Old plus new

I am living in my rv while building a home. I have four 100ah AGM SLA batteries wired in parallel, three 120 watt solar panels and a 3000 watt inverter. The inverter shuts off at 11.9 battery volts.  I am somewhat limited on power and would like to add four more batteries. My batteries are only 7 months old. I know mixing old and new batteries is bad. But at only 7 months, would this be a problem?


  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    If you add more batteries, you need to add more charging capability.  Otherwise, in 2 days you have 8 dead batteries

    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 ,

  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,743 ✭✭✭✭✭✭
    Yes on more solar or charge amps before you add batteries.

     Since this is short term it may be best to just buy the $600 dollar Yamaha inverter genset at costco.
    If your inverter does not have a charger built-in then that will not work.

     Be sure and do this safely! You would not be the first to have fire while building. Too many of those lately so be aware of rodent damage and have a non flamable area for all of this gear. Get a cat! Good Luck with the build!
    "we go where power lines don't" Sierra Nevada mountain area
    E-mail offgridsolar@sti.net

  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    edited May 2021 #4
    A 3,000 Watt inverter on 12 volt battery bank is pretty large... In my humble opinion, you should be looking at ~1,800 Watt max inverter at 12 volts... A 3kWatt inverter at full load would draw:
    • 3,000 Watts * 1/0.85 inverter eff * 1/10.5 volt cutoff = 336 Amps @ 12 volt bank
    Also, 11.9 volts is pretty high for a cutoff voltage... 11.5 to 10.5 volts is pretty typical (note: at 10.5 volt cutoff, the battery bank is pretty much dead and you have or may have killed the battery bank).

    I think you are expecting "too much" energy from your solar system as presently configured. Just some quick math (I have to leave soon):
    • 400 AH battery bank * 14.4 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 748 Watt array "nominal" for your battery bank
    A 400 AH battery bank can supply a recommended 2 days of storage, 50% max planned discharge of:
    • 400 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 1,020 WH per day
    • 1,020 WH per day * 1/5 hours of use at night = 204 Watt average 120 VAC for 5 hours a night
    I.e., a 300 Watt AC inverter would be a much better fit here...

    Say you are in the Dallas Texas area, and have 360 Watts or 748 Watts of solar panels. Fixed array facing south:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
    (For best year-round performance)

    For May:
    • 360 Watt array * 0.52 AC off grid system eff * 5.26 hours of sun per day = 985 WH per average May day
    • 748 Watt array * 0.52 AC off grid system eff * 5.26 hours of sun per day = 2,046 WH per average May day
    Anyway... With a 3kWatt AC inverter you can run it at ~1,000 Watt AC load for something like 1 hour a day (on average) with your present 350 Watt array... When you have a partially sunny day/cloudy weather, much less harvest.

    Really need to start with your loads--And design a system around those needs.

    Watch the inexpensive inverter-generators... Many appear to be MSW (Modified Sine/Square Wave) AC inverters (you may have to call the mfg and ask--MSW is avoided in the literature... Or ask for THD (total harmonic distortion specifications... 5% or less is TSW. MSW may be in the 25-29% range... Not nearly as nice as the (more expensive) PSW/TSW type (Pure/True Sine Wave inverter).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,743 ✭✭✭✭✭✭
    It is temporary ! Use a genset for building!  Much better to not run anything electrical that is valuable when building that you can't afford to lose. Building is super hard on equipment. Contractors have the worst tools !  I am not overstating this!  Good Luck and where are you building?
    "we go where power lines don't" Sierra Nevada mountain area
    E-mail offgridsolar@sti.net