drawing 12v from 24v

jimbob01jimbob01 Solar Expert Posts: 63 ✭✭
hi ...question...can i have 2 banks 0f 12v so i can connect my 12v inverter to both at the same time................... then connect the 2 banks at the top in series so i can charge the banks @24v? jim

Comments

  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: drawing 12v from 24v

    No, because this will short batteries.
  • BB.BB. Super Moderators, Administrators Posts: 30,485 admin
    Re: drawing 12v from 24v

    Also, when you discharge the batteries (upper/lower 12 volt string) they will discharge unevenly. And when you charge with a 24 volt charger, some of the batteries will over charge and others will under charge.

    There are ways of getting 12 volts from a 24 volt battery bank "safely" (this is frequently done on RV Bus and Truck chassis conversions where the vehicle runs at 24 volts and the house runs at 12 volts)--But the costs/efforts to do that are generally more costly (and less efficient) than just getting a 24 volt AC inverter instead. And if you have some 12 VDC loads, use a 24 to 12 volt or even a 120 volt to 12 volt converter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jimbob01jimbob01 Solar Expert Posts: 63 ✭✭
    Re: drawing 12v from 24v

    thanks gus for the reply...so a new 24v inverter it is...any recommendations? 3.5kw pure sine wave.
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: drawing 12v from 24v
    jimbob01 wrote: »
    thanks gus for the reply...so a new 24v inverter it is...any recommendations? 3.5kw pure sine wave.

    XW4024? I use XW6048 and it worked well for me so far.
  • BB.BB. Super Moderators, Administrators Posts: 30,485 admin
    Re: drawing 12v from 24v

    I like to start at the beginning... What are your loads (Peak Watts, Average Watts, Watt*Hours per day). That tells us the size of the battery bank needed to run those loads. Then we can work out the AC and Solar charging requirements.

    Once we get the basics, then we can spec. the hardware (inverters, charge controllers, AC battery charger, backup genset, etc.).

    We get a "balanced or optimum system" design out of the process...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jimbob01jimbob01 Solar Expert Posts: 63 ✭✭
    Re: drawing 12v from 24v

    hi bb... i have narrowed down the system today
    my loads vary depending on the time of year,mainly lights water pump 12v fridge ,vitamix etc.generally only run the generator from october to march..would like to minimize this.i i live in the middle of a river on a boat(lived off grid the last 18 yrs) i filter the rain and try to harvest the sun...in london uk there isnt so much sun
    i have room for;
    8x 200ah 12v powersonic batteries sealed
    8x 160ah yuasa endurance sealed
    today i bought 2 mstar mppt 45s
    6x 235 sharp monos ov 38v
    im going 24v system definately ...
    i also got 8x17v 80w panels
    whats the best way of configuring this lot?
    i now found out i got to get a new inverter @ 24 v
    thanks for your time, jim
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: drawing 12v from 24v

    If you been off gird all those years, you obviously are doing ok! But I would be slowing up on the purchases, until the big picture is resolved.

    Are you planning on running those different banks on sep controllers? That will work, but you have to hook the inverter to one bank at a time. They are too different to connect together. You have two controllers so that will work, and use a battery switch to swap banks.

    How old are the batterys? At 34kWh total, they are pretty big for that amount of solar.

    If your inverter is 3.5kw, thats pushing it for a 12 volt system, so the move to 24v is timely. Now that you have 1200+W solar ditto.

    Have you done a load analysis?
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • BB.BB. Super Moderators, Administrators Posts: 30,485 admin
    Re: drawing 12v from 24v

    You will have to look at the solar panel Vmp/Imp closely...

    You can put panels in parallel if they have the same Vmp (within 10% or better--I.e., two 17.5 volt Vmp panels in series for Vmp~35 volts; in parallel with your 38 Volt Vmp panels). And similar with Imp's.

    If the panel Vmps/Imps are too far apart, you may have to put them on two different arrays on two different charge controllers.

    Otherwise, you may look at the value of your existing smaller panels. The 80 Watt panels can be replaced by a single 250+ watt panel--Fewer panels, fewer connections, and the new panels are frequently not too expensive on a $$$/Watt basis (many folks sell their old/smaller panels and get pretty good prices for them).

    Regarding batteries--If you have good experience with batteries in general (first time off grid folks can kill their first set or two of batteries), sealed AGM batteries are pretty nice--But about 2x as expensive and usually not quite as long as service life vs good quality flooded cell batteries.

    If you go with flooded cell batteries, I would highly suggest a Battery Monitor (Victron is a very good European brand). If programmed correctly, much easier to see what the state of your battery banks are (and sealed batteries, you cannot measure specific gravity).

    Lastly, make sure you check the charging current supported by the sealed batteries. In the US, our GEL batteries are frequently limited to C/20 (5%) maximum charge rate--Higher charge rates cause permanent bubbles in the the GEL--And kill battery performance. AGM types are better here.

    Although, there are some European GEL batteries that seem to support higher charging rates--Be careful and check the specifications/manuals.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jimbob01jimbob01 Solar Expert Posts: 63 ✭✭
    Re: drawing 12v from 24v

    thanks guys for the informative replys..
    my current bank of batteries was bought 6 years ago and was second hand ...i think 2-3 yrs old
    so that makes them around 8-9 yrs old:-\
    i have bought another bank...1600ah of powersonic sealed lead acid
    these are about 2-3 yrs old
    also i have 12 yuasa endurance 100ah each these are 2-3 yrs old
    all the batterys are ex stand by batteries and have sat float charged.
    i think 2 banks with 2 ccs and a changeover switch sounds great..
    load i think in winter is around 100 - 150ah a day
    im a fabricator welder so i use the gene when making stuff on board
    in the summer i can weld (at low amps) from the battery bank for small jobs with the tig.
    here in the uk sun is not out so much..hence my thinking towards larger bank of batterys.
    so i think i should split the 6x 235w panels in to the 2 ccs and a bank each?
    im left with 8 x 80 w x 21.18 ocv
    also i got a c40 and a c60 left over..feel like im getting close
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