Battery bank & Inverter compatability ?

I'm running a magnum 4448 inverter with a 420ah battery bank. If my load is 4000 watts for 5 minutes, that would be 333 watthours of energy, for a battery that stores 20,160 watthours.  The battery would have to supply 4000 watts ÷ 48 volts = 83.3 amps. Maybe another 8 amps or so for inverter inefficiency and I'm drawing over 90 amps from the battery. Seems harsh for long term battery life.
So in reality, the only time that I can use my inverter to it's capacity without battery damage is during full sun?  If this is correct, whats a safe "max load" for this battery bank without incoming panel wattage?
Also what size battery is needed to maximize this inverter at night?


8-420AH US Battery L-16s, Midnite Classic 150cc, Whiz Bang Jr, Magnum MS4448PAE inverter/charger,  4590w Canadian Solar panels. Honda EU2000i generator.

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    The 420 ah capacity is only if you draw the power down over 20 hours... In other words, your battery bank has a 420 ah capacity only if you draw 21 amps (420 amphours ÷ 20 hours = 21 amps). 

    You can safely draw more than 21 amps, but due to Peukert factor your capacity will be less than 420 ah.

    Your 4000 watt load for 5 minutes does represent 333 watthours of energy, but your battery bank is much less than 20,160 watthours when you draw over 90 amps from the battery.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • ArkansasoffgridArkansasoffgrid Solar Expert Posts: 118 ✭✭
    edited November 2015 #3
    So whats my safe "max load" without incoming PV? 
    And what size battery would be required to safely produce 4400 watts for 10 minutes?
    8-420AH US Battery L-16s, Midnite Classic 150cc, Whiz Bang Jr, Magnum MS4448PAE inverter/charger,  4590w Canadian Solar panels. Honda EU2000i generator.
  • oil pan 4oil pan 4 Solar Expert Posts: 766 ✭✭✭✭

    The 20 hour C rate is the best case scenario. If you use a more realistic discharge rate, say 5 times that overly slow rate you can expect to about 15% less amp hours.

    I am going to estimate the 20 hour C rate for that bank is around 20 amps. If you were to use say 5 times that power or 100 amps it would drop your amp hours to about 370.

    83 amps should be fine.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • Dave AngeliniDave Angelini Solar Expert Posts: 4,840 ✭✭✭✭✭
    How long have you been living offgrid please? This is the time of year that we shift our loads in order to not use 4KW at night.
    You battery should do this though and if it does not when it is charged, you have a problem. Could be a wrong LBCO setting, a bad cable, a bad battery. One of the nice things about the XW/SW and Outback electronics is you can log them. It makes it easy to see weird loads in a 24 hour period.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    edited November 2015 #6
    These are good questions. The easy rules for FLA is 0.25C peak discharge rate, and 0.1C peak charge rate.
    I'm running a magnum 4448 inverter with a 420ah battery bank. If my load is 4000 watts for 5 minutes, that would be 333 watthours of energy
    So peak discharge amps is: 4000W/48V/0.9 (eff)= 92.6A DC.
    For 420Ah thats a discharge rate of 92.6A/420Ah= 0.22C. That is within (at the top of) the recomended discharge rate for FLA. (100Ah per kW of inverter).
    And, running your inverter at high power like that will (ignoring peukart, and exact voltage) flatten your battery to 0%SOC ie DEAD flat in: 420Ah/92.6A= 4.6 hours. Or to the recommeded 25% daily DOD, in = 420Ah/92.6A*0.25= 1.1hr

    So, as Dave said, not something you want to do much after dark at any time of the year.
     The battery would have to supply 4000 watts ÷ 48 volts = 83.3 amps. Maybe another 8 amps or so for inverter inefficiency and I'm drawing over 90 amps from the battery. Seems harsh for long term battery life.
    So in reality, the only time that I can use my inverter to it's capacity without battery damage is during full sun?
    So interestingly, when you design a system to have a peak FLA charge rate of 0.1C you get peak charge amps as:
    420Ah*0.1C= 42A
    Solar needed to generate this is: 48V*42A/0.77=  2620Wp. Which, (quandaryily) is not enough to power the inverter at max load on its own. (But using AGM or LFP, you could get 48V*(420Ah*0.2C)/0.77= 5240W. Better huh?)
     If this is correct, whats a safe "max load" for this battery bank without incoming panel wattage?
    Also what size battery is needed to maximize this inverter at night?
    Its all a balance of elements. From the above you can see that to increase after dark Amps, or Watt hour requirements you need to increase the size of... everything.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • ArkansasoffgridArkansasoffgrid Solar Expert Posts: 118 ✭✭
    edited November 2015 #7
    Thanks for all responses. We live very conservatively, and have never been below 84% batt after 3 overcast days.
    However a buddy, whose family is FAR less conservative, is dead set on going offgrid in the house we're building for him. I was trying to show him scenarios of my systems abilities/limitations. I didn't know all the calculations/formulas for the batt/inverter interactions so I figured I'd ask you guys.
    8-420AH US Battery L-16s, Midnite Classic 150cc, Whiz Bang Jr, Magnum MS4448PAE inverter/charger,  4590w Canadian Solar panels. Honda EU2000i generator.
  • PhotowhitPhotowhit Solar Expert Posts: 5,023 ✭✭✭✭
    ...and have never been below 84% batt after 3 overcast days....
    That might be doing more damage to the batteries than a bit of heavy use. Batteries need to be used, I think your Midnite CC can even skip charging some days to allow your battery bank to cycle.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
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