diy solar

diy solar

Grounding, disconnect / combiner questions

jdz

New Member
Joined
Feb 19, 2021
Messages
20
Howdy y'all!

Can't tell you what a great resource this has been, between the forum itself, watching Will's videos, etc... Great community and super happy to be here.

Here's what I've got put together (ordered) so far:

- 16s 100ah fortune (Overkill) batteries (48v)
- Overkill 16s BMS
- 2s3p (6) Hanwha 425w 72 cell panels (10a/42v)
- Victron MPPT 100/45 TR
- Victron Quattro 48/5000
- Victron Cerbo GX
- MRBF (2 lug) fuses attached to battery positive and 150a fuse for inverter, 50a fuse for charge controller

This is all going on a 20' shipping container using unistrut racking - code compliance is not an issue on this particular setup but still want it to be safe and well thought out.

- I know a combiner (midnite or otherwise) would be "best", but since I'm maxing out at 100v/33a short circuit (more like 80v/25a real world) and the runs are relatively short (everything lives in the container) would doing MC4 15a fuses to protect each series and then a 3-1 connector be sufficient? It represents a pretty significant savings, ease of installation, etc. Do I still "need" a disconnect between the controller and the panels?

- Grounding... The plan is to install a ground rod right next to the container, which will be bonded to container itself (probably a WEEB lug) and all the equipment. Was going to use WEEB lugs to daisy chain each panel to the next, then throw a WEEB lug (or 2) on the unistrut. Is this sufficient? Fortunately the container is up against a bank and not at all a "high point" in terms of lightning strikes, which lessens that risk to a certain extent.

Really appreciate your feedback on this!
 
If you switched to 3s2p and a Victron 150/45 you wouldn't need a combiner box, just a Y connector to bring the parallel strings together. No fuses needed. But you would still want a two-pole circuit breaker inside the shipping container before the solar charge controller.

An MRBF fuse is probably not appropriate on the battery. A Class T fuse is a better choice.
 
Hey Jim,

Thanks for the reply.

Originally was planning on doing 3s2p, but it looks like that may push me over 150v on cooler mornings. (These particular panels are 49v voc so they're right on the edge at 147v). Is there a particular two pole DC breaker that can handle 100v that you recommend?

Interested to learn more about the MRBF -- is the interrupt current rating not high enough for a lifepo4 battery? Speaking of which how do you actually calculate a sensible short circuit current potential for a battery pack? The fortune cells I'm using are M10 (3/8) stud so the MRBF lug / bar is a good fit.

Cheers!
 
This is the breaker I'm using. Specifically rated for DC, but maybe not enough amps for your case.


The AIC isn't high enough on the MRBF fuse. Check out this ongoing thread for more info. I don't have enough knowledge on the topic to discuss the fiddly bits. I went with what was recommended (Class T).


I thought the panels volts you provided earlier was Voc, so the 150/45 would have worked. You would have to step up to the 250v line instead. I really didn't want any more crap on the roof of my RV, so I did everything I could to avoid a combiner box. For a container, a combiner probably isn't a big deal. Midnite Solar has a bunch of combiner boxes.
 
Hey Jim,

Thanks for the reply.

Originally was planning on doing 3s2p, but it looks like that may push me over 150v on cooler mornings. (These particular panels are 49v voc so they're right on the edge at 147v). Is there a particular two pole DC breaker that can handle 100v that you recommend?

In that case, isn't 2s just below 100V, will push 15% over on cold mornings, too high for Victron MPPT 100/45 TR?
Hmm, does 100/45 even exist or was that a typo? I'm only finding 150/45 which should be fine.



There are plenty of 2 pole and even 1 pole breakers for 100 VDC. Midnight, Schneider. Even Square-D QO as 2-pole (not UL listed above 48VDC but "Square D certified" up to 125 VDC)

Interested to learn more about the MRBF -- is the interrupt current rating not high enough for a lifepo4 battery? Speaking of which how do you actually calculate a sensible short circuit current potential for a battery pack? The fortune cells I'm using are M10 (3/8) stud so the MRBF lug / bar is a good fit.

Cheers!

Based on 0.25 milliohm spec and 0.17 milliohm typical for some lithium cells, I calculate up to 20kA short circuit current from a single string. Class T is rated for that but MRBF is not. Wire resistance to first possible place for a short could reduce fault current, but takes a surprisingly long or thin run of copper to do that.
 
Last edited:
Sorry! It is in fact 150/45 and that is a typo -- correcting now.

Actually -- for whatever reason, I can't edit my original post... But was able to edit this one.

Thanks for the explanation and estimate of interrupt current, will look into Class T.
 
Sorry! It is in fact 150/45 and that is a typo -- correcting now.

Actually -- for whatever reason, I can't edit my original post... But was able to edit this one.

Thanks for the explanation and estimate of interrupt current, will look into Class T.

You're allowed to edit your posts for a limited amount of time.
 
Back
Top