Battery Wiring Option A or B????

Farmerboyz1721 · July 2016

Which option in the picture would be the best way to wire these batters.

Batters are 6 volt 210 amp hours each.

Image: https://us.v-cdn.net/6024911/uploads/editor/1w/08micjro7ifs.png

BB. · July 2016

Personally, I prefer "A"... I am concerned about solid bus bars connecting batteries together (thermal expansion vs bus bar creating stress on battery terminals).

I also, sort of, like the little bit extra of series resistance in each battery string... This acts like "ballast resistors" that helps balance similar current flow through each battery string (the battery string with the lowest resistance will carry the most current--And with I^2 heating, it can cause over heating in wiring/terminals in the string with the lowest resistance).

You also do not show where your battery bus common connections are. The plus common connection should be on the opposite end of the bus bar with respect to the negative bus bar (so that all battery strings see the same resistance to the loads/charging points--I.e., top left negative bus bar and bottom right on positive bus bar).

Lastly, you do not show any fuses/breakers (one per series string)... Each string (when you have three or more parallel strings) should have its own breaker/fuse to limit short circuit current (say a wrench falls across the center battery string--Then all of the battery strings on each side can put short circuit current into the middle string and possibly cause a fire).

https://www.bluesea.com/products/category/16/72/Fuse Blocks/Terminal Fuse Blocks

MRBF Terminal Fuse Block - 30 to 300A

These BlueSea fuse holders are pretty nifty (you still need to order fuses too). Note that fuse holders+fuses are not cheap--Many times, it is not much more expensive to use circuit breakers instead. Also adds a manual switch if you need to work on a specific battery string or want to turn off the battery bank (winter shutdown, resetting charge controllers, etc.).

-Bill

vtmaps · July 2016

BB. said:

Lastly, you do not show any fuses/breakers (one per series string)... Each string (when you have three or more parallel strings) should have its own breaker/fuse to limit short circuit current (say a wrench falls across the center battery string--Then all of the battery strings on each side can put short circuit current into the middle string and possibly cause a fire).

Firstly, why so many parallel batteries? No matter how you wire them you are going to have current imbalances.

Regarding fuses, it's not just falling wrenches you must worry about. Sooner or later you may have a shorted cell, and without a fuse you may have thermal runaway and a fire. As the batteries age and become imbalanced, some strings will take less current than others, and then you may have too much current through the lower resistance strings.

--vtMaps

Farmerboyz1721 · July 2016

Vtmaps
I needed the amp-hours and chose to use the affordable costs batteries.
In a previous post you have educated me on not having to many parallel connections.
Unfortunately i have to stick with this battery bank for a bit till i can afford a new one.
So i am trying to make the best of the situation that i have.

vtmaps · July 2016

Farmerboyz1721 said:
So i am trying to make the best of the situation that i have.

OK. Bus bars are indeed making the best of the situation. In addition to fuses, get hold of a DC clamp ammeter and a IR thermometer. Use the clamp meter to check for imbalances when charging, and use the IR meter to look for batteries that are warmer than the rest.

--vtMaps

Farmerboyz1721 · July 2016

Vtmaps.
Thanks again for your information. Which option do you recommend?

BB Thank you for your information.

jonr · July 2016

> No matter how you wire them you are going to have current imbalances.

Technically, I agree with this. But don't take it to mean that you can't measure such imbalances and by adjusting wire lengths, get rid of most of it. Despite sources that claim "perfect', you can do much better than "equal length paths".

vtmaps · July 2016

Farmerboyz1721 said:

Vtmaps.
Thanks again for your information. Which option do you recommend?

What option? You mean IR thermometer (about $30) or DC clamp meter (about$60)? I would suggest both

--vtMaps

mike95490 · July 2016

wiring option A ver B can cause stress on the terminals

littleharbor2 · July 2016

How would you fuse each string with a solid buss bar linking all batteries?

BB. · July 2016

At the mid point of the two battery string (jumper cable).

If they were 12 volt batteries on a 12 volt bus, not as easy.

-Bill

Farmerboyz1721 · July 2016

@vtmaps I meant which wiring option do you recommend in the photo of the first post.

And yes i am going to get me an IR Thermometer. I have a DC clamp.

vtmaps · July 2016

Farmerboyz1721 said:

I meant which wiring option do you recommend in the photo of the first post.

I agree with Bill and Mike. (option A)

--vtMaps

Marc Kurth · July 2016

I am a fan of single string battery banks. It's the best way to go unless there other reasons not to.

But I have to say that I have seen many, many off grid battery banks with four (or more) parallel connections run well. BUT they often use bus bars and the owners occasionally rotate battery positions. Generally they are 3-4 strings of 8 are they still running 6-7 years later. Keep in mind I only have experience with AGM's.

The oldest that I can recall was 10 years old when they swapped the batteries:
48v system with 3 strings of 12v 255 ah designed for a 10%-15% average daily DOD. The batteries were shuffled around annually based on standing voltage measurements. I personally capacity tested each of the old batteries and found that 2 were in the 80% range, most were around 50%-60% with a couple of dogs at 10% 20%. Obviously the system still "worked" because of it's large capacity to load ratio. But the owner got 10 years of service from them.

This one is about 5 years old and going strong in a very large residential system: http://marckurth.com/BatteryRoom1.jpg

littleharbor2 · July 2016

Nice battery room! I'd like to see more pix of it if possible.

Marc Kurth · July 2016

I will ask my customer for permission to share more. Not my design, I just supplied some stuff.

It is a 48v battery bank with 4 strings of (24) L16, 2v cells for a 233 KWH total storage capacity. His design has a shallow DOD and the battery room is climate controlled because he felt that PV was cheap compared to batteries.

The PV arrays are part ground mount and part roof mount on a new metal building/shop that he put up during the process.

littleharbor2 · August 2016

Looks very well done. Climate controlled, how cool, no pun intended. Are those 2 volt L-16's AGM type?

Marc Kurth · August 2016

Very clean work indeed by the contractor. I only helped with sizing and I supplied some stuff for the project.
Yes, Sir. There are (96) Concorde PVX-12150T 2v, 1200 ah AGM L16's.

porch13 · August 2016

Lastly, you do not show any fuses/breakers (one per series string)... Each string (when you have three or more parallel strings) should have its own breaker/fuse to limit short circuit current (say a wrench falls across the center battery string--Then all of the battery strings on each side can put short circuit current into the middle string and possibly cause a fire).

-Bill

Quick question on this. I'm looking at the "Outback Power VFXR 3,975 Watt Off Grid Solar System Kit with 3,600 Watt Inverter" kit from NAWS, which is a 48V system and will have two parallel strings of 8 batteries. With two strings does each string need to have a fuse/breaker or only when you get to three strings?

BB. · August 2016

I am trying to be safe--When you have three strings--2 strings in parallel can feed the third shorted string and overheat the wiring...

Just a suggestion. Not a lot of people seem to fuse/breaker per string. Shorts in the battery bus scare me.

-Bill

Marc Kurth · August 2016

Yes, Sir. Bill, you are right on track as usual. I try to get people to look at each segment of system as a complete subsystem and run through a few "what ifs" to decide if that section is adequately protected.

When you have seen the results of "stored energy gone wrong" you quickly gain a strong affinity to fuses!

vtmaps · August 2016

One thing fuses won't help with is high current bulk charging. If one or more strings develops high resistance, bulk charging current can get pretty high in the remaining string(s). But not high enough to blow the string's fuse. With any luck you will have put your remote temp sensor on the overheating string.

--vtMaps

porch13 · August 2016

I certainly don't mind spending a bit more for added safety. I like the Blue Sky Fuse that Bill had shared. If I put one on each string, which size fuse would be appropriate?

vtmaps · August 2016

porch13 said:

If I put one on each string, which size fuse would be appropriate?

I am not sure... I try really hard not to ever be in the position of needing to know

If you use too high a value, you don't get all the protection you might need from a shorted cell or bulk overcurrent events.

If you use too low a value, you run into the risk of cascading failures. If one string's fuse blows, the diverted current from that string may cause another string's fuse to blow, which causes them all to blow in a cascade of dollars sent to Blue Sky.

--vtMaps (not a fan of parallel batteries)

BB. · August 2016

The issue for fusing is that holders+fuses+spare fuses is not cheap--And you need to keep spare fuses handy.

For circuit breakers, they are not much more, but do take up a lot more room. But you also get a "free" on/off switch for debugging your system/battery strings.

For fuse ratings, there are two major standards in north America. The first is NEC. It is conservative. And remember to use 0.80 (80%) derating for wiring/fuse current. A 20 amp wire+fuse * 80% = 17 amps maximum continuous current (breakers/fuses will eventually trip at 100% of rated load). Here is a basic NEC wire rating chart:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

And then there is the Marine standard. Much less conservative and less controlled by standards (also uses SAE wire gauge which is slightly smaller in diameter than AWG standard wiring). I would use the Marine standard as an absolute maximum wire rating (till need 80% derating for fuses/breakers):

http://www.acbsphl.org/Tips_and_hints/ABYC_Wiring.htm

For solar/off grid power, we tend to have much heavier wire than needed because we want low voltage drop for our wiring (less energy losses, and for low voltage battery like 12 volt banks, you do not want a 10% voltage drop or 1.2 volt drop--11.5 volt battery voltage and 10.5 inverter cutoff voltage--Your inverter will turn off with a 1/2 discharged battery bank when under heavy load).

-Bill

porch13 · August 2016

So it sounds like a larger fuse just to avoid a catastrophic failure. I would rather avoid the parallel strings, but can't seem to beat the price to get a 48V 520AH setup.

Marc Kurth · August 2016

While maybe not optimum for every project, parallel strings are applied successfully every day all over the world. It has to be done right, but it is not something to be avoided at all costs as folks seem to think. Depending upon the design parameters, parallel strings can have distinct advantages.

A good approach is to go for a single string when feasible - within reason. The panic around here when parallel strings of batteries comes up is unjustified. In the real world beyond the internet, it is done all of the time and works very well when done properly.

In the hundreds (yes, literally) of battery banks that I have been involved with, parallel connections are not the reason for early demise of the battery banks.

softdown · August 2016

BB mentioned the advantages of circuit breakers over fuses. I have bought large "salvage" circuit breakers only to find that the maximum wire size was ~#4 AWG.

My learning is that #2/0 find strand copper is good for battery interconnects with #4/0 being even better. I use #4/0 here. I don't believe that circuit breakers that can handle these enormous wire sizes will be easy to find.

EDIT: Just looked at my main 125 amp circuit breaker. Looks like the max wire size would be ~#0/0. Not sure that size would fit.

vtmaps · August 2016

softdown said:

I have bought large "salvage" circuit breakers only to find that the maximum wire size was ~#4 AWG.

Are those breakers rated for DC current? --vtMaps

softdown · August 2016

Doubtful....and I don't recall the discussion for using AC breakers on DC. Seems that is not by the book but could work if some hazy formula was applied.

A big reason that I took a big break is that I have no strong interest in spending years learning and worrying about the tens of thousands of off grid details. Lots of pros post here...they sometimes come off like "code inspectors".

I just tell interested people that solar gets complicated. Pretty complicated.

vtmaps said:

softdown said:

I have bought large "salvage" circuit breakers only to find that the maximum wire size was ~#4 AWG.

Are those breakers rated for DC current? --vtMaps

vtmaps · August 2016

Do NOT use AC breakers on a DC power circuit. Really bad things will happen... and I don't mean that really bad things might happen... they will happen.

--vtMaps

Battery Wiring Option A or B????

Comments

MRBF Terminal Fuse Block - 30 to 300A

Categories