Biting the bullet. Doubling my battery capacity $$$$$$

[Zwy]

My system is running well. I have 6 EG4-LL's for 31kwh of total storage. My daily consumption is 25-30kwh.
When there is back to back sunny days all is good. When it turns cloudy for a few days I have to run off the grid.
I can run only about 1 day without sun. I don't think my investment will ever pay for itself but I think the added security for grid down senerios is worth it.
I'm contemplating going further into the solar $$$ hole by buying another 6 EG4-LL batteries doubling my capacity to 62kwh.
I'm sure I can trim my energy usage some but I live in Ohio and feel like my system sits idle too much. Double battery capacity would give me a lot more use before I need to go to grid, not 100% but a lot more. Should I bite the bullet ? The batteries are on sale now with the 6 slot rack "free". If I'm going to do it , might as well get it on sale - vs. full price. Any thoughts on this ? Thanks

My opinion after building a bank over 50Kwh, you capture more yield from panels with a larger bank. This extra yield can be put to use.

We normally use between 15Kwh to 20Kwh average on a daily basis. Yesterday, we used 54Kwh, I'm taking advantage of the extra PV once the batteries are full by running electric heaters (mini splits to be installed this summer). I look at it as free energy just waiting to be harvested. With the large bank, you can use this power thru the whole day, not just when the sun shines.

Part of the reason I moved to solar was to be able to move most home heating in spring and fall by using the excess. It is working out well.

 

[Zwy]

I just went down and checked on input. I have 16 530W panels, split into 8S, each string goes to one inverter.

PV on inverter 1:


PV on inverter 2:


And total amps into battery bank. With the larger bank, you can really push some amps in. 123A which is 30A per battery. Everything runs cooler this way. Shunt Ah is off, Batrium tripped this morning, never created a log and reset the Ah to 100%.

 

[EastTexCowboy]

I have built two with LF280K from 18650batterystore, so far. At least 12 more in the plans. And considering another 7, because I found some more space. In a bathroom closet that is only 10ft away. lol

I got to thinking about this. How do you handle getting all the cables the same length if it's that much farther away? I guess you could calculate the resistance and size the longer cables accordingly, but other than that I don't see an easy answer. I'm looking at a similar issue if I add more than one more rack of batteries, which I almost certainly will at some point.

 

[timselectric]

I got to thinking about this. How do you handle getting all the cables the same length if it's that much farther away? I guess you could calculate the resistance and size the longer cables accordingly, but other than that I don't see an easy answer. I'm looking at a similar issue if I add more than one more rack of batteries, which I almost certainly will at some point.

I will use larger cables for the longer distance. But I'm not worried about getting it perfect. Right now I have 3 batteries scattered in different locations. The closest one starts discharging or charging first. And the others catch up as the voltage delta changes. They all get to 100% or 10% at the same time. They just play a little leap frog in the middle.

 

[OffGridForGood]

I got to thinking about this. How do you handle getting all the cables the same length if it's that much farther away? I guess you could calculate the resistance and size the longer cables accordingly, but other than that I don't see an easy answer. I'm looking at a similar issue if I add more than one more rack of batteries, which I almost certainly will at some point.

Why not just match the cable lengths?
Say farther rack needs 6 foot cabes, just use 6-foot cables on the closer rack as well?
Or, each rack has matching cable lengths to a mid way bus bar set up, then matching length cables from this mid bus to each inverter?

 

[Quattrohead]

Tim was thinking of putting the other batteries in his neighbor's cow shed. ?

 

[timselectric]

I have thought about it a lot. And decided that I don't want all of my batteries to have the same life span. It would be very expensive to replace 336 cells at about the same time. Granted, it will probably be my grandkids problem. lol

 

[timselectric]

Tim was thinking of putting the other batteries in his neighbor's cow shed. ?

I have definitely been eyeballing their unused inground pool. It would be the perfect place for a sand battery.

 

[JJJJ]

Seriously, I can see how that addition I considered years ago could have been used for something like that.

Somehow, the budget got away from that idea, kids, food, clothing, housing, cars, you know.... At least we have moved away from the "used car lot" when it seemed everyone had a vehicle (not really, but felt that way)

 

[EastTexCowboy]

Why not just match the cable lengths?
Say farther rack needs 6 foot cabes, just use 6-foot cables on the closer rack as well?
Or, each rack has matching cable lengths to a mid way bus bar set up, then matching length cables from this mid bus to each inverter?

I think it's more than 6 ft for him. Kinda that way with me. I have 10' cables to the existing rack. If I end up just putting another EG4 rack of six I can maybe get by with another set of 10' cables if I do some shuffling around. But if I go with a DIY on a shelf or if I add a third rack or shelf of batteries later I'll have to have at least 15' cables and on the opposite side of the inverters. I don't want to replace ALL the cables with 15' so I may take a page from @timselectric and put larger cable for that rack and call it close enough.

 

[timselectric]

The current plan for the two remote battery banks (7 & 5 batteries), is parallel 500kcmil cables. Leftovers from a job. So, already paid for. (Not by me)

 

[Zwy]

Why not just match the cable lengths?
Say farther rack needs 6 foot cabes, just use 6-foot cables on the closer rack as well?
Or, each rack has matching cable lengths to a mid way bus bar set up, then matching length cables from this mid bus to each inverter?

Most of the time my batteries are just idling. With an example of 4800 watts draw, that is 100A total from the bank, 25A from each battery. If I added another bank the same size 10 feet away, the draw drops to 12.5A per battery. How much will they go out of balance with oversized cables? Not much, a much larger cable is just like a busbar, the resistance is very low.

If there will be larger loads, then it might get an imbalance between banks over a long time frame.

 

[Zwy]

One other way is the banks can be connected by having a busbar at the positive of bank 1 with a short cable from bank 1 positive and a long cable from bank 2 positive. The negative cables will be opposite, short cable at bank 2 to busbar and long cable from bank 1.

 

[TorC]

As I make cables and reading this, I'm now wondering how much it really matters to keep my 4/0 battery cables matched in total length. Probably 12' round trip for the shortest, 20' for the longest if I go neat rather than alike. Maximum sustained transfer 41A per 280Ah string based on STC array production (which I have regularly seen with some of the panels, and the rest are similar from same mfr). Under load, estimated daily consumption/24*3=12A/battery string.

4/0 is already purchased and here, not that easy to get more/different cable, so it's not changing.

 

[timselectric]

It's more important to be balanced. If you need all of your batteries to cover your max instantaneous load.
If either bank can handle it on their own. Then I would say close is good enough.
Each of my banks will be able to take care of everything. Extra banks are just adding to autonomy.

 

[TorC]

Well, looks like I need two of four to carry the maximum possible instant load. After trying to calculate total series resistance of each battery string, it looks like the extra 8' of 4/0 makes about a 5% difference in total resistance. I think that's probably within acceptable. Thanks.

 

[SilverbackMP]

Great discussion here as I am finishing up my DIY eight batteries in the next week and want to figure out possible future expansion. I think I will make those 4/0 cables the same length and try to make any future expansion the same length.

 

[Quattrohead]

This is becoming an interesting question.
Keep battery cables as short as possible or keep them the same length.

 

[timselectric]

It all comes down to what you need from your batteries.
Shorter is more efficient.
Same is more balanced.

 

[EastTexCowboy]

This is becoming an interesting question.
Keep battery cables as short as possible or keep them the same length.

I suspect the answer will vary depending on your setup and needs. I believe Tim is right about it not being as important if any of your individual banks can cover the maximum instant load. I'm sure there are a ton of folks on here who can do the math but you know someone is going to make a video and test now that it has come up. As for me, I'm just tired of buying a bunch of expensive copper only to find out that my solar addiction expansion means I have to replace half of it again. I'm acquiring a nice little pile of wire or cable that got replaced with something bigger. I shouldn't complain. That stuff will be a reason to do a separate solar setup later so it doesn't go to waste.