Using 2 strings of FLA batteries in parallel versus 1 string 48V 710 Ah bank

[Living Large]

I am posting this as its own thread, though it came up in another I posted here. It is a stand-alone topic.

I need about a 710 Ah 48V battery bank. I located a 375 Ah 6V battery, and was planning to create a 750 Ah bank with 2 strings of 8 batteries in parallel. Sixteen for $5600. I had seen advice to never use more than 3 strings in parallel.

A user here suggested I use a single string of batteries, since parallel strings can cause problems. With research, I found examples of people who said "I will never parallel batteries again". One novel description said the issues with paralleling increase with the number of strings as:
Issues = (# strings)^^2

OK, I looked and found some 2V cells. One (Crown 2CRP1100) was around $900 per battery. I stopped looking at this point, since that is $19400 for twenty four.

This leads to a question. Are the disadvantages of two parallel strings so great as to go to a single string? Should I keep looking for 2V cells that are more economical? A gut feeling, not based on fact, is that unless I could get in for a 50% premium (~$8000), it wouldn't be worth it. On the other hand, these 2V cells may last twice as long. Maybe I should be trying to find 24 2V cells for $11,000?

Perhaps someone could address the issues I might face at some point with two strings instead of one.

Thanks!

PS More than once, someone has suggested initially using golf cart or cheaper batteries at first - this question is for a permanent solution.

[Amy@altE]

I looked at a few Trojan Industrial 6V and Surrette 6V options, and they are all around $9k - 10k. 24 2V Trojan L-16-REs gives you 1100ah for $8700, so that may be an option.

[Living Large]

I looked at a few Trojan Industrial 6V and Surrette 6V options, and they are all around $9k - 10k. 24 2V Trojan L-16-REs gives you 1100ah for $8700, so that may be an option.

Thank you, Amy. I thought there were others in that middle ground. I had seen the L-16-Re @ 1100Ah, but the Ah increase I wasn't sure I can handle. That is a jump. Right now I have 4975W solar and the generator is 6000W and the XW5548 has a 110A output max. Someone else here can figure that out a lot faster and more accurately than I, but I think both the generator and PV are undersized for 1100Ah. The XW maybe just satisfactory.

[Amy@altE]

If you don't use the extra capacity, then your depth of discharge will be smaller, and your batteries will last longer.

[Sunking]

Use the best with a 10 year warranty Rolls 5000 series 6CS25p a 6 volt 820 AH battery. Or use Rolls Battery Selector and don't get married to 6 volt batteries as there are 2 and 4 volt cells. There is no reason to use parallel cells unless you need more than 4000 AH.

[Amy@altE]

Those Rolls (Surrettes) are one of the ones I was calculating. I'm seeing $10,784. Very, very nice battery.

[thastinger]

Especially as a beginner, your life is going to be easier with one series string of batteries.

However, I can see the logic in a parallel string. If that is your only source of power and you loose a battery you can reconfigure to one string and still run at a reduced capacity until you can replace the dead battery.

Since I don't require my battery bank to live, I have one series string. I've only have to EQ them 3 times in 2 years.

[Living Large]

Lots of interesting information here. Thanks, folks.

At a glance, the consensus appears to be to go for the single string. I had not considered the advantage thastinger points out - that two strings is a form of redundancy. My only other backup will be the generator, which while far better than nothing, could be pretty rough until replacement of the bank. Now I'm looking for a reason to NOT go with parallel strings, because having the battery bank fail doesn't sound good.

What problems might I face with two parallel strings, and how hard would they be to avoid/overcome, if it is easy to describe?

If I do go single string, will batteries normally give signs they are going prior to an abrupt failure?

[thastinger]

I think total and sudden failure on an off-grid system is pretty rare. The batteries aren't getting tossed around and live in a relatively controlled environment.

Your new part time job, and that one that is going to give you advance warning of pending failure, is checking SG and water levels. When the batteries begin their downhill slide you will not longer be able to get the SG to what the battery manufacturer specifies and you'll notice a reduced capacity.

[Living Large]

I think total and sudden failure on an off-grid system is pretty rare. The batteries aren't getting tossed around and live in a relatively controlled environment.

Your new part time job, and that one that is going to give you advance warning of pending failure, is checking SG and water levels. When the batteries begin their downhill slide you will not longer be able to get the SG to what the battery manufacturer specifies and you'll notice a reduced capacity.

That is what I assumed. I remember my dad slaving over LA batteries in our garage, as I related in a thread here. Like a mother hen, his collection of hygrometers and bottles of distilled water nearby. I really don't like that image, other than it is one of the few visions I have of dad. I remember him nursing weak batteries, and bringing them back to life. I guess I'll have a slightly higher class affair, and when batteries become suspect it will be time to take out a loan and buy a new bank. If I follow through with this insanity, dad will be smiling. Or perhaps laughing at me.

Based on my feeling right now, I'd lean towards the advice to go with a single string, and hope to develop my "A game" at maintenance immediately, and not make a very expensive mistake. Heck, I have my training of when I was 8 years old, looking over dad's shoulder.

[Living Large]

Use the best with a 10 year warranty Rolls 5000 series 6CS25p a 6 volt 820 AH battery. Or use Rolls Battery Selector and don't get married to 6 volt batteries as there are 2 and 4 volt cells. There is no reason to use parallel cells unless you need more than 4000 AH.

I did check this battery out a few days ago, and was impressed until I saw the 313 pound weight. No pain, no gain.

The cells are said to be "bolt on" (removable?) on this. Would this mean conceivably one could repair a bad cell?

[Mike90250]

here's the article about parallel strings. The differences in resistance between strings, forces the stronger string to take the most load.

http://www.smartgauge.co.uk/batt_con.html

You can go with 2 strings and connect on the diagonal.

2 strings gives you twice as many cells to fill and check. Terminals to check and keep clean.

If you know that you have the right AH bank chosen, pay the price for the easier to maintain single string.

[thastinger]

I did check this battery out a few days ago, and was impressed until I saw the 313 pound weight. No pain, no gain.

The cells are said to be "bolt on" (removable?) on this. Would this mean conceivably one could repair a bad cell?

When people visit and ask questions about my off grid system, I've found it much easier to explain the batteries in terms of "pounds of energy storage capacity" because that is really what you're doing. If you need X Kwh of capacity, that battery bank is going to weigh very close to the same whether it is configured as 12, 24 or 48V.

[Living Large]

here's the article about parallel strings. The differences in resistance between strings, forces the stronger string to take the most load.

http://www.smartgauge.co.uk/batt_con.html

You can go with 2 strings and connect on the diagonal.

2 strings gives you twice as many cells to fill and check. Terminals to check and keep clean.

If you know that you have the right AH bank chosen, pay the price for the easier to maintain single string.

I have seen that page before (but forgot the link - thanks). Question - unless I am mistaken, the examples are all multiple single cells in parallel.

How would this be adapted for the equivalent of 2 strings of 8? The only thing I could think of is 8 pairs of 2 in series, with each pair wired as shown.

[Mike90250]

Imagine that their example of a 12V cell, is your 48V string. (4, 12V in series or 8, 6v in series)
Then tie each end of the strings - to -, + to +. Then connect your power connections on the diagonal so the + goes to 1 terminal, and a jumper to the other +, the - lead goes to the - terminal for the + Jumper.