Fusing a battery bank

plongsonplongson Solar Expert Posts: 113 ✭✭
Cruising the forum and reading some threads that had comments on fusing battery banks got me really thinking if I have enough protection built into my system. I'd sure hate to have a failure and burn the place down...LOL So maybe you can give me some suggestions, I don't have any pictures loaded to show my setup, but it's not too unusual.

Essentially I have:
(16) L-16's Surette's. Two strings totaling 800Ah/48vdc with no protection between strings.
120" of 4/0 leading to the inverter unprotected until it hits the breakers in the Magnum panel. All else is breakered (PV-combiner-inverter) but I'm wondering about the batteries.

Should I install a fuse between strings and on the 4/0 lead to the inverter? And what size fuse is recommended for both? I liked the look of the Bluesea stuff but wonder about correct amp sizing for this setup.

Do you guys have any pictures of what you have done to give me a idea?

THANKS!

Paul
3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 

Comments

  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Fusing a battery bank

    ideally each string should have a fuse or breaker on it, but if you don't do that then at least put one there where the battery strings combine to go off in your 4/0. your magnum will draw 144a max continuously so a breaker or fuse with a higher rating than that is in order. minimally a 175a could be used, but if you want to you could go higher (up to 250a) as your wiring would allow it and this extra could be seen by a current surge from the inverter even though it won't be there continuously. see page 18 of your manual.

    if fusing or putting circuit breakers on individual battery strings you should divide the current up the max would be to the main 4/0 cable. if the main is between 175a and 250a then you can use a fuse or cb on each string that is 1/2 that value being you have 2 strings. 100a and 125a are both common values you could use and if done on the individual strings you won't need the main breaker or fuse unless you want that option for disconnecting purposes.

    edit to add;
    if you fuse the individual battery strings you can put a switch in as a main disconnect so a cb or fuse isn't needed to act as a disconnect.
  • plongsonplongson Solar Expert Posts: 113 ✭✭
    Re: Fusing a battery bank

    Thanks! Niel...I'm really going to look into it. I don't like being on the short side of protection...I'm going to order up some fusing now.

    Paul

    EDIT

    Can this Bluesea double fuse arrangement be used like this?

    fuse.jpg
    3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 
  • BB.BB. Super Moderators, Administrators Posts: 29,509 admin
    Re: Fusing a battery bank

    Assuming that the Fuse Block is connected to the positive terminals of both batteries (i.e., 12 volt batteries in parallel)--I probably would not do that. I believe in keeping the current path for charging/discharging batteries to be equal. In this case, you have an extra fuse in series with the right hand battery bank.

    From my point of view, fuses have pretty significant voltage drops under higher current (you need V*I, V2/R or I2R heating to "pop" the fuse). With 12 volt batteries, I would not want much more than 0.05 to 0.10 volt difference between strings (and ideally, close to zero volt difference for proper current sharing).

    You could connect one battery to one fuse top terminal. And the second fuse top terminal to the second battery. Then the bus bar output goes to the loads/charging sources...

    Note, this means that you have 100/175 amps (depending on which fuse you choose) from each battery, but 200/350 amps out the common terminal (you either need a heavy cable/bus bar common, or another fuse/breaker on the output to limit to 175 amps maximum (or whatever you decide).

    If you have a heavy cable/bus bar that can carry 200/350 amps and then use specific fuses/breaker for each + cable leaving the bus bar (i.e., 100 amps to an inverter, 80 amps to a solar charge controller, 15 amps to a DC lighting circuit, etc.).

    As always, try to balance wire resistance for parallel strings (per Smartgauge website). And, I always like to put a heavy on/off/disconnect of some sort from the battery bank. Both for emergency power off (before fuses pop), and to turn stuff off when in storage.

    Note an A/B/Both (or 1/2/both) switch can be handy. Both for normal operation, "A" to your loads and "B" to your charging device(s). Allows you to turn off your loads but leave your solar/shore power available for charging batteries--But you can turn off everything to isolate problems/servicing.

    wind-sun_2203_7473556Blue Sea Battery Switch 1-2-OFF 350 Amp


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Joe_BJoe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    Re: Fusing a battery bank

    You will want to specify a type "T" fuse, these are recomended for inverter protection.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Fusing a battery bank

    no not that way. out of curiosity, in what are you combining the 2 strings at this time?

    ideally string 1 will have a 100a to 125a fuse or breaker as well as string 2. from the fuse or breaker it then goes to be combined either on a bus or in a combiner box. from there it could be switched, but as was pointed out already the blue seas switch can accommodate two separate sources and it can act like a combiner. if you don't put the individual fuse/breaker on each string you will need a 175a-250a fuse/breaker as close to the batteries as possible after they were combined.

    i'm not good at descriptions or pc drawings so maybe somebody could draw you some pics of what i'm describing. for there to be no doubt i recommend a fuse/breaker in series with the last + of each string and ideally you should take that to the blue seas switch bb described and the output of that switch to go to the controller. of course, keep symmetry between the battery strings so that no differences appear.
  • plongsonplongson Solar Expert Posts: 113 ✭✭
    Re: Fusing a battery bank

    Hey all,
    Here is a picture of the system as it exists now.



    [URL="[IMG]http://i1199.photobucket.com/albums/aa471/plongson/IMAG0105.jpg[/IMG]"]IMAG0105.jpg[/URL]

    Is this more like what you are talking about??



    Attachment not found.
    3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 
  • BB.BB. Super Moderators, Administrators Posts: 29,509 admin
    Re: Fusing a battery bank

    No, I intended to connect each string to its own fuse.

    The 1-2 switch would connect to the fuse common bus connection, then (for example) to switch common. Switch one to your solar/generator charger and switch two to your loads. Allows you to isolate loads (when on vacation or power down the charging circuits for service)...

    We also have talked about being able to isolate battery strings for equalization or isolation if a cell fails.

    Sounds good in theory, but in practice more complexity can cause confusion and more things to go wrong.

    Six of one, half dozen of another.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • samuelsamuel Solar Expert Posts: 80 ✭✭✭✭
    Re: Fusing a battery bank

    Paul,

    Bluesea makes a sturdy product. I have a fuseblock+fuse from them in my system (with a 350Amp class T fuse, but knowing what I know now I'd have gone with a 250Amp fuse - in the event of a short it probably won't matter much).

    I'm also using a small fuseblock from Bluesea as a distribution panel for some 12V lighting and automotive outlet in the cabin. Overall I'm impressed with the build quality and very pleased with their products so far. In fact, I wish I would have picked up the battery isolater switch Bill pointed out instead of the one currently in use at the cabin (my Dad and I have had one or two conversations about replacing our current switch with something better; we don't like how the current switch will stick in the "ON" position in the cold).

    Pictures can be found via the link in my signature or here is a link to a picture:

    Sam
  • plongsonplongson Solar Expert Posts: 113 ✭✭
    Re: Fusing a battery bank

    I hope I'm not driving you guys nuts but I'm really trying to get this straight.

    Look at this change, I added a fuse to each string and re-drew the line/load from the switch. I presently have a 175 breaker inside the Magnum panel feeding in from the batteries so ditched the Blusea 175 T-Fuse. Does this look like adequate protection? Does it create a weird charge imbalance?

    I like the option to switch from bank 1 to 2 (shown here) but I already have a transfer switch to go from the from the inverter to genset so I see no need for another option.

    THANKS FOR THE FEED BACK!!

    Attachment not found.


    Attachment not found.
    3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 
  • BB.BB. Super Moderators, Administrators Posts: 29,509 admin
    Re: Fusing a battery bank

    Switch/fuse schematic is fine... You do have one possible issue of "resistance imbalance". The Negative jumper from the bottom string to the upper string adds ~12" of extra resistance.

    If the rest of the +/- wiring is equal--In theory, you should make one of the battery jumpers or the fuse to switch block ~12" longer to balance out the extra negative length. Or create a little negative/common bus connection (even a simple brass nut and bolt to bring all three connections together at a common point):
    [FONT=fixedsys]
    
    Upper String-----------\
                         BusBolt=========Negative DC power panel
    Lower String-----------/
    [/FONT]
    

    Just trying to keep your banks in balance. Batteries have very low internal resistance and a few feet of wire can make a large difference in current sharing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • plongsonplongson Solar Expert Posts: 113 ✭✭
    Re: Fusing a battery bank

    Something more like this?? And keep all cables on the red side equal as well?

    Attachment not found.
    3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 
  • BB.BB. Super Moderators, Administrators Posts: 29,509 admin
    Re: Fusing a battery bank

    Perfect. That will do nicely.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Fusing a battery bank

    by jimminie, i think he's got it.:D:p
  • plongsonplongson Solar Expert Posts: 113 ✭✭
    Re: Fusing a battery bank

    Thanks guys! I'm not always the fastest horse in the race, but I usually finish!! LOL

    I really appreciate the help.

    Paul
    3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 
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