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  • battery fuses

    I have an Off Grid system that has 14 L-16 6 volt batteries. They are for a 12 volt system so there is a string of 7 with two batteries in series. The only fuse is between the batteries and the inverter. From reading this forum I feel that for safety I need to put fuses on them. I would like some advise on the size of fuse to use and the placement of the fuses. Since two 6 volt batteries are connected together to get 12 volts I was thinking of attaching a fuse between the batteries. The connection on the battery is a 3/8 inch bolt, if I get a fuse that has a 3/8" connection at each end could I just connect the fuse between two batteries. Using 7 fuses. Would that work?

    I am interested any suggestions and ideas concerning size and placement.

    Thanks

    Dan
    Grid Tie 9 175w Sharp panels with Fronius 2000 on tracker and 22 bosch 230w fixed mount with SMA 7000.

  • #2

    Re: battery fuses

    Re: battery fuses

    Dan,

    Welcome to the forum...

    Yes, paralleling lots of big batteries together has the potential for lots of short circuit current (for me, I am probably more scared of your size battery bank than working on a home breaker panel...

    Fuses are there to protect the wiring and be the weak link in the system. And in the case of paralleled battery systems (with 3 or more strings), it prevents the other batteries from feeding a dead short on one string...

    So, the questions:
    • What is the maximum current you expect to pull (i.e., 1,500 watt inverter with 3,000 watt maximum surge).
    • What is the gauge / cable type of the individual battery cables.
    You will have all the + and - battery connections end at a common +/- bus connection point. Each cable leaving that point should have a fuse or breaker there to protect the wiring leaving the + bus connection point (assuming negative ground).

    For example, you have 60 amp charge controller (solar/AC/etc.) on a:
    • 60 amp * 1.25 NEC safety factor = 75 amp minimum circuit
    Next size fuse/breaker would be 80 amps (note, Outback Solar rates some breakers for ~63 amps for a 60 amp circuit--It sounds like they are really closer to 80 amp breakers--you will need to look at that closer if you choose Outback hardware).

    And, you will need to size the cable for 80 amps using NEC or other equivalent cable rating chart (aka boating chart, etc.). NEC is conservative, others not so much.

    Note that with solar, and especially 12 volt solar, you have very little room for voltage drop (i.e., 12 volt battery, 11.5 minimum voltage and 10.5 cutout on the typical inverter => 1.0 volt maximum drop). For long cable runs, you will probably need to use a voltage drop calculator and up-size the cables for low voltage drop.

    Personally, when I look at paralleled power circuits--I would size the fuse/breaker for your bank to be at least ~1/2 of the maximum current you expect to draw from the bank (and not 1/7tth even though you have 7 parallel strings). This should reduce "nuance" trips.

    Current sharing can be difficult on paralleled battery banks... The battery and cable resistance if very low, so small issues (loose/dirty connection, bad cell, etc.)( can dramatically affect current sharing.

    Lastly, as you reconfigure your parallel battery connections, you should aim for equal path length through all of those various parallel battery paths... An extra long path will add resistance and reduce current load on that string. An extra short path will increase charging/discharging current on that string.

    A nice explanation of how to do the path balancing:

    www.smartgauge.co.uk/batt_con.html

    And, if you can justify, I would suggest a DC Current Clamp meter (such as this one for ~$60, same as this model--just private labeled)... It can make the monitoring of shared current flow in your bank much easier and quicker (bad connections, shorted/open cells, etc.)... Just do your measurements both under heavy load and heavy charging currents (note: meter is just a low cost suggestion--pick your own based on your needs).

    Fuses, breakers, switches... some starting points:

    High Current Fuses and breakers
    Battery Bank Switches

    Remember one issue about cable termination--Taking terminal tabs and soldering them on cables is usually not a good idea--Cables can work harden and break at solder point and/or in a short circuit, the solder can melt and the cable comes loose--causing more issues.

    If you want to terminate your own, you might find a reasonable priced hydraulic crimper or a "hammer crimp tool".

    Questions?

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

    Comment


    • #3

      Re: battery fuses

      Re: battery fuses

      Now I have a question. The diagrm shows one row of batteries, what happens if there is two rows of batteries? is each pair of batteries treated as a single battery or is each group wired as showen? What my mind is telling me is each group is wired as diagramed and then they geat seriesed?

      Chuck
      3000 watts solar pannel, 12 Crown CR430 6volts @ 24 volts 2-VFX3524,FM 2 FM80 charge controllor, (Turned off Bergey XL1 wind gen), 11KW Kolar gen 15 plus years off grid in AZ and still learning

      Comment


      • #4

        Re: battery fuses

        Re: battery fuses

        Chuck,

        I am not sure I understand your question... But if it is, for example, you have 2x 6 volt batteries in series for 12 volts--Yes, that pair of batteries is treated like "one battery"--And then each pair of series connected 6 volt batteries is then wired in parallel according to the Smartguage chart.

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

        Comment


        • #5

          Re: battery fuses

          Re: battery fuses

          Originally posted by Chuck46 View Post
          Now I have a question. The diagrm shows one row of batteries, what happens if there is two rows of batteries? is each pair of batteries treated as a single battery or is each group wired as showen? What my mind is telling me is each group is wired as diagramed and then they geat seriesed?

          Chuck
          Other way 'round. When you connect two batteries in series they become one higher Voltage battery, so to speak. As in the common practice of two 6 Volt "golf carts" making "one" 12 V 225 Amp hour string. Parallel those as per the SmartGauge diagram. As Bill said, each string would have a fuse for best safety. And yes it can be between the batteries, as that connection is also part of the circuit.

          Attempting to wire them with extra inter-battery connection (paralleling the inter-cell links) isn't a good idea as it creates alternate current paths and eliminates the efficacy of fusing as described above. Someone on the forum recently tried that, with firework-like results.
          1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

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

          Comment


          • #6

            Re: battery fuses

            Re: battery fuses

            Bill:

            Thanks for the reply. I have a Trace 2512 inverter that was put in service in 1999. The batteries are also 12 years old. I purchased the cabin a couple of years ago. For most of the life of the batteries they we only charge with a 6kw generator but last summer I added 4 175 watt sharp panels with a FM80 charge controller. I also put up a Whisper 100 wind turbine. Between the solar and wind my diesel consumption was cut in half. The cabin is in the mountains and is use mostly for 2-3 days on weekends. My thoughts were since in the middle of winter use is much less (dark and cold) giving the batteries a little charge may help the out. We get many more -30 degree days than 70 degree days over the year.

            The battery bank is wired like method # 1 in the smartgauge web site. There use to be 8 strings on the battery bank but one battery was leaking and another was not putting out much and bulging, it probably had frozen. These two batteries were the furthest from the inverter. There is a large fuse between the inverter and the first battery but I do not know what size it is but will find out the next time I am there.
            The wire from the inverter to the batteries is 2/0 and the jumpers between the 6 volt batteries is 1/0.

            I understand the the batteries are getting toward the end of there service life but at the present time they seem to be serving our needs very well. The specific gravity is good in all the cells and when charged they are pretty close. We use between 100 and 200 amp/hours per day when there. The loads are mainly lights and some electronics. There are a couple of fuel oil heaters that have small fans. The stove and fridge are on propane. I also added a well last summer. It has a one HP pump but I only use it to fill the tanks in the garage. I always start the generator when pumping out of the well and the tanks are good of 2-3 days. We have a 1/2 hp pressure pump to charge the system that run off the batteries.

            I consider this set of batteries to be my learner set. If I can keep them going a little while longer I can probably take good care of a new set when I need them.

            So my present plans are to switch the battery cable set up to method # 2. I also need to fuse all the battery strings. Advise on how to do that would be welcomed.

            Thanks

            Dan
            Grid Tie 9 175w Sharp panels with Fronius 2000 on tracker and 22 bosch 230w fixed mount with SMA 7000.

            Comment


            • #7

              Re: battery fuses

              Re: battery fuses

              Well what I ment is I have 4 6volt batteries in series for 24 volts this is row one row two is the same 4 6volt for another 24 volts, now they are paralleled to up the amps. So is each row of 4 for 24 volts treated like one battery?

              Chuck
              3000 watts solar pannel, 12 Crown CR430 6volts @ 24 volts 2-VFX3524,FM 2 FM80 charge controllor, (Turned off Bergey XL1 wind gen), 11KW Kolar gen 15 plus years off grid in AZ and still learning

              Comment


              • #8

                Re: battery fuses

                Re: battery fuses

                Originally posted by Chuck46 View Post
                Well what I ment is I have 4 6volt batteries in series for 24 volts this is row one row two is the same 4 6volt for another 24 volts, now they are paralleled to up the amps. So is each row of 4 for 24 volts treated like one battery?

                Chuck
                Yes; the series string of four works like one big battery. In essence, and single 6V battery is actually three 2 Volt cells in series, it's just that the connections are internal rather than external.
                1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

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

                Comment


                • #9

                  Re: battery fuses

                  Re: battery fuses

                  Dan (and Chuck),

                  My computer ate my post (actually windows decided that it was the perfect time to run updates).

                  Anywho... As Caraboo/Marc said.

                  Shorter reply on fusing/breakers... I am a hacker--and even if I try my wiring still looks messy. Here are some suggestions to start looking:

                  Solar Panel Array Combiners, Breakers, Fuses
                  High Amperage Inverter Fuses & Breakers

                  Marine fuse holders are also good for 12/24 volt systems (typically 32 volt maximum rating):

                  1 or 2 fuse holder for mounting on a battery directly (very compact)
                  Various Marine fuse holder systems
                  Some Marine holders also have a negative bus too

                  You can look at automotive fuse holder systems... Those stereo guys can use a lot of power:

                  Various automotive holders

                  And a very simple/inexpensive fuse block for 6x AGC glass fuses:

                  Prime Products

                  If you are making up your own large gauge battery cables, a hammer crimping tool can be useful (if you cannot justify the hydraulic crimper).

                  Hammer Crimp Tool

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

                  Comment


                  • #10

                    Re: battery fuses

                    Re: battery fuses

                    Bill thanks for the info. I think I will use these

                    http://bluesea.com/category/5/21/pro...e/overview/378

                    I am planning on putting them on the jumper between the 2 six volt batteries.

                    I was thinking a 100 amp fuse

                    Does that sound like a reasonable approach??

                    Dan
                    Grid Tie 9 175w Sharp panels with Fronius 2000 on tracker and 22 bosch 230w fixed mount with SMA 7000.

                    Comment


                    • #11

                      Re: battery fuses

                      Re: battery fuses

                      Assuming these are 420 AH flooded cell batteries, the maximum useful surge current of C/2.5 would be:
                      • 420 AH * 1/2.5 = 168 Amps
                      You have 7 parallel strings of batteries driving a 2,500 watt 12 volt inverter (assuming 85% efficient inverter):
                      • 2,500 watts * 1/10.5 volts * 1/0.85 eff * 1.25 NEC factor = 350 Amp Circuit
                      For a battery bank with lots of paralleled strings--I would, personally, try to design for 50% of the load to be carried by one battery (lots of paralleled batteries do not always share current well--designing fusing to 50% will prevent blowing fuses unnecessarily vs 1/7th of load or 50 amp fusing):
                      • 350 Amps * 0.50 = 175 Amp circuit per battery.
                      Depending on lots of things, a 1/0 cable is good for ~150-170 amps or so (even upwards of 245 amps in free air--NEC is pretty conservative).

                      So--I would probably pick the 175 amp fuse myself... This will limit issues with false trips if you have current sharing issues (bad cables, cells, etc.) but still protect wiring/batteries from excessive current (you still need to protect wires leaving the + bus bar/battery common point).

                      Since this is an old battery bank and system--Before you put a lot of money into the system wiring up the fuses/parallel battery bank... Does this make economic sense to you? New battery bank, new inverter, changing to 24 or 48 volt bank?

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

                      Comment


                      • #12

                        Re: battery fuses

                        Re: battery fuses

                        Bill:

                        Thanks for the help. I know that some day they will have to be replaced. It just comes down to a few hundred dollars for fuses and battery cable or a few thousand for a new battery bank and inverter.

                        I will probably go with the fuses at this time but I have been thinking about what I will replace them with when the time comes. Since we use 100-200 amp/hrs per day and are there 2-3 days at a time we could use a battery bank that would supply 3-400 amp/hr over two days at 20-25% DOD. I was thinking of 8 L-16 batteries in two strings at 24 volts. Is there any advantage in using one string at 48 volts? Also I do not know how to figure the size inverter I would need.

                        Thanks

                        Dan
                        Grid Tie 9 175w Sharp panels with Fronius 2000 on tracker and 22 bosch 230w fixed mount with SMA 7000.

                        Comment


                        • #13

                          Re: battery fuses

                          Re: battery fuses

                          One simple way to assess your AC loads is with a Kill-a-Watt meter... They are cheap and easy to use. You want to:
                          1. Measure running Watts
                          2. Measure running VA (Volt*Amps)
                          3. Estimate Peak Watts and VA (k-a-w is not good at peak power)
                          4. Measure kWH per day/week/etc. of your intended AC loads.
                          Running Watts are used to size your inverter / battery / array...

                          VA and Peak VA (Watts) is used to size your wiring, inverter rating, and wiring--and one of the checks on Battery AH sizing.

                          kWH per day will be used to estimate the AH rating of the Battery Bank and the Size of the solar array.

                          In general, for off grid power--conservation is king. For a small system, a 12 volt 300 watt TSW inverter is something good to aim at... A small system can drive a well pump or other short usage items, but everything has to be much larger to handle the short term loads/surge current.

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

                          Comment

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