Newbie needs help

javid

Hello,

I think I have most of the main things I will need for my small, off-grid system I am looking to install this spring, but I could use some help with wiring/breakers/surge protection/etc questions.  I know there is a lot I don't know!

This is a small system, and will be installed in somewhat of a temporary manor (since we are camping in our Motorhome currently, but are looking to build a cabin soon).  I do have a shed to put batteries/inverter/breakers, etc inside.

I plan to use a 48v system.  Batteries are (8) 12V Interstate DCM0075U (they come lightly used from UPS system and are double the storage I will need currently).  For Charge controller, I have MidNite Solar Kid MPPT with Temp Sensor.  For Inverter, I have 48v Exeltech XP600.  

For Panels, I have (4) panels that were damaged that I got for next to nothing and I plan to put them in series.  They have Voc of between 27.6-28.9 (nameplate is 38.7Voc) and Isc of between 8.02-8.92Amps (nameplate is 9.05).  The fellow I bought them from told me they were damaged in grid-tied system and replaced, but I can't remember exactly what was wrong with them except output is lower.

I was thinking of basically following 48v wiring diagram that MidNite solar has here http://www.midnitesolar.com/pdfs/Kid_48V_3p_Series_REV_B.pdf.  I would be interested in any feedback on this layout.

My panels will be around 30-40' from the shed and I was planning on putting the batteries, inverter, and breakers all inside shed close to one another.

A few questions I have:

- Do I need SPD300 lighting arrestor?  This is off grid and I am not worrying about NEC compliance, but I do want safety.  This arrestor is expensive and I guess I am wondering if it is worth it.  If it isn't there, what am i risking?  And what are the chances it will actually save something?  If it is just the inverter I am saving, is it worth $130 arrestor to save $300 inverter if chances are low that it will be effective?  I am just trying to understand risk/reward benefit.

- What kind of wiring and size of wiring should I use between panels and shed?  I would like it to lay on the ground at this point, since our building site,etc is still being decided...

- I am going to have (2) strings of (4) batteries each.  What is best way to connect these to charge controller and do I need two of these MNEPV30 Battery Breakers?

- What size wiring between busses, breakers, charge controller and batteries assuming they are all within 6' of each other (at most).

- Any help with which bus bars, alternate breakers/other equipment recomendations?

If you have waded through all this, I really appreciate it!  Any help or guidance would be great - this is a great forum and I have learned so much!  Thanks!

Estragon

That voltage drop in the panels worries me a bit. Wouldn't that suggest a high resistance connection in the panel?

Figuring out how to wire ... Normally you would do a string of 3 for a Vmp around 90v, Voc around 115. At nameplate Voc, a string of 4 might overvoltage the controller if really cold. Actual Vmp for 2 strings of 2 likely too low. I don't know what Vmp to expect with Voc that low.

UPS batteries aren't usually ideal for deep cycle use. Hopefully you got them cheap enough that if they don't last it's not a huge problem.

Lightning protection is up to you and really comes down to probabilities. If your area/location is lightning prone, it's probably worth it. Naws carries cheaper suppressors I think. I went that route as my location isn't super lightning prone. Lightning could fry pretty much anything electronic, not just inverter.

Wire from panels is typically 10ga with mc4 ends on the panel side. Likely 15a breakers (panel spec should say). You don't really have to use pv breakers at all, but I'm a fan of being able to easily disconnect stuff if need be.

One breaker from batteries to +bussbar is okay with 2 strings.

Wire and breaker size for controller and inverter should be spec'd in manuals. I like to use the biggest that will fit.

I used a midnite big baby box and bussbars, and it worked fairly well.

javid

Thanks a lot for the response!  A few responses below:

That voltage drop in the panels worries me a bit. Wouldn't that suggest a high resistance connection in the panel?

<<< I am not sure - my recollection was something about a one of the diode's being bad.  I don't understand the ramifications if there was high resistance... I will try to contact person who sold them to me (conventional grid-tie system installer)>>>>

Figuring out how to wire ... Normally you would do a string of 3 for a Vmp around 90v, Voc around 115. At nameplate Voc, a string of 4 might overvoltage the controller if really cold. Actual Vmp for 2 strings of 2 likely too low. I don't know what Vmp to expect with Voc that low.

<<< I should check what will happen to charge controller if it gets overvoltage - will it just shut down or be destroyed?  I could probably get by on just using 3 panels and keeping it safe....>>>

UPS batteries aren't usually ideal for deep cycle use. Hopefully you got them cheap enough that if they don't last it's not a huge problem.

<<< I think these batteries are mostly suitable for deep cycle use - at least when I read the specs they seem that way.  They are Interstate DCM0075U batteries, and I got (9) for $300, which was cheap enough for me to take a chance - they seem in OK shape going by float voltage>>>>

Lightning protection is up to you and really comes down to probabilities. If your area/location is lightning prone, it's probably worth it. Naws carries cheaper suppressors I think. I went that route as my location isn't super lightning prone. Lightning could fry pretty much anything electronic, not just inverter.

<<< My site is somewhat lighting prone, but not too bad I think.  I wonder do the suppressors even work?>>>

Wire from panels is typically 10ga with mc4 ends on the panel side. Likely 15a breakers (panel spec should say). You don't really have to use pv breakers at all, but I'm a fan of being able to easily disconnect stuff if need be.

One breaker from batteries to +bussbar is okay with 2 strings.

Wire and breaker size for controller and inverter should be spec'd in manuals. I like to use the biggest that will fit.

I used a midnite big baby box and bussbars, and it worked fairly well.

<<<Thanks for the info and the recomendation on the midnite big baby box>>>>

Open to more feedback!  Thanks!

Estragon

Midnite classic controllers can take some overvoltage - they call it hypervoc. I think Kids can too. They stop charging at spec high voltage (eg 150v) but don't get damaged unless voltage gets to 162v IIRC.

Couldn't find specs on that battery, but the price sounds right. At least if they don't work you didn't pay much more than what you should save on core charge for new.

You can't be totally safe from lightning, but the arresters presumably help reduce the risk some.