Battery charging with generator

I have a grid tie-in with battery backup system. It is an Outback system with 3.2kw solar panels. This winter in northwest Illinois we had 12 days with little if any sun. I would like to be able to use a generator to recharge my 48v battery bank during these long periods of no sun if the grid goes down. I have an input for my generator to my subpanel but this would require running the generator all the time. If I could connect it direcly to the DC side of my system I would only need to run it for a short period of time to just give me enough battery power to carry me to the next sunny day. Any suggestions?


  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Battery charging with generator

    You need to use the genset to get above 75% charged, to keep the batteries from sulphating. Does your outback not have a battery charger that can run from the genset?
    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

    gen: ,

  • BB.BB. Super Moderators, Administrators Posts: 33,327 admin
    Re: Battery charging with generator

    You don't say what AH capacity your battery bank is and how much power you use in a day...

    Sounds like a might be an Off-Grid system?

    Anyway--generally size the battery charger current based on 5% - 13% of your banks AH capacity (we like the 20 Hour rate).

    Size the generator to 50-75% load of battery charger (x1.25 or so to allow charger losses)... You can go with a lightweight (inexpensive) genset if this will only be used during wither (propane, gasoline, diesel)... Or a prime power rated genset if this is an off-grid application and you use the genset a lot (say to power your welding/metal shop).

    Just to pick some numbers... Say you use 5kWhrs per day. Battery bank would be roughly:
    • 5,000 WH per day * 1/48 volt bank * 1/0.85 inverter losse * 3 days of no sun * 1/0.50 maximum batt discharge = 735 Amp*Hours
    Assume 5-13% rate of charge, 59 volt battery charging and 80% efficient charger, the load on the genset would range:
    • 735 AH * 59 volts batt charge * 1/0.80 charger losses * 0.05 = 2,710 watts min
    • 735 AH * 0.05 = 37 amps min charger
    • 735 AH * 59 volts batt charge * 1/0.80 charger losses * 0.15 = 7,047 watts max
    • 735 AH * 0.13 = 96 amps max charger
    So--Roughly, find a battery charger (or programmable battery charger) and size a genset from ~5kW-14kW range...

    Just a rough range and how I would look at the question... Plug in your own "real" numbers.

    If you don't have one--I would suggest a Battery Monitor so that you can just look at the meter and read out battery capacity in XX% (not 100% accurate--but should be close enough to keep you out of problems... For sealed batteries, where you cannot measure specific gravity, almost a must have).

    If you have a choice--try to pick a battery charger with a remote battery temperature sensor (battery voltage is temperature sensitive) and an AC charger with Power Factor Correction (lowers the "VA" required by the charger and can allow you to use a smaller genset).

    Avoid a "too large" genset if there will be a lot of hours on the genset. Most gensets have minum fuel flow around 50% of rated electrical output. If you run the genset at 10% rated output to charger your battery bank--you will consume ~5x as much fuel as you really need.

    Diesel gensets should be operated at 50-60% more so minimum load for a long/low maintenance life.

    "Suggestions" for running genset:
    • 75% state of charge, start the genset next morning and/or shed loads
    • 50% state of charge, start the genset now
    • Charge until 80-90% state of charge. And/or watch current and when it drops off by 1/2--turn off the genset--let solar panels continue the charging.
    Anyway, the above is just some basic recommendations and rules of thumb. If you have different needs, please feel free to ask. Nothing is written in stone.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery charging with generator

    some gt inverters are sensitive to frequency and voltage from any ac source. generators tend to vary enough to be a problem for the inverter to lock to it and allow power to pass
  • blackswan555blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Battery charging with generator

    Which Outback inverter`s, What gen ? have you tried to run the OB on the gen ? The OB "Grid" inverters do not like most "normal" generators, they seem ok with inverter type gens,

    Have a good one
  • BrockBrock Solar Expert Posts: 639 ✭✭✭✭
    Re: Battery charging with generator

    Depending on the size of the battery bank and generator I would choose an iota charger. They are really tolerant of input voltage and just use the genset to power the charger and let the Outback do its thing to make power. I do something similar using either an old genset to power an iota charger or an idling car and 1750w inverter which in turn powers the iota charger and I let the inverter power all the loads.

    The big advantage is the inverter continuously powers the loads and the genset / car / solar charges your battery bank via the iota charger.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
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