Howto disconnect battery bank

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bighornram
bighornram Registered Users Posts: 8 ✭
in Solar Beginners Corner #1
While connecting a panel and Morningstar SunSaver 10 it is advised to connect the battery first then connect the panel. I need to have my battery disconnected for a while while work is being done to my rv. I've removed the fuse that is inline from the solar controller to the battery (+) terminal and disconnected the (+&-) wires from the battery connection. The panel is still connected to the controller. Is it safe to leave this arrangement for several weeks until repairs are made to my rv? Specificly I connect the controller to the terminals on the converter unit in the rv. The converter is going to be replaced and I want to keep my solar controller disconnected until they replace the charge unit. I know I can crawl up the rooftop and disconnect the panel but that is I guess my main question: Do I have to disconnect the panel/controller if the battery bank is not connected?
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #2
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    Re: Howto disconnect battery bank

    I heard a long time ago that if you hooked panels to a controller without batteries already hooked up it would fry the controller. I don't know if that is still the case with the newer controllers.

    Is there a breaker of a fuse between your panels and controller? If so, just pull/trip it.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #3
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    Re: Howto disconnect battery bank

    Can you disconnect the solar panels from the controller side?

    In general, solar charge controllers should be connected to the battery first.

    This 1) lets the charge auto configure to 12/24 volts (some controllers do this--if you have solar power already, then the controller may configure incorrectly). And 2) it is possible to damage the controller if hit with unregulated solar energy (should not--but good to avoid).

    In the end, follow whatever the manual says for best results.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #4
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    Re: Howto disconnect battery bank

    Unless a system is more or less a toy, there should be either a breaker, easily removed fuse, and/or switch, (all properly rated for the DC voltages available of course) between the PV and controller. There can be many reasons for needing to quickly and easily disconnect the PV from the controller. Same thing goes for between controller and battery/s.
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #5
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    Re: Howto disconnect battery bank

    That's a bit of a generalization. My power pack is not a toy-it's a tool that provides 8.5 amps of modified AC for all the tasks in my shed. When I sought help wiring it correctly, moderators here didn't seem to think that extra step was necessary at low amps of my PV.

    Link to Owners Manual
    waynefromnscanada wrote: »
    Unless a system is more or less a toy, there should be either a breaker, easily removed fuse, and/or switch, (all properly rated for the DC voltages available of course) between the PV and controller. There can be many reasons for needing to quickly and easily disconnect the PV from the controller. Same thing goes for between controller and battery/s.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #6
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    Re: Howto disconnect battery bank
    bmet wrote: »
    That's a bit of a generalization. My power pack is not a toy-it's a tool that provides 8.5 amps of modified AC for all the tasks in my shed. When I sought help wiring it correctly, moderators here didn't seem to think that extra step was necessary at low amps of my PV.

    Link to Owners Manual

    Your system, your decision. It's just that personally, I wouldn't do it without a way to quickly and safely disconnect the panel/s from the controller. Wouldn't risk it. Example: Something starts arcing or smoking in the input circuitry of the controller, can I pull a switch, or do I start running around looking for wire cutters or a hacksaw. A DC arc can lead to a fire real quick. While I know the chances are very slim that this one example would ever happen, perhaps I spent too many years as a firefighter. Peace be with you.
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  • bighornram
    bighornram Registered Users Posts: 8 ✭
    #7
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    Re: Howto disconnect battery bank

    waynefromnscanada ==> Point well taken, though I'm with bmet in thinking my system is not a toy either. I think I came away from this forum thinking also that I only needed to protect the controller from the battery bank. All is good though that is why I came back and created this thread. The benefit of forums and the internet is that it provides us all with a means to learn based on experiences of others. We all want to learn and do our installs the safest and most efficient, cost affective way we can. The manual explains pretty well that the battery bank needs to be connected first followed by the PV's. It also documents nicely the functionality of the controller. Unfortunately it doesn't talk about this particular situation. I did shoot off an email to Morningstar's support people about this so will get back here with my findings. There are several circumstances that could lead to the battery bank's disconnection from the controller. My application is in an RV. I intend to cover this RV as one of my BIG mistakes with my old RV was to let the UV's cook everything. Decals were ruined, many plastic items (RV's have many) became brittle. My new unit will be covered when in storage. I also enlisted the services of Mr. Google and found a couple of interesting documents.

    Truck Mounted PV System (Thanks Gary, well done)!

    Photovoltaic System Overload Protection, Cooper Bussmann (Page 5 has a flow chart to determine the system's protection requirement)

    Define solar panel spec: Ipm Isc Max System Voltage
    Conductor/fuse size per string 1.56 x Isc
    Number of panels/strings = N
    Does array max Isc exceed current? Max Array Isc = (N-1) *Isc

    Isc = 8.02 amps
    Ipm = 7.63 amps
    Max System Voltage = 17.7 vdc (max value of series panels)
    Conductor Size Formula = 1.56 * Isc = 1.56 * 8.02 = 12.51A
    Conduct Size = 10AWG = 33A @ 60*C
    Controller Current rating = 10A
    N = 1 (1 Solar panel in 1 string)
    Array Max Isc = (N-1) * Isc = (1-1) * 8.02 = 0
    Array Max Isc is less than conductor withstand, therefore string fuses are not required.

    *The weak link is the charge controller which is rated at 10A so I should protect the controller with a 10A fuse rated >= 18vdc which should protect the controller.:cool:

    The Bussmann document recommends this fuse be on the + and - connection.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: Howto disconnect battery bank

    On small sized systems where there are no more than two parallel connected PV's (or strings thereof) fuses or breakers are not needed on the PV circuitry because the Isc can not exceed the wire rating (if the wire is properly sized). PV's can be shorted in full sun without fear of over-heating the wiring.

    That said, all PV connections will have some form of disconnect, preferably quick. It may be a properly rated switch or simply a plug that can be pulled easily (accessible and not requiring a special tool, which lets out MC4's). Having to use a screwdriver to loosen a terminal and pull a wire doesn't count as "quick". Very small PV systems may not even require this; it's a judgment call.

    The Cooper Bussmann reference is incorrect in stating circuit protection is needed on both (+) and (-) wiring. There is no point to doing this. Current on positive and negative wires in a PV circuit is the same, and interrupting that circuit at any point stops current flow. Standard practice is to ground (-) and put all switching/fusing on (+) lines. The NEC has recently added requirements for ground-fault detection to PV wiring which has caused some confusion and consternation and may be clouding the issue here.

    Charge controller connections to batteries should always be fused, regardless of system size. Batteries can push immense amounts of current that can easily exceed wire ratings if something should go wrong (like a short in the controller). Again, only the (+) side need be fused.

    Before anyone starts picking nits, circuit over-current protection may be fuses or breakers or a combination of both (as in multiple PV strings being individually fused and the array as a whole being fed to the controller through a breaker which is used mainly as a disconnect). I have used the word 'fuse' here because it's shorter than writing the whole thing out over and over, all right?

    Everybody happy now? :roll:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #9
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    Re: Howto disconnect battery bank

    And note that fuse holders (such as touch safe holders) are not good disconnects. If somebody just "pops the holder" to cut the power, you can get DC arcing which will melt/burn the fuse holder/block.

    And remember that you should always use a 1.25x safety factor for fuses/wiring... If you have a 10 amp circuit, then the fuse/breaker/wiring should be rated for a minimum of 12.5 amps. This is a safety factor by NEC and will also reduce nuisance tripping of protective devices (fuses/breakers are not supposed to open at 80% of rated capacity and will, eventually, break at 100% of capacity--Although, Outback Solar does have a set of breakers that seem to be rated for "100%" load--not sure how all that works).

    Also, you need to check the DC rating for switches and fuses/breakers. Normally, the DC rating is much less than the equivalent AC rating. DC does really well at sustaining arcs (great for arc welding too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bighornram
    bighornram Registered Users Posts: 8 ✭
    #10
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    Re: Howto disconnect battery bank

    Helpful explanation. Thanks
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #11
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    Re: Howto disconnect battery bank

    I'm all for fuses, chances are you won't standing at the CC to disconnect when something starts to smoke. That does bring up an interesting question of whether CC could have some "human alert" built into them if there is an unsafe condition.

    waynefromnscanada wrote: »
    Your system, your decision. It's just that personally, I wouldn't do it without a way to quickly and safely disconnect the panel/s from the controller. Wouldn't risk it. Example: Something starts arcing or smoking in the input circuitry of the controller, can I pull a switch, or do I start running around looking for wire cutters or a hacksaw. A DC arc can lead to a fire real quick. While I know the chances are very slim that this one example would ever happen, perhaps I spent too many years as a firefighter. Peace be with you.
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  • marsofold
    marsofold Solar Expert Posts: 45 ✭✭
    #12
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    Re: Howto disconnect battery bank

    I'm for putting a heavy duty inverter fuse (110 amps) on the + line from the batteries to the charge controller. But I'd feel threatened by a circuit breaker or disconnect on that line. Reason is, if at noon somebody decided to flip off the circuit breaker or pull the disconnect, then my $500 mppt charge controller would fry. At least an inverter fuse can't be casually disconnected by an ignorant party.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
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    Re: Howto disconnect battery bank
    marsofold wrote: »
    I'm for putting a heavy duty inverter fuse (110 amps) on the + line from the batteries to the charge controller. But I'd feel threatened by a circuit breaker or disconnect on that line. Reason is, if at noon somebody decided to flip off the circuit breaker or pull the disconnect, then my $500 mppt charge controller would fry. At least an inverter fuse can't be casually disconnected by an ignorant party.

    Just a point: you may not want a 110 Amp fuse on the wire from the charge controller to the batteries. Reason being: The NEC 1.25 rating for a 60 Amp controller would be 75 Amps. An 80 Amp controller would be 100 Amps. The Midnite Classic 150 is rated at 93 Amps output on a 12 Volt system; that would require 116 Amps of protection. (You round up to the nearest available size.)

    And this is one of those situations where the wire size used might be much larger than current handling alone would require in order to reduce V-drop (and thus inaccurate readings of battery Voltage by the controller, especially under load conditions). The circuit protection would still be according to the max current expected.

    The "$500" MPPT controllers are unlikely to fry because someone disconnects the battery while the PV is still active. Get confused and shut down, yes. Or display a little message like "Got Batt?" :D The cheap ones are likely to go "pfft!" (or other cartoon noise of your choice).
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