Basic 12v Two Panel Setup ... diodes?

Bass-O-Matic

I have two 85 watt 12v poly panels. different brands. Slighty different dimensions.

I want to wire them in Parallel to keep 12v.

Then connect them to a 6 guage wire for a 12 foot run to the 30 amp pmw charge controller.

I hear I need Schattke ??? Diodes? one for each right? is that it? what is this circuit breaker business?

Is there a two panel junctio box out there will this crud in it already?

Both panels have 10 gauge MC4 connectors/cables on them already.

Thanks fo rany advice

BB.

No, you should be fine.

If you have 3 or more similar panels in parallel, then you normally need a series fuse/breaker per parallel string. This stops 3 good panels feeding too much current into a shorted panel.

-Bill

Bass-O-Matic

Thanks Sir! 

Bass-O-Matic

Sorry, so I don't need the diodes OR the fuse with my wee little system?

BB.

Unless I am missing something, no fuses for the solar array to charge controller.

On general, any positive wire leaving the positive battery terminal should have a fuse (or circuit breaker which can also be a power switch).

Fuses are there to protect wires from over heating (high current from battery back to short).

Playing around, you may skip fuses. A system that is permanently installed and operating without you there, you want to be careful (avoid fires).

Even a smallish system plus battery can start a fire if something goes wrong.

-Bill

mike95490

No diodes.  

Who is spreading this mis-information around.  Modern solar gear since the 80's, has not needed any extra diodes

BB.

Diodes could be used in place of fuses for parallel array strings. But diodes are not a good idea. They can fal fail shorted (not good for a fuse). Also they have voltage drop and waste energy.

Another reason may have been older systems that used dump controllers? Panels connected directly to battery bank and prevented current back flow through the panels at night. Most systems these days use standard series solar charge controllers which prevent back flow themselves.

Only two reasons I can think of using diodes... Unless it was an attempt to prevent back flow through shaded panels/strings.

Never heard anyone say why diodes were used.

-Bill

Bass-O-Matic

Ah yes, thanks gents. I popped open the little panel on the solar panel and there were diodes. So, built in. 

Regarding fuses. I've been pouring over this... https://www.windynation.com/jzv/inf/how-properly-fuse-solar-pv-system

It sounds like there are 3 possible fuses. One from each panel into the combiner, a combiner to charge controller, one on the battery.

and that blade fuses are not recommended.

So, I'm sorry to be dense but I don't like fire and you know - I don't know what I don't know!

I'm thinking of popping one of these 10 amp inline fuses on EACH panel. Since I have MC4 Y connectors for positive and negative already.

https://www.amazon.com/gp/product/B00J96UG46/ref=ox_sc_act_title_3?ie=UTF8&psc=1&smid=AI4V8J406TIRL

Then from there... should I have an inline 20 amp glass fuse after the y connectors join the panels and before the charge controller? I don't see a fuse on the charge controller.

Then of course a fuse on the battery + terminal. Can I use a blade fuse there? I'd probably go 20 amps there.

Or am I fuse crazy?

Thanks

BB.

The diodes inside the panels are "bypass diodes", not "blocking diodes".

Bypass diodes are for use in modern systems that are over ~12 volt battery banks. basically, solar cells themselves are diodes. If you put several panels in series and block the sun to one (or more) individual cells, these cells go "high resistance". And if you have >~12 VDC across a single cell, it over voltages the solar cell and ruins it.

The bypass diode is installed to "bypass" "dark cells" in multi-panel setups (lets current "go around" dark cells). For various reasons, it is still not a great idea to install solar panels with shading or partial shading--But bypass diodes are there for safety/reliability.

Series diodes where there to prevent current back flow. But, in modern systems, they are not needed now (solar charge controllers prevent back flow, Grid Tied systems also do not back flow the arrays).

Blade fuses (automotive type fuses) have been known to overheat and melt the holder/cabling even when operated significantly under rated current capacity. One thing that is different with solar and battery charging systems is that they operate near rated current for hours at a time (frequently in hot environments like roof tops and such). Most people are use to working with AC power systems which are designed to supply high current for short periods of time (starting a refrigerator, well pump, etc.), and then current drops back to "nominal" current levels (A refrigerator may take upwards of 1,000 VA to start, then fall back to 120 VA running). Because a solar system provides maximum power for hours to recharge a battery bank--A "poorly designed" fuse assembly, a bit too small of diameter of copper wire, poor cooling, etc., when operated at 100% (or even 80%) of rated current can overheat and fail over time.

Regarding your system... Solar panels cannot "surge current" -- They only supply current based on amount of sunlight. So a 5 amp Imp rated panel will only output ~5 amps maximum, even into a dead short (we do have a 1.25x margin of safety for reflected sunlight, refraction from clouds, etc. for solar systems).

So, if you have a panel with a 5 amp (Isc=1.25x5=) 6.25 Amp Isc rated output and a 10 amp series fuse rating--One panel into a short or  will not pop a 10 amp fuse, but two panels into a shorted panel (13 amp Isc from two panels) will (barely) pop a 10 amp series rated fuse. 1-2 panels in parallel will not need series fuses.

Many people put a circuit breaker on the input to their Solar Charge controller with solar panels. Not because it is needed to prevent high short circuit current--But because it is cheap and handy "on/off" switch to service/check/reboot the charge controller.

Now batteries can supply high surge current into dead shorts--So every positive wire that leaves the battery positive post should have a fuse/breaker near the positive post/bus bar to protect wiring going to other DC devices (charge controller, lighting, AC inverter, etc.) have their wiring protected.

For small systems with heavy wiring--It has been "accepted" that the battery will discharge before it can start a fire in heavy wiring (i.e., no fuse between the car battery and the starter motor--Heavy cables will not catch fire if the starter shorts--As the battery will go dead first).

Fuses/Breakers are sort of a extra cost/hassle device that many folks do not bother with. "There are fuses inside the Inverter--I don't need a fuse/breaker on the battery bank + bus too"... And to a degree, that is true. However, if a cable fails (crimp connector pulls out, piece of metal cuts insulation, animal eats insulation, etc.)--And you have a dead short back to the battery bank and the fuses in the inverter did not do a thing.

So, for me, I really push the idea of safety here. Most people are "use" to having 120/240 VAC coming into their home from a 10,000 Amp maximum pole transformer--But there are a series of breakers/fuses from the home's utiltiy drop until it hits the 15 amp rated 120 VAC outlet in your bedroom.

When you put a 700 AH @ XX volt battery bank in your home--You have something that can feed 1,000's of amps into a dead short--And no breakers/fuses scares the heck out of me (even a single 80AH @ 12 volt car battery can feed close to 1,000 amps into a dead short).

Most people do not realize that DC current is much more difficult to turn off (breaker/fuse) than AC current. Basically, 24 and 48 volt battery banks are "perfect" arc welding sources--DC current sustains arcs (over ~12 VDC) very nicely. And it is not just 1,000 amps that is a danger--Even 10 amps of sustained current is a great ignition source. Here is an example or two:

https://www.youtube.com/watch?v=Zez2r1RPpWY

AC Breaker on a 290 VDC rated solar array and 10 amps--Just being turned off:

https://www.youtube.com/watch?v=HLX9cdB4TFQ

When you are designing your own off grid power system--You are taking on a lot of personal responsibility here. And placing this into your family home/cabin. Dangers to people when occupied--Things going wrong when nobody is there to turn stuff off.

-Bill

Bass-O-Matic

Amazing response! I really feel like I learned something here.

So in summary:

1. Everything coming OFF the battery post should be fused. I'm already planning that with a nice Blue Sea DC Distribution Panel.

2. A DC Disconnect is a great idea between the panels and Charge controller.

3. Otherwise I'm good to run the panels in parallel and connect them to the Disconnect and then Charge controller.

Sound about right for my install?

BB.

Fuses or breakers are fine. Breakers are nice because they make nice on/off switches too.

-Bill

Bass-O-Matic

OH, DC breaker... like this one

https://www.amazon.com/MidNite-Solar-Photovoltaic-Circuit-Breaker/dp/B004EQK7YA/ref=sr_1_1?ie=UTF8&qid=1476217242&sr=8-1&keywords=10+amp+dc+circuit+breaker

BB.

Yep. And our host Northern Arizona Wind & Sun has them too.

https://www.solar-electric.com/residential/circuit-protection/circuit-breakers.html

-Bill

Bass-O-Matic

I ordered it on Amazon but canceled to order from Northern Sun in thanks for your excellent advice.

Question.

10 amp breaker right?

And since I don't have a "box" I think I can get DINRAILS and install them under the bench in the RV where all this is going. So I can get a DINRAIL mounted breaker.... I mean I need one that I can screw the 6 guage wires into and that I can mount somehow. If you don't know I can call Northern.

Thanks again!

BB.

Thank you for ordering from NAWS--I am not connected with NAWS, other than being a moderator here (volunteer). However, they do fund and maintain the forum for all (folks do not need to purchase from NAWS to post here).

Regarding the correct breaker... Can you confirm your setup (number/size/rating of solar panels, rating of charge controller, size of battery bank, gauge of wiring, etc.

Details matter here.

-Bill

Bass-O-Matic

The devil is in the details! So, while I have some pros around here is my setup. Note. I'm not putting in an inverter on purpose.

Let me know if you can see that pic. otherwise I'll put the larger file here. It should be scalable and about 1 meg.


https://drive.google.com/file/d/0B8j3DxVF_IAHVlVsOVdUVzI4NkU/view?usp=sharing

westbranch

You can cut out one fuse and some wire by going straight to the Battery from the Charge controller,  and then to the fuse panel and that charger/ converter... but what you laid out will work.

BTW you should check that the converter wizard is giving a proper charge to those batteries as specified by the maker....

Bass-O-Matic

Thanks. Well, I'm going through the wall of an RV. The batts are outside so that would mean more holes in the wall I think?

Slight update to schematic...

BB.

The 10 AWG cable for the Progressive Dynamics Power Converter is way to small for a 45 amp output charger:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
http://www.boatus.com/boattech/articles/choosing-cables-and-terminals.asp

If you follow NEC, 8-6 AWG minimum. If you follow the Marine wiring table, yes you can--But I would suggest going heavier. Less voltage drop, less power loss, and cooler running wiring.

6 AWG sounds pretty small for your battery bus cabling too... What is the maximum current you expect to draw from the bank and what fusing/breaker value?

-Bill

Bass-O-Matic

https://drive.google.com/file/d/0B8j3DxVF_IAHaUpONmotZ0ZxUkU/view?usp=sharing  Link to XLS




Updated schematic. Yes, I sent the schematic to progressive and they suggested 8ga. 

"6 AWG sounds pretty small for your battery bus cabling too..." 

Are you talking about just the run from the Battery to the DC Panel? Would 4ga. work?

" ... what fusing/breaker value?"

30 amps fuse? I hear blade bad but maybe not in this application and I like this Maxi Fuse holder.

Blue Sea Systems MAXI Fuse Block - 30 - 80A

https://www.amazon.com/gp/product/B000THTBZO/ref=pd_sbs_263_2?ie=UTF8&psc=1&refRID=MZDRXDAFFJ0JH506AK4E

"What is the maximum current you expect to draw from the bank ..." 

40 amps WORST case scenario on draw? That's with ALL the lights on and everyone on laptops and smart phones.... I've been trying to work this out for years actually... I've maxed out panels and batts. My little trailer can only so much so it will recharge as it may.... 



RV is only 13 feet long (tongue length, 10 x 7 floor plan) . Built it myself. From scratch. 5 years in. weekends only...  

Bass-O-Matic

Any thoughts on the update?  removed a fuse and replaced a breaker with a fuse + switch combo for more sensitivy.

I read that solar should have fuses on negative line as well? news to me...