Results 1 to 7 of 7

Thread: 48 V battery bank and two 24 V inverters ??

  1. #1

    Default 48 V battery bank and two 24 V inverters ??

    Let me describe my situatiuon - i need to upgrade my current PV system that is suffering from a poor design and not adequately chargeing my 24V battery bank; due to a lower than needed Vmp. See my other post for more info and discussion and responses if needed. The batteries are now configured to one large 24V bank that supply TWO 4,000 watt inverters.
    I want to rewire and series up the voltage on my panels and then use MPPT charge controllers to charge my battery bank which consist of sixteen L-16 batteries. From the specs on MPPT controllers i see the 48 volt units can supply twice the wattage.
    OKAY --heres the question -- can i reconfigure the batteries in such a manner that they can supply the 24 volts to the inverters -- and YET be in series to make a 48 volt bank for the charge controllers? Is this feasable or fantasy?---advisable???

  2. #2

    Default Re: 48 V battery bank and two 24 V inverters ??

    You may have got a bit confused by the MPPT function. It can take a higher-than-system Voltage input and downconvert to system Voltage, turning "extra" array Voltage into additional charging Amps.

    It is not a good idea to try to split a 48 Volt battery bank to run two 24 Volt inverters: the load on the two "halves" of the battery bank will inevitably be uneven. So whereas you'd be charging a 48 Volt bank evenly, it's individual batteries would be discharged at different rates. This will rapidly destroy the batteries. Nor would there be any power advantage to doing so.

    There's nothing wrong with upping the nominal array Voltage to "48 Volts" and feeding that through an MPPT controller to a 24 Volt battery bank and running one or two inverters off that.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3

    Default Re: 48 V battery bank and two 24 V inverters ??

    Thanks for the prompt reply ---yes i am confused !! ---easily so the older i get.. Heres what i find confusing from the tech specs on a Tristar MPPT i read the following:
    Maximum Battery Current : 45 amps
    Nominal Maximum Solar Input :
    At 12 Volt - 600 Watts
    At 24 Volt - 1200 Watts
    At 48 Volt - 2400 Watts
    Peak Efficiency : 99%
    Nominal System Voltage : 12, 24, 36 or 48 volts DC
    Max. Solar Open Circuit Voltage : 150 volts DC
    Battery Operating Voltage Range : 8-72 volts DC
    Maximum Self-consumption : 2.7 Watts
    Transient Surge Protection : 4500 Watts/port
    Doesnt that indicate if its a 48 volt SYSTEM it will handle twice the wattage????????
    Thanks for pointing out the pitfalls of the 48 volt battery bank and (2) 24 volt inverters: i can see thats not acceptable at all.
    Now getting back to selecting the correct size and number of MPPT controllers. The way the panels are now arranged there are 10 strings. When i series them i will wind up with 5 strings. I will have to send 3 strings to one controller and 2 strings to the other. The OPEN CIRCUIT VOLTAGE at the combiner box is 36 Volts. Total amps available should be around 116.
    Should i use one 80 amp controller for the 3 strings and one 60 amp controller for the 2 strings?

  4. #4

    Default Re: 48 V battery bank and two 24 V inverters ??

    Charge controllers' Amperage rating is based on their output. An MPPT controller that can handle 45 Amps can handle 45 Amps @ 12 Volts, @ 24 Volts, or @ 48 Volts. The corresponds to more Watts on the input side:
    45 * 12 = 540, 45 * 24 = 1080, 45 * 48 = 2160
    And yes the array Wattage given by the manufacturer is higher than that because they actually consider the panel efficiency. You can to some extent always "over panel" a controller; "excess" power simply doesn't get used.

    Open circuit Voltage (Voc) should not exceed the controller's maximum input Voltage, including allowances for cold temperature compensation (when panels get cold their Voltage goes up). When you put panels in series the Voltages go up. If the Voc on your panels is 36 you could put four in series without exceeding the 150 Volt input limit. When reconfiguring an existing array it becomes a matter of trying to divide up the existing number of panels into a new arrangement of equal numbers. It doesn't always work. Your ten panels limits you to five strings of two each like you said, or three strings of three and leave one panel out.

    I can't remember your panel specs 'cause I'm old and confused too and answer about 50 questions a day. They all start to run in together after awhile. But that's the basics. Repost the panel specs and we can juggle the numbers about.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  5. #5
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
    Posts
    21,235

    Default Re: 48 V battery bank and two 24 V inverters ??

    Solar charge controllers are output current limited... So, the controller will output 45 amps at 12 volts, 24, or 48 volts... Because power is:

    • Power = Voltage * Current

    If you change from a 12 volt to 24 volt bank, the power through/managed by the controller will double. And if you go from 24 volts to 48 volts, it will double again.

    So, with a high voltage battery bank, you will need fewer/smaller charge controllers for the same wattage array.

    For your system, with a 150 VDC maximum input charge controller, you are able to put 5 of your panels in series--and almost able to do 6 (depending on how cold it gets in your area--perhaps 6 is good down to near 14F or so--depending on the exact panels you have).

    6 panels would give you 7 parallel strings which you can divide among several controllers (depending on battery bank voltage and specific controller output current rating).

    The maximum array / minimum output rating would be:

    • 4,200 watts * 1/59 volts charging * 0.77 panel+charger derating = 55 amps day to day expected maximum current

    So--depending on your choice for derating (0.77 is about the maximum derating I would suggest before it becomes an expensive over-paneling to charge controller ratio)... You could, if you wanted do all this on one 60 amp MPPT charge controller or a pair of 45 amp MPPT controllers.

    I am getting a little confused--In your earlier thread you talked about 42x 100 Photowatt panels--Is this a different installation?

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  6. #6

    Default Re: 48 V battery bank and two 24 V inverters ??

    Can MPPT charge controllers and PWM controllers be used together on the same battery bank --- since i have 10 strings at my combiner box now , could i series up 8 strings --That would give @ 72Voc on those 4 strings. i measured the Open Circuit Voltage again today (midday 90 f.) and it 35.8 volts. Put those 4 strings into one combiner box and send to a new 60amp MPPT. Leave the other 2 strings where they are (in a different combiner box) going down to the existing C60?

  7. #7
    Join Date
    Oct 2010
    Location
    Nova Scotia canada
    Posts
    2,720

    Default Re: 48 V battery bank and two 24 V inverters ??

    Quote Originally Posted by mrleemus1 View Post
    Can MPPT charge controllers and PWM controllers be used together on the same battery bank --- since i have 10 strings at my combiner box now , could i series up 8 strings --That would give @ 72Voc on those 4 strings. i measured the Open Circuit Voltage again today (midday 90 f.) and it 35.8 volts. Put those 4 strings into one combiner box and send to a new 60amp MPPT. Leave the other 2 strings where they are (in a different combiner box) going down to the existing C60?
    Shouldn't be a problem using different controllers as long as they are not in any way connected together until their output hits the batteries, or the big cable feeding the batteries. Just that the MPPT controllers will be able to take advantage of extra voltage, tuning it into extra amps, while the PWM controllers will not. Other thing to note - - - it's when the panels are at their coldest that they produce the highest voltage. The warmer they get, the lower their voltage. Just mentioned that as you stated you checked their open circuit voltage at mid day and 90 deg F, when the panels would be quite hot.
    1900 watts PV, (1000 watts PV feeding MidNite Classic 150; 900 watts PV + 160 watts micro hydro both feeding into a single shared Morningstar TS-MPPT-60) ; Xantrex Pure Sine 1800/12 for heavy loads; Xantrex Pure Sine 1000/12 on 24/7 for everything else; six Rolls Surrette 2 volt L16 @ 12 volts.
    Domestic hot water totally provided by the sun 8 months out of every year via thermal panel.

Similar Threads

  1. Is there a way? 24v battery bank and two 12v inverters?
    By tom-from-texas in forum Solar Beginners Corner
    Replies: 4
    Last Post: November 18th, 2013, 11:44 PST
  2. Help...Newbie in town...two cheap inverters on one battery bank ?
    By Pangea in forum Solar Beginners Corner
    Replies: 21
    Last Post: June 27th, 2013, 8:11 PDT
  3. Using two inverters connected to same battery bank?
    By nginx in forum Solar Beginners Corner
    Replies: 6
    Last Post: September 17th, 2012, 12:25 PDT
  4. Replies: 7
    Last Post: July 3rd, 2012, 0:14 PDT
  5. Two inverters on one battery bank?
    By Tulumtam in forum Off Grid Solar & Battery Systems
    Replies: 12
    Last Post: September 22nd, 2011, 7:43 PDT

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •