Cable size?

Erinscot · October 2014

I am wiring up a new system that will eventually include solar. I will be using 6 T-105's connected to a magnum MSH3012 on the 12V side. The inverter specifies 4/0 cables through a 400A fuse between the bank and the inverter. I have the fuse, will be having the 4/0 made up, but question what size I truly need for the battery interconnects. 4/0 would be wonderful, but spendy! Think I can get away with smaller? Or will any benefit of the 4/0 just be washed away but undersizing the interconnects too much?

I will be working a trimetric into the system at a later date.

Blackcherry04 · October 2014

Re: Cable size?

2/0 $3.70 a foot shipped, 4/0 $5.30 a foot shipped in 10' lengths. If You make all the Interconnects 9 1/2 " cable length plus the lugs and gives you a nice air space between the batteries easy room for expansion and contraction. The series cable goes in a "S " shape and the outside buses loop from battery to battery.

You always going to have a voltage sag with 12v , why tempt any drop for a $1.60 a foot.

Added: Every thing about being successful with batteries is about attention to detail. It's every crimp, Lugs that fit the battery lug posts. 2" of adhesive lined heat shrink on each lug, proper nut and washer. You have to reduce resistance in every connection that is possible. It's just not place to skimp. You drop a 180 amp load on a 12v bank everything has the best it can be. Even the charger in your inverter produces a lot of amps overtime ( 125 amps) one weak connection you'll have big trouble, that kind of amps will cause it to glow cherry red on a bad connection. Everything on 12V paralleled batteries is about getting them to charge and discharge evenly as possible.

I recommend you number each battery, the first pair on the positive end take the brunt of the current demand. Once a year tear the bank apart and shuffle the position of the batteries in the bank and brush every connection and coat every connection with a corrosion protectant. The vent caps on your batteries will clock 180°, make vent holes in the sides face away from the connections, attention to detail.
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vtmaps · October 2014

Re: Cable size?

Blackcherry04 wrote: »

the first pair on the positive end take the brunt of the current demand.

He has 6 batteries (three parallel strings). Do you mean that the three (not two) batteries on the positive end will take the brunt of the demand?

--vtMaps

inetdog · October 2014

Re: Cable size?

vtmaps wrote: »

He has 6 batteries (three parallel strings). Do you mean that the three (not two) batteries on the positive end will take the brunt of the demand?

--vtMaps

The way that I read it, he means the string (pair of batteries) whose + terminal is closest to the inverter and CC. Assuming that the batteries are wired diagonally, that pair would be the last pair on the negative end.
I do not think that he means that the two batteries in one series string will behave differently from each other, ignoring the other 4 batteries.

Blackcherry04 · October 2014

Re: Cable size?

He has 6 , 6v Trojan t_105's. They will be set's / pair's of 2 batteries in 3 parallel strings. As I said the first pair on the positive end always will use more water than any of the rest and they will always have less capacity as they age if you don't shuffle them around. My observation comes from dealing with 100's of banks in the same configuration of 8-10 pair strings for 30 + years. Anytime anyone care's to set up a bank and run them for a few years, please post back and let us know what you found. My synonymous is they take a bigger demand while charging and discharging.

Yes they are wired diagonally.

Next will we will be talking about Dip and Recover and what it's called and if it effects batteries in paralleled strings.

vtmaps · October 2014

Re: Cable size?

Blackcherry04 wrote: »

Yes they are wired diagonally.

As explained at Smartgauge, diagonal charging is uneven charging for three or more parallel strings. Better, would be to use battery bus bars or combiner (smartgauge method 4).

Your wiring method may charge and discharge the middle batteries differently than the batteries at either end, but the batteries at the positive end and the negative end should be the same as each other. I guess that the reason the batteries in the positive end of your setup charge differently is because of temperature difference at that end of the battery bank.

--vtMaps

Blackcherry04 · October 2014

Re: Cable size?

vtmaps wrote: »

As explained at Smartgauge, diagonal charging is uneven charging for three or more parallel strings. Better, would be to use battery bus bars or combiner (smartgauge method 4).

Your wiring method may charge and discharge the middle batteries differently than the batteries at either end, but the batteries at the positive end and the negative end should be the same as each other. I guess that the reason the batteries in the positive end of your setup charge differently is because of temperature difference at that end of the battery bank.

--vtMaps

Thanks for the recommendation. I'll pass, as it is it works just fine and it's a good excuse to disassemble the bank and do the required maintenance on the cabling.

Erinscot · October 2014

Re: Cable size?

FYI, I did elect to go with the 4/0 interconnects. The batteries are wired similar to the smartguage, with inverter connections at opposite corners of the series'.

Cariboocoot · October 2014

Re: Cable size?

Erinscot wrote: »

I am wiring up a new system that will eventually include solar. I will be using 6 T-105's connected to a magnum MSH3012 on the 12V side. The inverter specifies 4/0 cables through a 400A fuse between the bank and the inverter. I have the fuse, will be having the 4/0 made up, but question what size I truly need for the battery interconnects. 4/0 would be wonderful, but spendy! Think I can get away with smaller? Or will any benefit of the 4/0 just be washed away but undersizing the interconnects too much?

I will be working a trimetric into the system at a later date.

I'm surprised no one has mentioned you don't want 3 parallel strings on a 12 Volt system especially, nor do you want a 3kW 12 Volt inverter (if you really need that amount of power).

Too late now I guess.

Technically the interconnecting wires do not have to be the same size as the run from the batteries to the inverter. It's usually done because it's easier (pull a few more inches off the roll, add ends). Otherwise the current is going to be around 250 Amps maximum which could be handled by 2/0 AWG, but why bother?

Now some will say that the current per string is 1/3 that or 84 Amps which can be handled by 2 AWG, but that relies on the current being even. Don't bet on it happening. If you do wager that, each battery string should have its own fuse because you have to protect that short length of smaller wire in between each battery in the string. If you don't, it can easily fry should the current be unbalanced between strings and exceed the wire's maximum. Not much of an advantage over using all large size wire, eh?

Erinscot · October 2014

Re: Cable size?

The main reason for going with the msh3012 was to use the hybrid function to run our marine air conditioners. But now we have discovered that the generator is dying, and are looking at integrating solar much earlier than originally expected. We realize that the 16k btu A/C's are not reasonable now (might not have ever been). With those loads gone, the 3012 is definitely overkill now, but it is what we have.

Curious...why do I not want to have 3 strings? With the decreased loads, I can easily switch back to 4 t-105's with two spare if it would be better.

vtmaps · October 2014

Re: Cable size?

Erinscot wrote: »

Curious...why do I not want to have 3 strings? With the decreased loads, I can easily switch back to 4 t-105's with two spare if it would be better.

Here's a short discussion of why: http://www.wind-sun.com/ForumVB/showthread.php?14674

As for your idea of retiring two batteries for storage... it's use them or lose them. Might as well use them... you've made your mistake with this bank. One of the nice things about batteries is that when you make a mistake like this, you get to try again. Actually, even if you make no mistakes you will buy another battery bank, but not so soon.

What you should have is a $60 DC current clamp meter. Every month or so when you do battery maintenance check to see if the charging current is equal in the three battery strings... when it starts to vary, start a new thread here and we'll see what we can do to help.

--vtMaps

mike95490 · October 2014

Re: Cable size?

Erinscot wrote: »

.... But now we have discovered that the generator is dying, and are looking at integrating solar much earlier.....r.

But you will have some sort of generator to provide backup power for when the sun don't shine, and the batteries are low ?

Erinscot · October 2014

Re: Cable size?

I have a few of these around... Attachment not found.

zoneblue · October 2014

Re: Cable size?

This is kind of a FAQ here. Someone has a boat or RV that was historically a small 12V system, a few lights and electronic kit. They then decide to add a few things, then a few more things, and next thing you know they trying to run 5000W inverter off of 12v. A 5000W inverter will draw upwards of 550 amps. No average battery anywhere will cope with that. And least of all, not a few golf carts.

The investment in 12V gear prevents people from doing the rational thing, of going to a higher system voltage. But the fact is each step up in system voltage quarters the current related losses.

Following the forum rule of thumb, you need 100Ah of battery for each 1000W of inverter on a 48V system. That implies that for a 12V system you need 400Ah per kW. With AGM you can go a bit less, due to lower internal resistance.

Blackcherry has through long experience found a way to make it work, but read and reread carefully the preconditions he specifies.

inetdog · October 2014

Re: Cable size?

zoneblue wrote: »

T But the fact is each step up in system voltage quarters the current related losses.

Well, yes and no....

I throw that number around too, but a more accurate analysis says that for a given fixed wire size, each doubling of voltage cuts the IR losses in half.
What changes by a factor of four is the percentage voltage drop, because you have doubled the system voltage and cut the wire voltage drop in half.
If you go to smaller wire to take advantage of the lower current, and try to maintain the same percentage voltage drop, you will actually leave the amount of power lost in the wires the same.

Example:

12V, 5% Voltage Drop (%VD) and 8A would be .6V times 8A = 4.8W.

Change that to 48V and go for the same %VD:

48V, 5% VD and 2A would be 2.4V times 2A = 4.8W.

Keep the same wire size instead and you have:

48V, .3125% VD, and 2A, giving you .15V times 2A = .48W

So, you can either use smaller cheaper wire and get the same energy losses or half the losses or you can keep the same expensive large wire and get 1/4 of the energy losses.

zoneblue · October 2014

Re: Cable size?

P=I^2*R, thus for a fixed R, I/2 means P/4. Can you mitigate that? Sure you can.

gww1 · October 2014

Re: Cable size?

Is it already part of the math (When dealing with solar) that: The only time you actually lose the three percent drop is when you are hitting the maximum amprage you planed for in wire? If your panel only puts out 70% of its rating but you sized wire for a 3% drop at the panels 100% rating are you then no longer losing the 3% you planed for, OR am I all wet?
Thanks
gww

zoneblue · October 2014

Re: Cable size?

On the pv side the voltage is relatively constrained and its the current that varys. So yes, the power losses are lower at lower insolation levels. Also you are horse trading copper costs, extra panels, and controller losses and so on, so losses on the pv side arent quite as critical as elsewhere.

2% is only a guide, and you certainly want much lower percentages between controller and battery, and battery and inverter for obvious reasons.

Cariboocoot · October 2014

Re: Cable size?

The loss is based on V*I in vs. V*I out. So the panel Voltage & current under operating conditions is the real basis, but since we are looking at maximum loss potential then it is calculated off Vmp * Imp which is the maximum power of the panel.

Using nominal Voltage (which is less than Vmp) will give you a large amount of 'fudge room' on the loss.

Cariboocoot · October 2014

Re: Cable size?

zoneblue wrote: »

2% is only a guide, and you certainly want much lower percentages between controller and battery, and battery and inverter for obvious reasons.

This especially!

The controller to battery run is vital to beginning Absorb stage at the proper Voltage level; charge controller judges by V at its output which may not agree with actual battery Voltage due to V-drop on the wires as the V*I here will be at its maximum at the beginning of Absorb.

The battery to inverter run will have an effect on maximum output power vs. Voltage sag and possibly inverter cut-out due to "low Voltage".

Cable size?

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