Need expert help on new large battery bank for off grid..

bryhan67

Hey guys, I have a vacation home off grid and I am building a new battery bank.  The system will be 48v. My inverter will be 48v 3000 watts.  I got a good hook up on 12 volt AGM batteries they are 140ah at 20hr rate. I bought 24 of them.  I will make 6 strings of 4 batteries each. I know its overkill but I could not pass up the deal. We only go a few weekends a month so most of the time the bank will be at idle.
Here are my questions:

1. I have a buss bar on each end. Cable length from each string in 18". I will need to buy 12 cables for 6 strings. What guage cable do I need for this?

2. I would like to run breakers. Should I run a breaker on each string? Thats 6 breakers. 

3. What size breakers do I need for each string?

4. Coming off the bus bars combing all the batteries what guage wire should I run to the inverter? These two cables will be 32" each.

5. I have a midnight solar 200. How critical is it for the output cables to be the same length?  Would 18" pos and 50" neg be a bad set up?

6 In the middle of each string I will run a 1" x 1/4" copper flat bar about 4" long. so the battery have a 1" space between them. It gets up to 100deg here. Is this copper bar sufficient?



Thanks
Bryan

jonr

Be sure to use a clamp DC ammeter to check balance between each series string.  I have a Mastech MS2108 (cheap but adequate).

mike95490

Well, if you must parallel batteries, you are doing it right .    The battery wire from the Charge Controller, keep it short,  but they don't need matched lengths.  And go a size larger if you can, the Classic does not have a voltage sense line, and because of the high charge amps, it's sensed voltage will not be well calibrated,

3,000w inverter @ 46V would be 65 amps.  I'd put 50-70A DC breakers (from Midnight Solar) on each battery string. Mounting breakers might be an issue, because the arc chute wants to be oriented with the breaker upright, as if it were in a panel.  The leads in and out of the breakers count as part of the matched lengths.

Battery cable wires, assume only one string is working , 65A at full load in the inverter would be #4 wire, I'd use #2 - good for 95A, and you need margin so your 70A breaker blows before the wire does.

I'm sure there's some spec that says 60A circuit to be wired with 120% rated wire (72A)

Bus bar to inverter, at least 1/O cable.  There will be starting surges from motors, and you want the inverter to be well fed.

Battery - Battery interconnects, you want something that will flex, and not bend/crack a terminal or case,   Either tinned flat braid, or MT cable.

WaterWheel

I'd make sure you have enough panels to easily charge that bank.     Here is a suggested fuse that is easy to mount for each string. https://www.amazon.com/gp/product/B0019ZBTV4/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

Not as handy as a breaker but a lot easier to mount.

bryhan67

What are the odds all the current will passing through 1 string?  I can see thats our your setting it up..  If I was pulling max 65 amps that would only be about 11 amps per string?  Instead of fuses I would like to do individual breakers. Any tips on a good clean set up for 6 breakers?  Confused on Midnite Solar's breakers. They seem to be more for panels. 

What about somthing like this?
https://www.bluesea.com/products/category/14/32/Circuit_Breakers/285-Series

mike95490

If something happens, one or 2 or more strings may fail (likely in a massive parallel setup)
OR
in 4 years when you replace batteries, you may only purchase 2 strings, and work the snot out of the battery bank, saving money on excess bank capacity.

Raj174

Reread Mike's post, it's good information. You will not find a better breaker than those made by Midnite Solar.

Two of these enclosures can mount the six breakers, 3 in each.
https://www.solar-electric.com/mnedc-quad.html

This is the breaker used in the enclosure, I believe Mike suggested 70 amp.
https://www.solar-electric.com/pamodccibr.html

Remember, the breakers protect the cable and must be able to carry the full amperage draw of the inverter.

bryhan67

I like the enclosures with breakers. Pretty clean and easy to install. Since the enclosure will have 2 empty slots any danger running higher volts from the panels through another breaker and out out controler to the battery bank. That would be 8 breakers in the two enclosures.  What would be a good breaker size before the controler and after?  In the pic is what I am setting up. 

softdown

I'm curious about the decision to employ six strings on a part time vacation home. I would have employed more solar and less battery while using 2/0 cables. I'm not an expert of course.

I can't hardly imagine needing more than 12 of those batteries. In fact, I would be inclined to believe that 8 would work fine. FWIW....I have found that good batteries store OK if kept cool and charged over 12.5 volts.

Raj174

No technical problem mounting any size of this type DC breaker for any purpose in these boxes. Obviously, they would need to be clearly labeled. 

Breaker sizes - PV strings- 8 to 12 amps
                         PV to charge controller - 25 to 30 amps, THHN #6 AWG cable
                         Controller to battery bank - 60 to 70 amps, Recommend a THHN #4 AWG cable, although a #6 will do.

Could I ask why you are using such high voltage PV strings?  Are the solar panels mounted very far from the controller and batteries? Otherwise, three panels in series would have been more efficient.  

mike95490

For the simplest, I'd suggest the MRBF fuse blocks and fuses,   A wide selection of fuse ratings.
https://www.bluesea.com/products/2151/Dual_MRBF_Terminal_Fuse_Block_-_30_to_300A
With fuses, you loose an easy way to disconnect one string from the system.

bryhan67

Sorry for the delayed response. I have been in Rosarito getting ready for a upcoming desert race this weekend. The reason for so many batteries is I bought 150 of them to sell. So if anyone needs batteris in the south west usa or baja mexico let me know. Also I want to run a air conditioner 24/7 when I am there in the summer. I have in confined to one small room.

As for the panels what am I losing? I have the midnight solar 200v model. The way my roof is perfect wide for 4 panels. I made adjustable frame for summer and winter. Heck my little Honda eu1000 gen is all I need. I don't need any color or batteris. But what's the fun in that. I feeling of running my account for free off the sun.. When I am full time I will need to figure how to a/c all the house... dc mini splits will be the way to go.

Few times a year I do go for a 2 week stretch.

Anybody have a good source to buy tined copper cable lugs? I think I will get about 100 of them.

bryhan67

Could I eliminate my bus bar and do a enclosure with 6 breakers then one outlet to the inverter? This way would have 6 #2 wires going in and one 1/0 cable going to inverter?

Something like this? But cant find breakers that will except #2 cable. 

https://www.solar-electric.com/mndico.html

bryhan67

Any feedback on one of these? 
http://www.ebay.com/itm/10-Ton-Hydraulic-Wire-Battery-Cable-Lug-Terminal-Crimper-Crimping-Tool-9-Dies-E1-/221862199523?hash=item33a80480e3:g:DuYAAOSwTA9X2MAV

Or is this a more simple way to do it?
http://www.ebay.com/itm/Heavy-Duty-Battery-Welding-Cable-Hammer-Crimper-8-4-0-GA-5-YEAR-WARRANTY-/282168744327?hash=item41b2915987:g:xlEAAOxycERRjV1U

mike95490

hydraulic is the way to go.   The host site NAWS, has tinned lugs in the store AND the midnight combiner panels
https://www.solar-electric.com/36432.html

please don't use a hammer lug crusher

bryhan67

WIth that combiner is there a way to connect #2 wire or lugs from each string? Then a  1/0 cable going to inverter? 

jonr

 > I have it {AC} confined to one small room.

In many cases, I think this is a good way to go.  AC for the entire building might take excessive power - but one very well insulated and air sealed room will be much less while providing a place to escape the heat.  If you want to get fancy, consider adding lots of thermal mass and materials that absorb humidity (sort of a thermal and humidity battery).

Raj174

bryhan67 said:

WIth that combiner is there a way to connect #2 wire or lugs from each string? Then a  1/0 cable going to inverter? 

The largest cable that this PV breaker will take is a #6 AWG. So if you go with the other DC breakers and enclosures, you may have to find or make a 100 amp buss bar with enough terminal connections for the battery strings and a connection for the 1/0 cable to the inverter. It might a good idea to use one of the Blue Sea MRBF fuse blocks and fuses, that Mike posted previously, at the buss bar for the 1/0 inverter cable.  

bryhan67

I called Midnite solar and always those guys are great. Tough to set up 6 strings. Maybe more simple to run the double fuses. So thats the rout I am going. they said I might have some problems charging some batteries. I will go ahead and make up #2 cables just encase I upgrade to larger inverter in the future.   From controller to buss bar I will run the 6ga with 50amp fuse. I think I will eventually I will buy a deep fridge and keep the batteries at 77deg all the time.  Lots of great help here. I lean more and more everyday. Once I get it set up I will post some pics. 

Marc Kurth

What is the make and model of the batteries?

bryhan67

Here is what I built so far. Let me know if you see anything really wrong. All the metal in the rack is so the batteries don't get stolen.  I still need to finish making cables and heat shrink them. Is it ok if all 6 neg cables are about 2 inches longer than the positive? Of is it better to make the positives longer? Also is it critical to have both neg and pos cables same length going from bus bar to inverter?

Marc the batteries are made by data safe they are 12HX540-fr. Hard to find solar related data on them. If you look up trojans they are identical. http://www.trojanbattery.com/pdf/datasheets/12AGM_Trojan_Data_Sheets.pdf

vtmaps

bryhan67 said:

 Is it ok if all 6 neg cables are about 2 inches longer than the positive? Of is it better to make the positives longer?

it's ok

bryhan67 said:

  Also is it critical to have both neg and pos cables same length going from bus bar to inverter?

Not an issue. each cable should be as short as possible.

--vtMaps

mike95490

If these are indoors in a fairly constant temperature environment, the interconnect cables would be OK.  But if there is a lot of thermal excursions, I'd be concerned that those short interconnects could act like solid rods and stress the battery terminals. Short lengths of heavy wire are pretty stiff, when reinforced with heat shrink

bryhan67

Whats better keep them shorter or  longer and  put a little  bend it it.  For #2 cable is there a shortest length to go by?  now they are about 4.5" center hole to center hole. Even longer cable when bent gets stiff also and want to twist.  How much thermal excursions are we talking about? We are on the beach in mexico and this will be in my garage. summer temps outside are 100 high and 80 low then in winter 70 high and 50 low. But inside my garage wont change as much.  These batteries never heat up like my old led acid trojans. 

Marc Kurth

Those are AGM batteries made by Enersys for UPS type duty, rated at 123 ah based at a 20hr rate. They are optimized for high current, short duration output - but they will work fine if you don't deeply cycle them too often.

I could not readily locate DOD/Cycle rating on these, but Enersys batteries of this general type tend to run around 500 cycles at 50% depth of discharge. If you limit it to 25%-30%, you are likely to get closer to 1000 or more charge/discharge cycles from a new set.

In general terms, it is important to provide these batteries an extended float time as often as possible.

mike95490

Have you calculated the loss another inch for each interconnect would induce ? Adding an inch or two (the next time you build cables)
and putting a Z bend/kink in it, will allow stress to be absorbed in the flex of the wire, not the battery terminal.
But if you are only experiencing a 20 or 30 degree swing, that's not much.  Here I deal with 105F summer and 20F
See attached pic and link.  (expansion loop)
http://engineering.stackexchange.com/questions/569/what-is-the-purpose-of-these-diversions-in-a-natural-gas-line