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Thread: battery fuses

  1. #11
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
    Posts
    20,783

    Default 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.

  2. #12

    Default 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.

  3. #13
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
    Posts
    20,783

    Default 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.

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