Fuse for battery

jackbombay
jackbombay Solar Expert Posts: 46 ✭
in Solar Beginners Corner #1
What do people typically use for a fuse on a house battery in an RV? I have a 200 AH AGM battery, and I did have it set up without a fuse for a while, but I know that's bad, and I'm setting things up differently now and want to include a fuse, something like this seems like it would be fine, yea?

http://www.amazon.com/Earthquake-Sound-Fuse-Block-Holder/dp/B00CBB72AM/ref=sr_1_5?s=automotive&ie=UTF8&qid=1404748882&sr=1-5&keywords=100+amp+fuse

Seems like 100 amps would be ok, or should it be smaller?

Thanks!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #2
    Re: Fuse for battery

    What are your loads (and AC inverter, if you have one) ratings?

    And what is the size of your main wiring (gauge)?

    Adding a main battery bank fuse/breaker is a good thing (many people go with breakers, much easier to shut down power for servicing when needed). But you should also have fuses/breakers sized for each positive wire leaving for branch circuit/load...

    I.e., perhaps you want 100 amp main bus breaker, but a 15 amp breaker for a 14 AWG wire going to the DC lighting/fan in the system. And a 40-50 amp breaker for a 300 Watt 12 VDC AC inverter circuit.

    Fuses/breakers are their to protect the wiring against shorts/fires... And wires are sized to support your maximum planned loads.

    And wiring/fuses/breakers should be 1.25x (minimum) the maximum continuous expected current:

    15 amp circuit * 1/1.25 derating = 12 amps maximum continuous current

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #3
    Re: Fuse for battery

    Ahhh, all very important pieces of information!

    I do have an inverter, 1200 watt (it came with the van) but I rarely run anything off it that draws a "lot" of power, if I do I start the van and hit the combiner switch so the alternator can shove electrons at the inverter. The tag on the inverter says it pulls 120 amps at 12V.

    all the other loads are low, LED lights, efficient compressor fridge (truckfridge, 1.4 amps IIRC)

    the wire (cable?) from the house battery to the alternator/inverter is 4 gauge.

    Thanks for the info on the sizing of auxiliary circuits, that'll be pretty easy to set up and all makes sense of course.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #4
    Re: Fuse for battery

    You can choose what "standard" you want to use.

    NEC is pretty conservative (there is fill factor and ambient temperature deratings, but as a start):

    Wire     Current Ampacities NEC Table 310-16

    And the ABYC wiring chart which is much less conservative:

    http://www.boatus.com/boattech/articles/choosing-cables-and-terminals.asp

    4/0 is good for around 195 to 445 Amps... depending on your assumptions and risk tolerance.

    A 1,200 Watt 12 VDC inverter, assuming running at 1,200 watt continuous (not all really meet their marketing spec) would need:
    • 1,200 Watts * 1/0.85 inverter eff * 1/10.5 volts battery cutoff * 1.25 NEC derate = 168 Amp minimum Branch circuit + fusing/breaker

    A 200 AH AGM Battery can, in theory, output upwards of 2-4x its rated AH capacity (200 AH * 2 = 400 Amp output).

    If this was a flooded cell, it would be ~1/2.5 for maximum surge current for reliable operation (200 AH / 2.5 = 80 Amp).

    In any case, you can see that running a 1,200 Watt inverter on a 200 AH AGM battery will give you around 1 hour of power--That may, or may not, be useful for you.

    In general, you probably would not pull more than C/8 for average power (~4 hours of load, to 50% maximum discharge). That would give you an average "useful design" current of:
    • 200 AH / 8 = 25 amps nominal
    • 25 amps * 12 volts * 0.85 inverter eff = 255 Watt nominal maximum from AC inverter
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Mustang65
    Mustang65 Solar Expert Posts: 42 ✭✭
    #5
    Re: Fuse for battery

    The fuse between the house battery and the RV charge controller Should be a 30 amp inline fuse. The inverter should be wired directly to the house battery with its own fuse. I have a 1500 watt inverter and I used a 300 amp ANL fuse. My 1500 watt inverter documentation suggested a 300 amp.
    Don

    2013 Jayco Eagle 284BHS
    250Watt Grape Solar Panel, MorningStar MPPT 60 Charge Controller
    1500 Watt Ramsond PSI, 2 Trojan T145 Batteries (260Ah)
    2 - AirSight Wireless IP Cameras (used as rear view cameras)
    EnGenius WI-FI extender, D-Link wireless (n) modem
    MagicJack Internet Phone
    2012 Ford F150XLT, EcoBoost w/3.73
    157" Wheel base, HD Towing Package
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #6
    Re: Fuse for battery

    The inverter generally is used to recharge cordless tool batteries, 3 at a time at most, which is about 30 amps of draw on the house battery, but that would be for 30 minutes at most. Occasionally I'll use the inverter to power some of my 110V tools, but I do let the engine idle so the alternator does a bunch of the work in such a situation.

    Based on what you've posted, BB, I'm going to go with a 200 amp fuse from the battery to the inverter, the inverter will be located directly next to the battery so the complete run of wire to and from the inverter to the battery will be, maybe 3' total (18" pos 18" neg) so the 4 gauge will be plenty.

    The van does have a 150 amp alternator, but I think it would be a really rare situation where the alt would throw more than 100 amps at the battery, of course the battery really shouldn't have a whole lot more more than 30 pushed into it. I do have an amp meter and shunt that I'll be setting up with this new configuration so I can see how many amps are hitting the battery from the alternator under various conditions. At any rate, based on all this I think a 100 amp breaker from the alternator to the battery will be about right, sound good?
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #7
    Re: Fuse for battery
    Mustang65 wrote: »
    The fuse between the house battery and the RV charge controller Should be a 30 amp inline fuse.

    Thats what I was planning on, I have a bunch of inline fuses for that sort of stuff :-)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #8
    Re: Fuse for battery

    While the alternator will seldom output 150 Amps... I would still fuse it at 175-200 amps (150 * 1.25 = 187.5 amp rated fuse/breaker/wiring). Assuming the cabling is capable of >150 Amps.

    The problem with a breaker/fuse/fusible link that may be undersized, when the over current device trips, the output of the Alternator will probably go >>120 Volts and wipe out the electronic regulator and possibly the rectifier assembly.

    You really only want the fuse/etc. to open if there is truly too much current going through the cabling. Anything else may just cause you expensive grief.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #9
    Re: Fuse for battery

    Cool, I will be going with 200 amp fuses on the house battery and also from the alt to the house battery so the run of cable from the alt to the house battery will be protected on both ends as it should be because both ends can be hot.

    I ended up getting 2 of these fuse holders with some 200 amp fuses,

    http://www.amazon.com/gp/product/B0002PUI82/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
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