Fuse help needed

coominyacoominya ✭✭Registered Users Posts: 8 ✭✭
Hi all, nice to find a forum with such extensive information.
I am setting up my first offgrid solar power system, just a small 12V test project to power a 40foot HiTop shipping container with cooling fans and lighting.

I have a couple of 80W panels up on the top, a kemo 7amp regulator and two 90Ah vrsla batteries. I have adequate cables on the small 5m run down to the regulator and have fused the load output from the battery on the positive side ( I am bypassing the regulator load wire for now) but was wondering if it's normal to fuse the solar panel to regulator circuit and the regulator to battery circuit?

Comments

  • PhotowhitPhotowhit ✭✭✭✭ Solar Expert Posts: 5,008 ✭✭✭✭
    Re: Fuse help needed

    Those panels should be able to produce more than 7 amps in good conditions. Most of the charge controllers, perhaps the only one I've seen called a regulator, was not something I would trust.... might be worth looking into something better.

    2 panels can be used without any fuse or breaker between them and the charge controller.

    If your running 12 volt fans be sure to run reasonably heavy lines, you might be using much more current than the fan rating otherwise, due to voltage drop along the lines.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • BB.BB. admin Super Moderators, Administrators Posts: 29,634 admin
    Re: Fuse help needed

    Pretty much, you are looking for safety with fusing and breakers... And sometimes, breakers are preferred because they are also a handy on/off switch (plus, if you have big 100+ amp fuses, a fuse holder + a couple of fuses are frequently more expensive than a breaker).

    So--First look at the battery bank. It is the source of high current (at a relatively fixed voltage). So, anything that leaves the battery should be sized to handle the load + a fuse/breaker to prevent a short circuit causing excessive current to flow and giving red hot glowing wires.

    A good place to start is your local electric code (have the NEC--National Electric Code in the US as a popular standard). The NEC is pretty conservative, and with solar, you generally are worried about voltage drop (particularly with high current/12 volt circuits where about 0.5 - 1.0 volts drop is the most that most loads can take).

    Even if you used a standard that allowed higher current flow (like the American Yatching and Boating Association), with solar/off grid DC power, you usually want low voltage drop (efficiency and to send DC power longer distances than you may on a smaller boat/RV).

    So--You would run the battery wiring (series/parallel connections) to the common +/- Bus Bar/Connection points, and each wire that leaves that point should have a protective device. And that would include to the output of the solar charge controller. We typically only fuse/breaker the DC side of the system. And usually would ground the DC negative battery bus (to your metal container, plumbing, etc.) so a short circuit to ground would pop the fuse/breaker.

    Note that for when you have three or more solar panel (or series strings of solar panels) in parallel, each string should have a fuse/breaker too... Basically, for most solar panels, if you have two parallel solar panel strings, they can feed "too much" current to a shorted third panel string. You will find many solar panels have a series fuse rating on the data sheet. If yours does not have a fuse rating, you can use around 1.56x the Imp rating of the panel (round up to next common fuse value).

    Note for the solar charge controller wiring--Battery voltage during charging is pretty critical... Too low, it takes a long time to charge the battery bank (and can limit charging current). Too high, and you can cook the battery bank. For a 12 volt battery bank, you want around 0.05 to 0.1 volt maximum drop for charge controller to battery bank voltage drop.

    For your system, 2x 80 watt panels seem a bit much for a 7 amp charge controller... Guessing:
    • 2 * 80 watt * 1/17.6 volt Vmp = 9.1 Amp Imp-array
    And, for your battery bank--We recommend around 5% to 13% rate of charge... 5% is OK for a standby/weekend/seasonal system... For a full time off grid cabin/home/etc. system, 10% to 13% or so is a better amount (plus, batteries are getting expensive and solar panels are at a historic low for costs--It is much better to install "extra" panels these days to keep the battery bank "happy"):
    • 2 * 90 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 169 Watt minimum array
    • 2 * 90 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 339 Watt nominal array
    • 2 * 90 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 441 Watt array "cost effective" maximum
    Because you have VRSLA batteries, you may get away with a bit less than 5% rate of charge--But I would not suggest it if you plan on cycling the batteries (around 1% to 5% is usually a "maintenance" or for storage).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • scrubjaysnestscrubjaysnest ✭✭✭ Solar Expert Posts: 175 ✭✭✭
    Re: Fuse help needed

    I would use breakers rather then in line glass fuses. The fuses tend to fail over time due to heat from resistance at the fuse/connector interface. Here in the USA fuses or breakers are required if I remember the NEC codes correctly.
    If you prefer fuses look into Blue Sea fuses and connector blocks, very expensive but you won't have the connection resistance problems.
    For your 7 amp regulator, solar controller?, it is to small for your panels as 80 watt panels are usually capable of generating 5 amps in full sun. That would be 10 amps for two 80 watt panels.

    I see BB typed faster then I did and gave some very good pointers
  • coominyacoominya ✭✭ Registered Users Posts: 8 ✭✭
    Re: Fuse help needed
    Photowhit wrote: »
    Those panels should be able to produce more than 7 amps in good conditions. Most of the charge controllers, perhaps the only one I've seen called a regulator, was not something I would trust.... might be worth looking into something better.

    2 panels can be used without any fuse or breaker between them and the charge controller.

    If your running 12 volt fans be sure to run reasonably heavy lines, you might be using much more current than the fan rating otherwise, due to voltage drop along the lines.

    Ahhh, thanks, glad I came here! I mistook the 7Amp rating as being just the maximum load, I didn't even think to check the input. Lazy me for not reading the spec sheet. No damage done as yet because I only have one panel connected at the moment, about 4 Amp output, just to keep the battery topped up and run 400mW of muffin fans I have installed to draw a little hot air out. I am in your debt for correcting me.

    I have a powertech 32Amp regulator in a box here and will use it instead. I have been mucking around with solar for some years, just little portable systems for running telescopes and amateur radio gear in the field. This is my first dedicated system.

    I will definately investigate the circuit breakers, they sound much more practical.
    I will ensure I don't exceed the 13% abouts in input current, though for my 180AH battery that shouldn't be an issue. I would need 6 * 80W to exceed that.

    The batteries are old, the panels were cheap and the regulators, wiring and lights are all sitting around here in my garage so this first attempt shouldn't cost me anything. I have an appreciation of the dangers or electrical fires etc and want to get it right on this test installation so as when I put the cabin up on the land (beside the container/worshop/storage) I will have good practices in place and some experience under my belt.

    Thanks for the detailed speedy replies.

    Thanks for the heads up on these important factors.
  • BB.BB. admin Super Moderators, Administrators Posts: 29,634 admin
    Re: Fuse help needed

    You can go as high as 20-25% or so for lead acid batteries... But you really should have a remote battery temperature sensor to help reduce the chances of thermal run-away (batteries get hot, charging voltage goes down, charge controller jacks up current, batteries get hotter, etc.).

    ~13% is pretty safe and reliable... For more most folks that don't run heavy daytime loads (charge during day, use power at night), it usually works out to be about the most solar panels you want to buy (cost effective).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • coominyacoominya ✭✭ Registered Users Posts: 8 ✭✭
    Re: Fuse help needed

    Thanks for the info Bill. As I stated, up until now I was have just been doing small setups, one battery generally, a 20W panel or an 80W and small regulators, so it was easy to over engineer the system with big cables on short runs and small devices. The biggest I have done is actually my campervan which has 120W of fixed panels into a 20A Plasmatronics and down into a 200Ah exide N200 flooded semi deep cycle. The whole system is quite complex with lots of led lighting, fans, and amateur radios. There is a 600W inverter also but I keep this on a direct curcuit to the batteries not via the load terminal on the regulator.

    But that system is hobby stuff really, nothing like installing a proper offgrid system to run a workshop where long cable runs are the norm and devices run unattended for weeks at a time. We'll get there, 12V to begin with because I have all the gear for that, and eventually probably 24V in the cabin. Will be a fun excercise and a pleasure to be in control of my power and it's useage.
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