where to put fuses

johnelarue
johnelarue Solar Expert Posts: 33 ✭
in Solar Beginners Corner #1
Hello again,

I've been running my small solar system for about 6 months now, no problems. Just realized forgot something, fuses

Where exactly should I put my fuses, what kind etc?

Also what size battery to battery wire/cable should I be using? Now using 4mm2(square) metric, approx 10 gauge? Everything seems to work fine, nothing heats up. But then again the system is not used often.

where on the board?
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there are 3- 50watt, 12v panels. 2- 105AH Delco sealed RV batts.

Loads are 6- CFL's 13watt, on a 120watt car inverter.

Thanks guys,

john
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
    Re: where to put fuses

    4mm^2 wire is around 12 awg -- NEC (US National Electric Code) would rate the wire at ~20 amps maximum. For Chassis wiring, around a maximum of 41 amps.

    Some other questions... How long are the wire runs? Voltage drop is a very serious issue with 12 volt systems... Lots of current and relatively high resistance of wire can cause issues.

    120 watt car inverter:
    • 120 watt * 1/10.5 volts cutoff * 1/0.80 eff = 14.3 amps maximum current
    For 12 volts, if you drop the battery to 11.5 volts (discharged under load), and assume the inverter turns off at 10.5 amps means you can have 1.0 volt drop. Assume the fuse/breaker has a voltage drop, lets give the wire a maximum of 0.5 volt drop. Using a handy wire voltage drop calculator, we find:
    • 14.3 amps by 9 feet one-way run (around 2.7 meters) on 12 awg wire gives us 0.49 volt drop
    Also, it looks like the car inverter is connected to the Low Voltage Disconnect on the solar charge controller?

    Generally, it is not required for an inverter (they turn off is battery is dead anyway), many LVD cannot pass the 15-30 amp (surge) from the inverters (LVD will turn off or get damaged), and LVD+extra wiring causes more voltage drop.

    Usually, a short, heavy wire run to the inverter from the battery directly is best.

    Another question is the wire length from the charge controller to the battery bank. Looking at battery voltage vs state of charge, we see that a 0.1 volt difference in battery voltage represents around a 10% change in state of charge. Also, the difference between full charging voltage of ~14.4 volts and float voltage of ~13.6 volts is 0.8 volts--So, for proper battery charging current, we probably would not want more than a 0.1 volt drop between the charge controller and the battery bank for proper/quick battery charging.
    • 3x 50 watt panels * 1/17.5 volts typical solar PV Vmp = 2.9 amps max charge
    Voltage drop for 12 awg wire, 2.9 amps, and 0.1 volt drop gives us a maximum one way wire run of 9 feet.

    Fuses/circuit breakers are there to protect the wiring from shorts... In an off-grid solar system, the typical high-current energy source is the battery bank... So any wire leaving the positive battery post (assuming negative ground system) we would protect any wire leaving the battery bank with a fuse/breaker based on wire size. So place the fuse "close" to the battery bank (close is a relative term--for boats, I believe that is ~1.5 feet maximum).

    For national electric code, fuses/wiring is 1.25x higher than maximum continuous current... 14.3 amps from inverter:
    • 14.3 amps * 1.25 = 17.9 amps minimum fuse/wire rating
    For the inverter we would round up to 20 amp rated wire fusing (minimum).

    And 12 awg wire is rated for ~20-41 amp fuse/breaker. 20 amp fuse would be "conservative". Up to 40 amp fuse would be "safe" for your application.

    If you have a volt meter handy--you can measure the voltage drops (from controller to battery under full sun/charging current; and from battery to inverter under load) to double check your actual voltage drops are within your needs (if the inverter is never operated at 120 watts, then you don't need as heavy wire/fuses).

    Also, do you have a switch (you can also use a circuit breaker) between the inverter and the battery bank--Inverters typically take a few watts to run even if there is no load. On most small systems, you need to "turn off" the DC to the inverter to prevent battery discharge when the battery is turned off--Otherwise the inverter "idling" can kill the batteries.

    Also, how you wire the batteries can affect how well they charge/share loads... Take a look at this link for information:

    SmartGauge battery wiring configuration information

    Lastly (in this too long post :roll:), you need to pick fuses/breakers based on the maximum voltage and short circuit current. For a 12 volt system--good quality car fuse/breakers are usually OK.

    -Bill

    PS: I should add that charge controllers and inverters need good ventilation to keep cool. Also, charge controllers should be at the same temperature as the batteries so the charge controller is at the same temperature at the battery bank.

    The plastic box with the cover on may keep too much heat inside. Also, it is not a great place to run wiring (over heated wiring could set plastic on fire).

    Lastly (really lastly this time, this post :roll:), typically a battery bank should be charged at 5% to 13% of rated capacity. Roughly:
    • 2* 105 AH * 0.05 = 10.5 amps minimum
    • 2* 105 AH * 0.13 = 27.3 amps maximum
    Depending on the type of battery (AGM, Gell, etc.), you might want to get a nice 10 amp 100 VAC battery charger to recharge the battery after outages.

    Your 150 panels may be OK for "maintenance" charging--but at 3 amps is a bit small to provide much more power.

    Monitoring your battery bank state of charge (accurate digital volt meter) to insure the batteries are properly charged (and float charging) will help you give the batteries a nice long life.

    Deep Cycle Battery FAQ
    www.batteryfaq.org
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • johnelarue
    johnelarue Solar Expert Posts: 33 ✭
    #3
    Re: where to put fuses

    Thanks very much BB for the lengthy reply,
    BB. wrote: »
    4mm^2 wire is around 12 awg -- NEC (US National Electric Code) would rate the wire at ~20 amps maximum. For Chassis wiring, around a maximum of 41 amps.

    Some other questions... How long are the wire runs? Voltage drop is a very serious issue with 12 volt systems... Lots of current and relatively high resistance of wire can cause issues.


    BB wrote:
    • 14.3 amps by 9 feet one-way run (around 2.7 meters) on 12 awg wire gives us 0.49 volt drop
    Also, it looks like the car inverter is connected to the Low Voltage Disconnect on the solar charge controller?

    Generally, it is not required for an inverter (they turn off is battery is dead anyway), many LVD cannot pass the 15-30 amp (surge) from the inverters (LVD will turn off or get damaged), and LVD+extra wiring causes more voltage drop.

    I assume the LVD is where it says LOADS and has a picture of a lightbulb? So I should hook the loads to the battery itself it seems.? Sorry for my blinding obliviousness.

    BB wrote:
    Usually, a short, heavy wire run to the inverter from the battery directly is best.

    Another question is the wire length from the charge controller to the battery bank. Looking at battery voltage vs state of charge, we see that a 0.1 volt difference in battery voltage represents around a 10% change in state of charge. Also, the difference between full charging voltage of ~14.4 volts and float voltage of ~13.6 volts is 0.8 volts--So, for proper battery charging current, we probably would not want more than a 0.1 volt drop between the charge controller and the battery bank for proper/quick battery charging.
    • 3x 50 watt panels * 1/17.5 volts typical solar PV Vmp = 2.9 amps max charge
    Voltage drop for 12 awg wire, 2.9 amps, and 0.1 volt drop gives us a maximum one way wire run of 9 feet.

    From batteries to controller is only 3 feet or so , so looks ok.

    BB wrote:
    Fuses/circuit breakers are there to protect the wiring from shorts... In an off-grid solar system, the typical high-current energy source is the battery bank... So any wire leaving the positive battery post (assuming negative ground system) we would protect any wire leaving the battery bank with a fuse/breaker based on wire size. So place the fuse "close" to the battery bank (close is a relative term--for boats, I believe that is ~1.5 feet maximum).

    For national electric code, fuses/wiring is 1.25x higher than maximum continuous current... 14.3 amps from inverter:

    • 14.3 amps * 1.25 = 17.9 amps minimum fuse/wire rating
    For the inverter we would round up to 20 amp rated wire fusing (minimum).

    And 12 awg wire is rated for ~20-41 amp fuse/breaker. 20 amp fuse would be "conservative". Up to 40 amp fuse would be "safe" for your application.

    Sounds like the easy part :)

    BB wrote:

    Also, do you have a switch (you can also use a circuit breaker) between the inverter and the battery bank--Inverters typically take a few watts to run even if there is no load. On most small systems, you need to "turn off" the DC to the inverter to prevent battery discharge when the battery is turned off--Otherwise the inverter "idling" can kill the batteries.

    There is a switch on the inverter itself that starts a cooling fan and the inverter itself.
    BB wrote:

    Also, how you wire the batteries can affect how well they charge/share loads... Take a look at this link for information:

    SmartGauge battery wiring configuration information

    Lastly (in this too long post :roll:), you need to pick fuses/breakers based on the maximum voltage and short circuit current. For a 12 volt system--good quality car fuse/breakers are usually OK.

    Have some ventilation holes in the wiring box there, will make some more. Not too long at all, thanks.
    BB wrote:

    PS: I should add that charge controllers and inverters need good ventilation to keep cool. Also, charge controllers should be at the same temperature as the batteries so the charge controller is at the same temperature at the battery bank.

    The plastic box with the cover on may keep too much heat inside. Also, it is not a great place to run wiring (over heated wiring could set plastic on fire).

    I can take the cover off when running the inverter. It's more of an anti-kid device. In this case would metal be better than wood?

    BB wrote:

    Lastly (really lastly this time, this post :roll:), typically a battery bank should be charged at 5% to 13% of rated capacity. Roughly:
    • 2* 105 AH * 0.05 = 10.5 amps minimum
    • 2* 105 AH * 0.13 = 27.3 amps maximum
    Depending on the type of battery (AGM, Gell, etc.), you might want to get a nice 10 amp 100 VAC battery charger to recharge the battery after outages.

    Your 150 panels may be OK for "maintenance" charging--but at 3 amps is a bit small to provide much more power.


    Monitoring your battery bank state of charge (accurate digital volt meter) to insure the batteries are properly charged (and float charging) will help you give the batteries a nice long life.

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    In other words the batteries need and occasional heavy charge that my panels can't provide, thus shortening their life span?

    Thank you very much again BB!

    john
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
    Re: where to put fuses

    You are very welcome John,

    Regarding the Load Connection on the Solar Charge Controller--You need the manual for your brand/model of charge controller and read the instructions. I cannot see what brand/model you have, so I do not know what its specifications are.

    Many LVD/Load connections are rated for 8 amps or so (8 amps * 12 volts = 96 watts). If you run you inverter at full power, it may cause problems for your charge controller.

    Kid-proofing your setup is important. The top of the batteries need to be covered too (keep metal objects away from battery connections--Batteries can output 1,000 amps or more into a dead short). Keeping children away from your battery/electrical connections is very important. Make sure there is air circulation (holes, screen material in box)--Even sealed batteries can vent hydrogen gas (if over charged or getting old).

    A metal box would be nice to protect your electronics. Wood would probably be better than a thin plastic box. Plastics tends to be very easy to set on fire (unless it is designed to be fire resistant--which your box probably is not).

    Regarding the size of your solar charge controller and solar panels... Flooded Cell (liquid electrolyte) Batteries need ~5% minimum charge rate to "bubble" the acid and mix it. Gell and AGM batteries do not (I do not know which type of battery yours is).

    The issue is what you are expecting from your system. For example, lets say that you discharge your system by 50% and now want to recharge your batteries back to 100%. For most people, they get around 5-6 hours of "full sun" during a clear summer day. To recharge your battery bank it would take:
    • 2 * 105 AH * 0.50 discharge * (1/2.9 amps charging*5 hours of sun per day*0.77 system efficiency) = 18.8 days to recharge
    So, just using your solar panels, it would take almost 3 weeks to recharge your battery bank from 50% use (and no additional loads).

    In general, lead acid batteries do not like to be stored below 75% state of charge for very long. If kept days/weeks/months below 75% state of charge, the batteries will "sulfate"--The lead sulfate that forms during discharge will gradually turn into crystals ("harden") and reduce the capacity of your batteries. Once the battery "sulfates"--It is pretty much impossible to convert (recharge) those crystals back into active battery materials--and that "capacity" is lost forever.

    You need something like:
    • more solar panels
    • AC backup charger
    • Backup Generator + charger
    Your present solar panels are just a bit more than needed to keep the batteries fully charged. You don't have enough solar panels to properly recharge the battery after heavy use.

    More than likely, a 10-20 amp AC battery charger is probably your best deal for now. This allows you to use your battery bank in an emergency (a few lights during a power failure--then recharge the battery bank after the outage is over.
    • 2*105 AH * 12 volts * 1/(6*13 watt lamps * 1/0.85 eff inverter) = 27.5 hours of operation
    If you limit your battery to 20% state of charge (80% discharge) to prevent battery damage:
    • 27 hours * 0.80 battery discharge = 22 hours of 6*13 watts of lighting loads
    So, you can have a 4-7 nights of emergency lighting (or more, if you manage your loads and only use the lights when needed).

    Picking a charge controller for your battery will be a bit of a search too... You will probably recharge the battery with the AC charger and then disconnect it once full--Let the solar panels keep the battery charged.

    The batteries are typically recharged at ~14.3 volts or so (Bulk/Absorb Charging) and once full, then can be "maintenance charged" at ~13.6 volts or so (if you leave the charger permanently connected to the battery bank). This is typically called a "3 stage" charger.

    You don't want to leave an AC charge controller permanently connected 24 hours * 7 days a week if it will keep charging at 14.3 volts--it will eventually damage the battery bank from over charging (sealed batteries are nice--but do not tolerate over charging very well).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • johnelarue
    johnelarue Solar Expert Posts: 33 ✭
    #5
    Re: where to put fuses
    BB wrote:
    Lastly (really lastly this time, this post ), typically a battery bank should be charged at 5% to 13% of rated capacity. Roughly:

    2* 105 AH * 0.05 = 10.5 amps minimum
    2* 105 AH * 0.13 = 27.3 amps maximum
    Depending on the type of battery (AGM, Gell, etc.), you might want to get a nice 10 amp 100 VAC battery charger to recharge the battery after outages.

    Your 150 panels may be OK for "maintenance" charging--but at 3 amps is a bit small to provide much more power.

    Thanks again, I think everything is clear but the above. If there are 3
    50watt panels wired in parallel, I get 2.9Amps X 3= 8.7 amps. Correct? When I measure amps in full sun I get a reading of 8.7amps at the controller. And 13.8volts or so.

    More panels would put me over the 10amp minimum charge. (5%-13%)

    Thanks,

    john
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #6
    Re: where to put fuses

    Yes--In parallel, the panels' output current adds (in series, the output voltage adds instead).

    Remember, for daily use--the panels must also replace any energy used during the day (on average).

    On average, an off-grid system is ~50% efficient from solar panel to AC inverter output... For example, if your panels get full sun all day, a typical summer day may be 5 hours of "full sun equivalent" per day:
    • 150 watts * 5 hours of sun per day * 0.50 efficiency = 375 Watt*Hours per day
    But in a deep winter day, you may get 2 hours of sun or less:
    • 150 watts * 2 hours of sun per day * 0.50 efficiency = 150 Watt*Hours per day
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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