Starting battery pack build... Comments invited!

pfromero

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Joined
Jan 30, 2017
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Hi all,

I am starting my battery pack build. I will be using 240 cells / battery pack with an average cell rating of1650 milliamp hours. I want to use an 8 x 30 array. This I believe will give me 29.6 volts at 51 amp hours for a total power of 1.5 kilowatt-hours per battery pack. Please check my attach picture to see how I want to do it and leave a comment much appreciated. Am I correct in thinking this will give me the opportunity to have either a 24 volt or 48 volt system?

Paul

image_tahxng.jpg
 
you'll have a nominal voltage of 29.6 and i'm assuming you'll want to have a cut off voltage of 24? you should be okay from my understanding.
what parts are you using?


Also how many packs are you building? two?
 
hi firefrog,

I forgot to add the photo.....I measured my Home Depot Surplus stranded cable, and its dia. is 1/4", which went down to 3/8" after I twisted it more with a drill. I will be building 4 packs, and they will be made out of recycled 18650 cells from laptops and medical equipment. There is Mike's holders to keep them together, and Home Depot #2 gauge (1/4 inch diameter) stranded copper wire for the bus. According to Cerro Wire, the maker of the copper wire, their #2 wire is capable of handling 130 amps. I will be connecting 175 amp battery quick disconnects (generic unbranded) to the ends of the bus wire.

What do you guys think of hooking these up to a 24 volt or 48 volt inverter/charger. Will it work?

Thanks,

Paul
 
If you connect this to a inverter just make sure it fits the range of the input voltage, meaning don't over do it or be just shy of its limits other than that it should work.
Did you have a specific inverter in mind?

Are you planning to have cell level fuses?
 
Hi firefrog,

On the negative of the batteries I will be using Remington Industries 20 TCW tinnedcopper wire as fuse wire.

Yes, I have a specific inverter in mind. It is a 12 kilowatt inverter charger. The website to see it is www.sigineer.com
Only problem is I have to find some sort of automatic transfer switch while the battery pack is in use because I have a chilicon power micro inverter zero export to grid AC coupled system. The utility does not know it's there. :)
 
According to Cerro Wire, the maker of the copper wire, their #2 wire is capable of handling 130 amps.

Aspendell brought some interesting information to the forefront:Biggest bottleneck in an 18650 pack#21

It's worth noting, tinned copper wire has significantly more resistance for DC. For this reason we have to nearly double the cross-section of speaker wire that most want to sell on ebay if we are using it for DC current.

In AC current it travels primarily through the core of the conductor, but in Direct Current the electrons like travel along the surface of the conductor, hence the tinned part of tinned copper.

Just an FYI, and FFT (food for thought), is that #2 rating of 130 amps at AC or DC? Could make a huge difference if they are calculating at AC, and you're running DC through it
 
If the utility doesn't know can you be fined?

I'm still new at this, so with this micro inverter does it give back energy to the grid?
 
Hi Firefrog et all,

I never thought of it! Yes, they are calculating AC power. I'm counting on the fact that this is a 51ah battery pack, and the wire can go up to 130a AC. They are stranded cables with 7 strands which will help. But, you know, that "sizing them for being in series" scares me a little, for I may go to 48v DC and this will add additional current through my bus bars?!?

The utility probably would not fine me because my export is zero. They have electronic meters, so now they can monitor residential customers more closely, and I have a bit of (think a few watts) going outbound when power produced is greater than power used for a few seconds. In the case of my system. It is originally designed for customers in Hawaii that operate residential solar systems under their zero export regulations, but it works for anybody. In my case, the electrical rates are low to begin with, the power company's payment for exported energy even lower, and the combined construction and engineering costs so great if the utility was involved, that I had to cut my costs dramatically to make financial sense of my solar project.

If you have micro-inverter grid tied solar system, yes, you will give back energy to the grid, unless you can control the micro inverters to throttle back or even shutdown ...when export is greater than zero...and this is what my system does.
 
Yes the more voltage you add the more current you should expect to have
Due to ohms law V=I*R
V is voltage
I is current or amperage
R is resistance
 
Cells at 1650 can be bad yes! But if they are from 2000mAh cells they may be as good as 2100 coming from 3000mAh :)

But yes i agree 1650 in avg sounds low. But at same time you take what you get :)


Im going to use lower cells to but as extra packs only... Small packs of 40p and such so i easily can bring out those strings..... 14s40p and so forth...
 
Hi Firefrog and all,

FireFrog said:
Yes the more voltage you add the more current you should expect to have
Due to ohms law V=I*R
V is voltage
I is current or amperage
R is resistance

My battery pack has 51Ah, my voltage is 24volts , if I put batteries in series, it will bring voltage up to 48volts and double the resistance. Therefore I should go with a 24v inverter charger instead of a 48volt. They have a 10kw 24v inverter charger on the above mentioned website.

Paul
 
No. Double volt equals half the current. That equals less loss and thinner cables. Always go with as high voltage as you can!
 
Yes I concur,
The higher the voltage, the lower the current saturation on busses, sub-busses (the ones between each S in your 8s config), and your wiring connected to your pack connectors to get it where its going.
Every time you double the Voltage it increases the power you can put thru the same conductor while observing the same & voltage drop by 4X.
When they Rate wires it is usually based on the distance it can carry a specific voltage at a specific current while observing a 3% voltage drop.

Its good practice to at least double the capacity that would be needed at max current when choosing wiring. I shoot for at least 3X myself. There are many reason for this. Any voltage drop you have in your system will be wasted power in the form of heat. But if you decide to add another pack to this one later, or you are planning on say 2ga cables for a 3 foot run to your inverter now but you end up needing an 8 foot run later, then you already have the extra margin built into your design. And there is a direct relationship between voltage drop and current rise, to a specific draw. For instance, if you have an inverter that is trying to to pull 50amps at 48 volts from your pack but your wiring is undersized so you have a 10% drop in voltage, then the current will go up by 10% to try to compensated for the requested power needed, so it would shoot up to 55amps.

The biggest material costs we have in DC systems is the Cables. The easiest way to minimize this is to raise the voltage anywhere the current is traveling more than 6" or so. But there is a trade-off on the safety side. Most are building single cell parallel packs which keeps the +and - on opposite sides of the pack and never much over 4v if a short were to occur.
For this reason the series-parallel packs (use much smaller cables) much be designed with much more attention paid to safety.

My motto: Always oversize conductors ;)
 
Hi all,

Aspendell said:
But if you decide to add another pack to this one later, or you are planning on say 2ga cables for a 3 foot run to your inverter now but you end up needing an 8 foot run later, then you already have the extra margin built into your design.)

Very good reasoning! I hope that I'm going to be ok with 24v DC, and I plan on a 6000W inverter for now (drastically scaled down my plans from yesterday). If I go bigger, than I can put the 24v packs I am building into series for 48 volts. In any case, I need to email the manufacturer to see if they have a proper inverter/charger for my packs, as firefrog suggested.

In addition to the above drawing, I have put together a "whole system" drawing, with an automatic transfer switch involved because I cannot circumvent (go around) the design of my microinverter based zero net export system. In addition, I have added a "Critical loads" subpanel to seperate the output from the inverter from the main AC grid when it is in operation because there is no way to "throttle down" the inverter output, like I can the micro inverters, and this would result in excess power out to the grid if it (the inverter) was powering the main circuit breaker panel. I hope that is not clear as mud, and I hope you can understand the drawing.

Thanks for looking,

Paul

image_nkzmic.jpg
 
Looks like you have done a lot of planning.

If you can get an inverter that has the charger (AC and solar/wind) and transfer switch and communications port built into the main unit you will save a lot of money and time in the long run. A descent 150 amp charger could cost you as much as you paid for a descent inverter later.

If you haven't yet bought your inverter, go over to knurlgnar24 channel and watch several of his inverter videos. They are a year or 2 old but very relevant. And he works in the electrical engineering field.

He will explain why you never want to buy an inverter that has a peak output rating of more than 1.5X its rating. Basically they are all junk that use transformers to do the conversion, rather than switching and electronics. The transformer is not only inefficient and heavy but they get their peak ratings by using the power stored in the transformer in the magnetic fields to get their peak output figure they claim. He found that most of those inverters that claim 4000 to 12,000 watts are so inefficient that they can even sustain 1500 watts for 10 mins.
See, if you have even a 3000 watt inverter that is only 85% efficient, that means that 15% of the power you are putting into it needs to be dissipated in the form of heat. That would be 450 watts of heat thru a case and fan that is only designed for maybe 250 watts of dissipation.


So unless you are paying well over $4000 USD you will probably be buying an under-engineered inverter.
Also if you will truly be using 4000 watts, you are better off going to at least 48v on the DC side. 170 amps is a lot unless you are going to at least 0/0 cables, which are very expensive.
Hope that helps :rolleyes:
 
Aspendell said:
So unless you are paying well over $4000 USD you will probably be buying an under-engineered inverter.

Hi,

Can you say, "Outback 8084 Radian Inverter/Charger plus all the bells an whistles"....that is the one to get, I already have found out.

"open thy wallet"

Paul
 
Sweet
Outback is one of the best engineered systems out there, if you can afford them. So is that 2 inverters chained together?
 
pfromero said:
Hi all,

I am starting my battery pack build. I will be using 240 cells / battery pack with an average cell rating of1650 milliamp hours. I want to use an 8 x 30 array. This I believe will give me 29.6 volts at 51 amp hours for a total power of 1.5 kilowatt-hours per battery pack. Please check my attach picture to see how I want to do it and leave a comment much appreciated. Am I correct in thinking this will give me the opportunity to have either a 24 volt or 48 volt system?

Paul

I skimmed over the replys and didnt see this but if its mentioned disregard. Your bus bar seams kinda long from end to end. Its snaked through the cells like it should be but I would connect the ends of the pack together. In the picture where it says 8s on the left middle, those cells voltage have the longest run in the pack. Connecting the cells on the ends of the pack would help.
 
AveRageJoe said:
pfromero said:
Hi all,

I am starting my battery pack build. I will be using 240 cells / battery pack with an average cell rating of1650 milliamp hours. I want to use an 8 x 30 array. This I believe will give me 29.6 volts at 51 amp hours for a total power of 1.5 kilowatt-hours per battery pack. Please check my attach picture to see how I want to do it and leave a comment much appreciated. Am I correct in thinking this will give me the opportunity to have either a 24 volt or 48 volt system?

Paul

I skimmed over the replys and didnt see this but if its mentioned disregard. Your bus bar seams kinda long from end to end. Its snaked through the cells like it should be but I would connect the ends of the pack together. In the picture where it says 8s on the left middle, those cells voltage have the longest run in the pack. Connecting the cells on the ends of the pack would help.

Thanks Ave Joe!! I will add a length of #2 awg bus to the ends and double it up, plus some shrink wrap. Thanks again.

I have some questions though,

Ave Joe:
1) I originally had 28 gauge copper wire that I was going to use as fuse wire. When I would short a 18650 battery (+ to -) with this wire, it would blow within 1-2 seconds. Good, I thought, but when I asked you (PM message last week), you told me that you tested a 30 gauge, and it took 15 amps to blow. So, I dont understand!?!

hbpowerwall:
I bought the 20 gauge TWC (from Remington Industries), but Ave Joe tested a 30 gauge version from Mike, and it is not popping at even 15 amps! I tested this 20 ga wire on one of my 18650 batteries, and it didnt even get hot.

All:
With respect to the 30 gauge wire Ave Joe is testing: are we talking about the same wire...i.e. 30 gauge tinned copper from Mike in both above questions?

It looks like I am going to have to go with authentic fuse wire (100g) 5-6 amps. Is my thinking correct?

Thanks,

Paul
 
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