unless you are using loads during the day such as washing, vacuuming, pumping, etc.) to use excess solar panel capacity.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: DC generator to boost batteries

    fooddiva,
    you did not get into specifics on what kind of current levels you are reaching with those pvs that you have, but you should be able to have the current up to a 13% charge level for those batteries so, yes, adding more pvs is an option and may help. with the current levels you are seeing from these old pvs if your charge percentage is below 5% then you definitely need more pvs.