Off-grid system: 2x VMAX AGM 6V 225 aH batteries in series with a 4kW inverter ...

GrievousAngelGrievousAngel Registered Users Posts: 1
I do understand that 2x VMAX AGM 6V 225 ah batteries in series can not support a 4kW inverter very long (I purchased a new AIMS 12V 4kW inverter at huge discount). I am in the 'try-out' phase now. My camper is Jayco Hummingbird 17RK  < 20 ft. (120V @ 30A load center). The 120V loads are: Ceiling Exair AC unit that runs on a 20A CB (I installed an extra 'start-up' capacitor to reduce the AC start-up current), convection microwave (20A CB), Fridge (120V /LPG), other 120V loads are minimal. LPG runs furnace, tankless HW heater, stove, Fridge (if needed).  12V typical loads.

I can charge the battery system via: 1. * Tow trickle charge, (2) ** Solar - Renogy w/ 2x 100W Elite panels on roof plus a portable folding 100W panel (plugs into camper tongue), (3) + Progressive Dyn PD-9280 Converter 80A, (4) + AIMS Inverter (converter section - up to 110A). The charge system with * inicates  'charge while moving', ** while moving and stationary, + Grid dependent. Solar is the only 'off-grid' charge syststem. 

I am hoping we can enjoy some 120V devices while 'off-grid' . . . 

What might I expect in terms of 'off-grid' 120V load performance? With and with-out AC load?

Thanks for your input.

Maggie & Billy


  • BB.BB. Super Moderators, Administrators Posts: 30,651 admin
    edited December 2019 #2
    Welcome to the forum Folks,

    I can give you some very depressing math... And I will down below. But to size/understand an off grid power system, you need to understand your energy needs (average Watts, Watt*Hours per day of usage, maximum surge current, etc.).

    Getting a "kill-a-watt" type energy meter and running your loads (from shore power) can give you a better idea of how much energy you would use...

    Refrigerators and Air Conditioners are what, generally, push from small to medium size power systems to much larger power systems. And for RV systems, running an AC system is difficult from solar--Generally a Genset can be a better solution for weekend/seasonal camping.

    And some math... For example, say your A/C system takes 15 amps @ 120 VAC, and runs on a 50% duty cycle once you cool the space down (purely made up numbers for example. Assume that you run for 4 hours in the late afternoon/evening to cool things down. The numbers would be:
    • 15 amps * 120 VAC = 1,800 Watts running
    • 1,800 Watts * 4 hours evening usage * 0.50 duty cycle (runs 1/2 the time) = 3,600 Watt*Hours evening/night time cooling.
    • 3,600 WH per night * 1/0.85 A/C inverter eff * 1/12 volt battery bus = 353 AH @ 12 volt nighttime load from A/C
    • 2x 6 volt @ 225 AH batteries in series for 12 volts = 12 volts @ 225 AH battery bank capacity
    So, the "purely a guess" A/C system would use 353 AH from your 225 AH @ 12 volt battery bank. In general, would suggest that you (on average) discharge your battery bank by 50% (to 80% discharge can work, but you need to recharge right away).

    How much energy can 3x100 Watts of solar panels produce for an off grid battery based inverter power system... Pick some random nice place in Georgia: (note: Link database seems to be corrupted--mixing with the country of Georgia City list).

    Sandy Springs Georgia (USA)
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 71° angle from vertical:
    (Optimal summer settings)


    So, let us guess at 5 hours of sun per day during summer:
    • 300 Watts of solar panels * 0.52 off grid AC system eff * 5.0 hours of sun (average) = 780 WH per day
    • 780 WH of generation / 1,800 WH of loading = 0.43 hours per day of "cooling power"
    Anyway--Just a very quick look at what the basic math would look like with random numbers 

    to be honest, I would be suggesting  a 300 Watt AC inverter for your battery bank--That would run your loads for a few hours... A 4 kW inverter at max load:
    • 4,000 Watts * 1/0.85 inverter eff * 1/12 volt battery bus = 392 Amps @ 12 VDC
    Even if your battery bank could support 392 Amps (which it probably cannot)--You would use up the batteries in:
    • 225 AH capacity / 392 Amps = 0.6 Hours at 4,000 Watt AC loads
    And, in reality, your battery bank would be dead before that...

    Large AC inverters look like a steal (price wise)--But the reality of what an RV can support for Solar Panels and Battery banks (especially the smaller RVs), you are much better to reduce your electric loads to the bare minimum through choice of appliances (i.e., propane fridge, fan to circulate air vs A/C, etc.)...

    When you get into the details, large AC inverters running small loads are just terribly inefficient. For example (guessing at your inverter model) a 4,000 Watt @ 120 VAC 12 volt AC inverter takes around 44.5 Watts just powered on (without any loads)...

    Or 44.5 Watts / 12 volts bus = 3.7 Amps

    That is a very significant load for a small power system.

    A smaller 12 volt 300 Watt inverter takes something like 0.45 Amps (no load-MorningStar 300 Watt 12 volt PSW inverter): (spec sheet)

    Of course, this depends on your loads... If you are charging phones, laptop computer, etc... A 300 Watt inverter can work nicely. If you are running skill saws and A/C system, you may need a 2,000-4,000 Watt AC inverter--But you also need more solar panels, a larger inverter, and a 48 volt battery bank (and lots more/lots larger battery bank).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    A problem with some of the larger 12v Aims inverters I've looked into is a big draw for them just being on.  Just to keep it lit 24hrs/day could use as much as a full-sized AC fridge. 

    Running the air conditioning likely wouldn't last long depending on running current.  I can run a small microwave okay off a pair of similar batteries for a few minutes if they're fairly fully charged.  The voltage sags in use, but recovers well after.  If the batteries are lower SOC though, they can sag lower than the inverter low battery cut-off voltage.  I keep the big inverter off except to run the microwave.  Many LPG/AC fridges I've seen are really meant for running on shore power AC only, and are real hogs if trying to run on AC off-grid.

    For best battery life, try not to regularly discharge to less than 50%.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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