Using Both Panels and Generator to Charge Batteries?



  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    JRM, this is your discussion, so you pretty much guide it with your questions and answers. If something is getting too far afield--Please feel free to repeat/clarify your direction.

    With solar/off grid power/etc..., it pretty much all revolves around the energy usage, or in your case, the battery bank and assumptions/models/rules of thumb about how you will use it... Please note that the "rules of thumbs" for off grid home/cabin/RV design--We try to be conservative and give you a working system that will meet your needs. That is a "balanced" system design.

    AC inverters in general, are relatively cheap. And solar panels have come way down in price over the last decade or so... But batteries have been pretty much fixed or even rising in price. And folks tend to get a "smaller" battery bank and a large(r) AC inverter. And that can be a "miss match" in capacity/ability of the batteries to "source" the current/power.

    For example, "low cost" 12 volt batteries tend to be around 100 AH in capacity (equivalent in energy storage and weight/size to 6 volt @ 200 AH "golf cart" batteries).

    A 48 volt battery bank made from 4x 12 volt @ 100 AH batteries in series would support around a 500 to 1,000 Watt AC inverter "nicely". Our first rule of thumb is 1,000 Watts of AC inverter or Solar array rough maximum (rule of thumb for "quick sizing" of system).

    If you want to run a 3,500 Watt AC inverter at "full tilt" (power/shop tools, 1 HP well pump, etc.), then you are really looking at an ~350 AH @ 48 volt battery bank... Or 4x 100 AH @ 12 volt battery strings (4s x 4p = 16 batteries; or if using "Golf Cart batteries 6 volts @ 200 AH or 8s x 2p battery bank). That is a 48 volt @ 400 AH battery bank...

    The above rule is for Flooded Cell Lead Acid batteries--But is really a good start for any battery bank. AGM and Li Ion batteries can supply larger "surge/continuous" current than FLA batteries--But that leaves less "stored energy" to use overnight/cloudy day or two... Sort of like a large gas tank with a "small" fuel line, or a smaller gas tank with a large diameter fuel line... Can empty the tank "faster", but you will also "run out of gas sooner"...

    So--Back to your loads... Generally, "smaller loads" (most efficient you can find tools/appliances/lighting) and only run when needed (don't leave the the AC inverter on 24 hours per day if you are only using for short periods of time (as an example).

    The above talked about "maximum" current/power from battery bank based on voltage/AH rating. And there is "sizing" the battery bank for overall storage/capacity. For example, say you want to have 5 hours of power per evening (LED Lighting, TV, cell charging, laptop, battery based power tools, RV water pump). As an example, a 4s x 1p battery bank (12 volts @ 100 AH or 48 volt @ 100 AH total capacity). Say 2x nights and 50% max planned discharge for longer battery life (3rd day is sunny, backup genset, etc.):
    • 100 AH * 48 volts = 4,800 Watt*Hours of energy storage:
    • Daily available Battery energy = 100 AH * 48 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 1,020 WH per day
    • 1,020 WH per day / 5 hours of average draw = 204 Watts average 120 VAC loads
    So--An average power draw is closer to 204 Watts--Or a small fraction of what a 3,500 Watt inverter can supply... And a large AC inverter can draw 20-40 Watts just being "turned on/no loads" (Tare Losses)... 5 hours * 20 Watt Tare Losses = 1,000 WH or almost your entire "planned" battery loading--Leaving 20 Watt*Hours for your actual loads.

    Of course, I still do not know what your planned AC battery bank AH capacity was--And I don't know your loads. So the above is more of an "example of an un-balanced design--Large inverter, small battery bank, and expected loads).

    You can make different assumptions (such as you are only up there on weekends and only want 1 day of storage+50% max discharge--And use the genset to recharge when needed) and the math will be somewhat different (like 2,040 WH per 1 day/overnight)--But still need to know more about your usage (or, for this example, your battery bank sizing) to "build the back of the envelope model").

    I know you are eager to get the system up and running--But spending a day or two on getting the basic sizing of the system right (vs your expectations/needs) will save you money and frustration down the road.

    You might want to plan on "small solar system" for the trailer/overnight/quiet time energy usage... And go head with a genset (I like the 'inverter-generator' because they are relatively fuel efficient and quiet--especially with lower loads).

    As an example... A Honda eu2200i supplying 25% of rated load @ 8.1 hours @ 0.95 gallon tank
    • 1,800 Watts * 0.25 loading = 450 Watt average load
    • 1,800 Watts * 0.25 loading * 8.1 hours * 1/0.95 gallon tank = 3,837 Watt*Hours per Gallon
    • $4.00 gasoline (prices are rising quickly) * 3.837 kWH = $1.04 per kWH
    That is (very roughly) what a well designed/daily use/9+ months a year off grid solar system costs (all the upfront $$$ for panels+batteries+electronics+new batteries every 3-5 years)... For a "getting started" system, if using a genset during the day (power tools, lights, etc.) is "OK" with you--It is not a bad tradeoff to run genset during the day and solar overnight for "quiet time/overnight lighting" as needed). Something like a 2s x 2p Golf cart (6 volt @ 200 AH) batteries for a 12 volt @ 400 AH (same energy storage as 4s x 1p 12 volt @ 100 AH battery bank above)... And use a small (300-600 Watt or so) AC inverter, smaller solar charge controller, etc. plus array for the RV overnight power.

    Trying to buy "big now" and migrating over to your house system--It is a bit more difficult to justify. Like buying a Mac Truck to drive from hardware store to the homesite vs a car+trailer or mini-van/small pickup... Eventually, if you get into "trucking", the Mac is nice--But for small jobs, it is going to be expensive to "keep happy".

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    And to answer your question about mix/match charging sources... More or less, Lead Acid batteries can take quite a bit of charging current (at least up to 25% of AH capacity--100 AH -> 25 amps charging). At less than ~80% state of charge, FLA batteries can take even more than 25% rate of charge (they are very efficient when charging below ~80%). And above ~80% to 100% state of charge, the batteries "naturally" start to limit charging current--Eventually to 1% or less as they near/hit 100% SoC (assuming proper charging voltages).

    Also, with a the Midnite Classic controllers, they have an option called the WizBang Jr.:

    Which is a remote current sensor--When combined with a precision resistor (shunt), the Classic can measure charging current to the battery bank and limit its own charging current (just the solar portion) to the battery bank (cannot control, for example, the genset/AC charger portion). The WBJr also adds a battery monitor function to the Classic. (other Amp rating sizes should be available)

    More or less, FLA batteries are "happiest" at ~10% to 13% rate of charge (longest life). Especially when you have high rates of charge available (13%+), you want the Remote Battery Temperature Sensor option. You mount it to the battery bank and the Classic will reduce the charging voltage as the batteries get hot (either from charging, or ambient air temperatures)--Another good option to keep battery banks "happy" (some models of Midnite controllers may come standard with RBTS).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • wellbuiltwellbuilt Solar Expert Posts: 636 ✭✭✭✭
    Ok yes they can be connected together 
     not knowing how large your charger is 
     and the battery’s amp hours and type
     I would say I would not use them together . 
     Charge with genarator early 6/7am and get to 80% or a little more , then let the solar power your loads and finish charge the battery’s 

     I don’t  Disconnect anything I just turn off the generator and as the sun get up higher in the sky the solar takes over . 
     Your air conditioning may cause conflict 
    Now your genarator will charge the battery’s 
     and power the ac . 
     I think you have to put the inverter in the ac line to use it to run the The air conditioning and other items in the trailer . Witch is a huge load for 900 watts of panel and a few battery’s . 
      I would power the ac Completely separate and just run  agenarator to use it. 
       If that is the case you don’t need the solar really the converter will charge you batterys. 
       The cheep inverter live expectancy is not the real problem 
     3500 watts is a huge draw on a small system . 
        And the inverters use a ton of power just being turned on .
      We are talking 50 watts an hour 1200 watts a day before you use any power in the trailer 
     I have a small inverter 300 watts that I use in my trailer that draws 6 watts . 
        The question is not cut and dry . 
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • JRMJRM Registered Users Posts: 17 ✭✭
    Bill and Wellbuilt make good points about inverter size.  I jumped in at 3500 watts for two reasons.  One, what I read about loads is that motors take a lot more watts to start than to run (examples include mixers and vacuums, as well as the trailer air conditioner), so inverter sellers recommend getting at least twice the size of potential loads.  The other reason is that the basic trailer input is 30 amps at 110 volts, which comes out to nearly 3500 watts.  However the only times I have come close to this is when the trailer's converter is charging its batteries - which happens automatically, and the air conditioner comes on.  So, I have probably made a beginner's mistake with the inverter size.  Also, It appears that maximum inverter efficiency occurs at about 25 percent of capacity, and drops off rapidly for use below about 20 percent, which favors use of a smaller inverter, then decreases gradually above about 25 percent.

    I know I am short on solar panels.  Three  panels at 300 watts each, under the best of conditions, is probably not enough to fully charge the 4 batteries (at 12 volts and 100 amps each) that make up my 48 volt battery bank.  And this is not even accounting for keeping the trailer batteries charged.  But, for the time being, I am willing to accept that I will still be needing a generator.  And the way I hope to use these batteries (not exceeding about 50 percent of total capacity), I will only have about 200 amps of battery power (not counting trailer batteries that only run dc powered appliances).  But adding more batteries means more panels that would require building more support structures to keep cows away.  And for now, I would like to focus on getting things running.

    The comments about inverter power use and efficiency are very helpful.  I went back and checked the specifications for my inverter and found that its non-operation power drain is 0.7 amps and its efficiency number is 87 percent, which does seem like a lot of wasted power.  All of this is great information for the future.

  • JRMJRM Registered Users Posts: 17 ✭✭
    After reading Bills comments again, if I am able to limit generator run time to between 12.0 and 12.4 volts per battery, generator charging would not continue above about 75% of the battery bank's capacity, allowing the Midnite controller to "top off" batteries when the sun is shining (including the trailer batteries).  Since this generator charging would be limited to less than the 80% state of charge, where the batteries can handle a high rate of charge, it seems that the battery bank should be able to handle input from both my charger (about 17 amps) and the solar controller without creating problems.   With 4 batteries in a 48 volt battery bank, it does not seem possible with equipment that I have for the rate-of-charge to exceed 25% (100 amps) of battery capacity (400 amps).

    My electricity use in the trailer is usually low - some TV, making coffee in the morning, short periods (seconds to minutes) of 1000 watt microwave use, a few low watt led lights, thermostat controllers for ac and propane fueled refrigerator, fan motors, some laptop use, and a small amount of water.  Most of my use happens at breakfast and lunch, then in the evening at and after dinner.  Even my relatively small battery bank (with maximum of 200 amp hours from 100% to 50% state of charge) can probably handle this without being charged by a generator, at least after sunny days.  But this changes if the trailer's air conditioner is needed to cool things off (nearly always during sunny weather).  The ac takes about 1700 watts (15.5 amps at 110V) to start and 700 watts (6.4 amps at 110V) to run.  These numbers were found on-line and seem conservative, especially for the running watts.  This could deplete my battery bank and lead to generator use/charging when the sun isn't shining, but it is not as bad as I had been thinking.

    However, this does not consider the unknown efficiency of my large (oversized?) inverter when used for small (<700 watt) loads.

    Does this seem correct?  It is better than I had expected, which makes me wonder. 

  • wellbuiltwellbuilt Solar Expert Posts: 636 ✭✭✭✭
    That is what you have to do disconnect your converter at 80% and let the solar take over 
      Don’t charge the trailer battery with your 4 deep cycle battery’s it will kill them . 
     I believe your converter dc power feeds your battery’s
      and your ac power has a panel that’s fed from the generator . 
     If you pull the fuse on the converter it will
     not charge the battery’s but a/c power to The air-conditioning system and other outlets will still work. 
     700 watts is a lot of power for a trailer .
     My cabin uses 2/300 watts and is 2800sf 
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • JRMJRM Registered Users Posts: 17 ✭✭
    Wellbuilt - Thank you for hanging in with my questions.  I'm not sure about separating charging of trailer batteries from the ac accessories.  Travel trailers come with a plug and cord that supplies a 30 amp service (more modern trailers have a 50 amp service).  This goes to a panel in the trailer where it is distributed into breakers for plugs and major appliances.  Part of the this ac power is used by a converter that charges batteries.  So what I am faced with is getting dc from the sun, storing it in batteries that power an inverter to create ac for the trailer, part of which is converted back to dc to charge trailer batteries.  It seems overly complex and wasteful, but not compared the the hassle of plugging the trailer into a generator to use ac appliances and charge batteries while putting up with the noise.  Even if the generator still comes on, it will be almost 150 feet away.  I agree that 700 watts is a lot, especially for solar, but the trailer gets very warm without the air conditioner.  I'm not there about 2/3 of the time in the summer, so the sun has a chance to recharge batteries for my next stay.

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Remember that 12.0 to 12.4 volts is "resting voltage" (no loads/no charging for 3+ hours). When charging, more or less, are approaching 80%+ state of charge when the battery charging voltage his the "absorb" set point.

    Here is a chart that explains battery voltage vs load vs charging vs state of charge:

    Measuring state of charge vs battery voltage is just a rough guess... You might look at integrated inverter-chargers and see what they offer for control. That is where Schneider and others really shine--The integrated system. However, not cheap, not "simple" to configure and maintain.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • wellbuiltwellbuilt Solar Expert Posts: 636 ✭✭✭✭
     your battery’s will run your ac for a hour ? 
      Most trailers need a 2800 watt generator 
    to even start a ac unit .
     But your battery’s will supply 2400watts for one hour then they will be at 50% . 
     Your solar will supply 700watts +- per hour of full sun. 
     You can’t plug your converter into a inverter to run Air conditioning in the trailer .☹️
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • wellbuiltwellbuilt Solar Expert Posts: 636 ✭✭✭✭
    If I’m not mistaken one of these fuses  is for AC the other is DC 
     my buddy uses the converter  to charg the battery’s 
     then unplugs the fuse to stop the unit from charging . 
    The other pass thru power for his AC . 
     If I’m not mistaken 
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • wellbuiltwellbuilt Solar Expert Posts: 636 ✭✭✭✭

    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • JRMJRM Registered Users Posts: 17 ✭✭
    Bill and Wellbuilt - Thank you for the analysis.  I am headed out the door and will look more closely when I get back on Sunday night.  I had seen the attached article and had been trying to relocate it to save.  I will re-evaluate my charger start/stop voltages based on this - I want the generator/charger to start at about 50% SOC and shut off before reaching 75% SOC.  This should "save" my batteries while still leaving room for solar generated storage.  I know that using voltage as a proxy is not perfect, but probably is good enough.  The trailer ac power use I found indicates 1700 watts startup (a few seconds) and 700 watts running.  With inverter losses, this would seem to give about three hours of run-time to 50% discharge (2400 watts) not counting solar input during the day.  The ac runs about half the time starting in mid-afternoon, so I should get about a day out of it with some power left over in the batteries.  Wellbuilt is right about the ac startup - I have had my Yamaha 2800 watt generator that is supposed to provide a short-term battery boost to 3000 watts stop generating (it doesn't die, just runs on without the generating) when the ac starts while trailer batteries are charging.  This is very disappointing to come back to.

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