What size batteries for 48v and 30 amps?

BilljustBillBilljustBill Solar Expert Posts: 219 ✭✭✭
Let's say I'll have a 48 volt system with strings of four panels of the following:
135 watts
120 watts
100 watts
90 watts
75 watts

The total is about 30 amps @ 48 volts. What amp-hour size of eight 6-volt batteries would I begin with to be sure I have enough panel power to prevent the cells from going bad due to low charging? What battery brand would you buy?

I live in Texas and have a site that provides sunlight from 8:00 a.m. to 6:00 p.m.

Thanks,
Bill
Bill

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What size batteries for 48v and 30 amps?

    If I read you right you've got a total of 2080 Watts? With a typical 80% efficiency that would be 1664 Watts.
    At 58 Volts charging you'd get about 28 Amps. The standard recommendation of 5% - 13% means it could theoretically charge 215 to 560 Amp/hrs. A good size would be around 300 Amp/hrs - perhaps a set of L16 'A' Trojans (325 Amp/hrs).

    I have to say this is a somewhat backward way to design a system. But if it's what you've got to work with, it's what you've got.
  • BilljustBillBilljustBill Solar Expert Posts: 219 ✭✭✭
    Re: What size batteries for 48v and 30 amps?
    If I read you right you've got a total of 2080 Watts? With a typical 80% efficiency that would be 1664 Watts.
    At 58 Volts charging you'd get about 28 Amps. The standard recommendation of 5% - 13% means it could theoretically charge 215 to 560 Amp/hrs. A good size would be around 300 Amp/hrs - perhaps a set of L16 'A' Trojans (325 Amp/hrs).

    I have to say this is a somewhat backward way to design a system. But if it's what you've got to work with, it's what you've got.

    Thanks for your help, Cariboocoot.

    I have to buy eight batteries just to get the system up and running. "IF" things in our country should "Go to Hell in a Handbasket", buying enough batteries with the quality that will last through tough economic and inflationary times ahead, I can't afford to shorten battery life because of low panel output.

    Yes, it is a 'different' way of building a solar panel project. As the economic times have been somewhat "nail biting" in the last year or so, having "something" that works in case the bottom fell out was a good feeling.

    I've been collecting the parts to this Off Grid solar project as bargains are found. I'd found two used 120 watt American Signal/Siemens panels for $22.50 each, four new 100 watt 'China Made' panels from an Internet auction for $500, and four used Siemens 75 watt panels for $160.

    If I had a large cash budget, I'd buy all of them with the same wattage and of the same brand. But, alas, I collect the panels and support items as time, money, and bargains are found. I began this solar project after testing the 120 watt panels last August of 2009. Funding this project so it's paid in full as I go, takes time, work and flexibility. I used $265.00 in clad silver I've found while metal detecting toward two new 90 watt 'China Made' panels making their cost less than half price. Because I'm an "American Picker", I have had a large garage sale and two different months of selling at the local flea market, and raised a little more than $3,000 in cash.

    With some of those profits, I've purchased the TriStar 60 controller for under $195, reel of new #6 copper wire for $85, metal for array frameworks $50, four new black-frame Kyocera 135-SX panels (including shipping to Texas) for $385 each, and two sets of six carbon fiber wind generator blades for under $60 each.

    Since seeing the need for adding a 48 volt perm. mag. homemade wind generator into this project, and having an average to good wind site, I'll probably use the TriStar controller for it. Then, build the framework for two ground based tracking panel arrays, so I can more closely match each array with sets of strings and two smaller MPPT controllers to get the most out of this 'hodge-podge' of bargain panels.... ;>)

    What do you think?

    Bill
    Bill
  • RCinFLARCinFLA Solar Expert Posts: 1,469 ✭✭✭✭
    Re: What size batteries for 48v and 30 amps?

    You may not be able to yield the full potential wattage from panels. List the specific model numbers or Voc, Isc, Vmp, and Imp of each panel.

    There are two Tristar 60 controllers, one is PWM and the other is MPPT. From the price you paid I assume you have the PWM controller.

    A PWM controller just directly connects and disconnects the batteries to the panel to regulate battery voltage so you can lose a lot if the panels have significantly higher Vmp then battery voltage.

    As to mixing different type panels you need matching Imp to series connect them. You need matching Vmp to connect in parallel to match them for max power with an MPPT controller. This is pretty difficult with mix make panels.

    With 48v battery array, you will likely have to connect panels in series to get their combined voltage above battery voltage.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What size batteries for 48v and 30 amps?

    You might be better off putting the smallest panels on a separate PWM controller - the specs are likely to be quite a bit different from the larger (over 100 Watt) ones as RcinFLA says. Loss of power from connecting mis-matched panels can be significant.

    If this were a 12V system I'd suggest separate PWM controllers for each matched set of panels. Not sure of the availability of such controllers for 48V.
  • BilljustBillBilljustBill Solar Expert Posts: 219 ✭✭✭
    Re: What size batteries for 48v and 30 amps?
    You might be better off putting the smallest panels on a separate PWM controller - the specs are likely to be quite a bit different from the larger (over 100 Watt) ones as RcinFLA says. Loss of power from connecting mis-matched panels can be significant.

    If this were a 12V system I'd suggest separate PWM controllers for each matched set of panels. Not sure of the availability of such controllers for 48V.

    Okay, I understand some of what you are saying. If I might clarify as my learning curve continues, I plan for wiring of the panels' wattages, it might help you and other members.

    All the ratings I listed have 4 of equal wattages on each strings, i.e. Four 75 watt Siemens panels. Three of the five strings have matching wattages and brand names.

    Each string equals 48 volts and has as equal Amperages as the Spec. sticker/Amp Meter reflects. Unless I get a very good offer on some of the used panels, and sell them to buy new and matching panels, here is how I see using what I will have:
    The 75 watt string and 90 watt string on one controller, the 100 watt and 120 watt strings on another controller, and the new Kyocera 135-SX panels are wired to another controller. The distance between the two arrays and the batteries, and amount/low cost of #6 copper wiring isn't a problem.

    So, do you believe that there will still be a higher than normal loss? Would you have to put a blocking diode on the lower wattage strings to prevent the higher wattage from back flowing?

    Bill
    Bill
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What size batteries for 48v and 30 amps?

    It isn't just a matter of Watts or nominal Voltage rating; it's the Voc and Imp that will make the difference. In general, 10% tolerance is allowable. But a 90 Watt panel and a 135 Watt one are definitely going to be incompatible in the same string/array. It would be best to keep only like panels together, and each array of same charging the batteries through its own controller. Whether or not it's practical to do this is another issue.
    What's you nominal system Voltage? I'm thinking if it's 12 or 24 then the smaller panels wired through PWM controller(s) and the larger ones taking advantage of MPPT might be the way to go.
  • BilljustBillBilljustBill Solar Expert Posts: 219 ✭✭✭
    Re: What size batteries for 48v and 30 amps?
    ....
    What's you nominal system Voltage? I'm thinking if it's 12 or 24 then the smaller panels wired through PWM controller(s) and the larger ones taking advantage of MPPT might be the way to go.

    The batteries, panels, controllers, and inverter is all 48 volts operating at higher charging voltage in the 50's....hence the eight 6-volt batteries..............
    Bill
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