DIY 24v 8s or 8s2p LifePo4 Bank and BMS Design ?

MrM1

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I had been asking in a different thread I did not start so decided to start my own thread

I have narrowed my new LifePo4 bank size down to either an 8s 272 ah bank OR an 8s2p 564ah bank (8s 272 paralleled). I need 200 amp discharge and so picking a reliable yet affordable permanent install BMS has proven the most difficult part. I have a 2565w solar array of 9 REC 285w panels in 3S3P (95.7 vmp / 89.1 charging amps), 24v Conext SW 4024 24v 4000w inverter, Midnite solar Classic 150 CC and 4 225ah 6v Trojan T105s (too small) for my daily 23hr 3200wh need off grid.

If I go the 8s2p bank, @FilterGuy gave me some interesting things to think about back in this thread.

My 3 biggest concerns:
  1. That I have a large enough bank to supply my needs for about a 24hr period at 3200wh per day
  2. That I have a BMS that can handle a 240v 7amp pump starting up (my inverter can but it can do 4000watts)
  3. So it would seem I need a BMS that can possibly match my Inverter output although I seldom if ever average more than 2400 watts

Here is what I am considering:

Either An 8s 24v 272ah bank with 2 Overkill Solar 24v 100amp BMS system in parallel (I know not recommended but has been done)

Or an 8s2p 24v 564ah bank in this configuration:
8s2p.JPG
Which as I understand from @FilterGuy ... using two 24v 100amp OKS BMS systems would give me 200amp service. As I am seeing things now, this might not be bad. I did list some limitations if one of the two BMS systems shut down 1/2 the bank and I did not know it, but as I understand things right now, the BMS system is actually the 2nd layer of defense and that the Inverter LVC and the CC charge cut off is the primary (?). In other words, the BMS system really should not be shutting things off UNLESS there is a PROBLEM. So it appear this setup would work. Others (@Ampster and @Mac6792 ) listed a few other considerations.

My idea is to order 8 272ah cells now, and test my loads and the system with 2 OKS BMS's in parallel for my loads (I actually might try it with only one BMS and really see if I ever exceed 2400 watts except in surges like well pump startups). ...

BUT then if i discover that an 8s bank of 272ah is just not large enough and I am discharging too much each day (down to 20-25% dod) I would order 8 more cells (say in 6 months) and build up the 8s2p bank.

What am I missing?
  • What problems and limitations am I (in my very novice brain) not seeing?
  • Can 8 cells ins series be added for 2 parallel strings in say 6 months later to an LFP bank if needed for extra capacity with out issue?
  • Would my existing Array of 2565w 285w panels in 3S3P (95.7 vmp / 89.1 charging amps) be large enough to recharge the 8s2p 564ah bank in a day in NE FL USA? (4.3 average hours of solar sun daily)​
  • Based on all above, does it just make sense to go ahead and order 16 cells for the 8s2p bank from the start?

    Thanks in advance for your help. And thank you for letting me post in this new to me forum.​
Mike
 
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FilterGuy

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My idea is to order 8 272ah cells now, and test my loads and the system with 2 OKS BMS's in parallel for my loads (I actually might try it with only one BMS and really see if I ever exceed 2400 watts except in surges like well pump startups). ...
Are you talking about putting two 8S BMSs on one set of 8 cells? That might not work very well.
 

MrM1

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Reading this now. This looks really interesting in light of what I am trying to do. I am just trying to learn / understand as much as I can before I start ordering and building a 24v LifePo4 bank. I need it to be safe and meet my needs.

24v Battery Config ideas
 
Last edited:

MrM1

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Ok so Go Big or Go Home. I decided to order 16 cells so I will NOT be trying to Parallel 2 of the OKS 24v 100amp BMS units, but instead will be building up two 8s strings with one BMS on each string and the placing those in Parallel. (see diagram above)
 

Jimmynik1

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I had been asking in a different thread I did not start so decided to start my own thread

I have narrowed my new LifePo4 bank size down to either an 8s 272 ah bank OR an 8s2p 564ah bank (8s 272 paralleled). I need 200 amp discharge and so picking a reliable yet affordable permanent install BMS has proven the most difficult part. I have a 2565w solar array of 9 REC 285w panels in 3S3P (95.7 vmp / 89.1 charging amps), 24v Conext SW 4024 24v 4000w inverter, Midnite solar Classic 150 CC and 4 225ah 6v Trojan T105s (too small) for my daily 23hr 3200wh need off grid.

If I go the 8s2p bank, @FilterGuy gave me some interesting things to think about back in this thread.

My 3 biggest concerns:
  1. That I have a large enough bank to supply my needs for about a 24hr period at 3200wh per day
  2. That I have a BMS that can handle a 240v 7amp pump starting up (my inverter can but it can do 4000watts)
  3. So it would seem I need a BMS that can possibly match my Inverter output although I seldom if ever average more than 2400 watts

Here is what I am considering:

Either An 8s 24v 272ah bank with 2 Overkill Solar 24v 100amp BMS system in parallel (I know not recommended but has been done)

Or an 8s2p 24v 564ah bank in this configuration:
View attachment 40222
Which as I understand from @FilterGuy ... using two 24v 100amp OKS BMS systems would give me 200amp service. As I am seeing things now, this might not be bad. I did list some limitations if one of the two BMS systems shut down 1/2 the bank and I did not know it, but as I understand things right now, the BMS system is actually the 2nd layer of defense and that the Inverter LVC and the CC charge cut off is the primary (?). In other words, the BMS system really should not be shutting things off UNLESS there is a PROBLEM. So it appear this setup would work. Others (@Ampster and @Mac6792 ) listed a few other considerations.

My idea is to order 8 272ah cells now, and test my loads and the system with 2 OKS BMS's in parallel for my loads (I actually might try it with only one BMS and really see if I ever exceed 2400 watts except in surges like well pump startups). ...

BUT then if i discover that an 8s bank of 272ah is just not large enough and I am discharging too much each day (down to 20-25% dod) I would order 8 more cells (say in 6 months) and build up the 8s2p bank.

What am I missing?
  • What problems and limitations am I (in my very novice brain) not seeing?
  • Can 8 cells ins series be added for 2 parallel strings in say 6 months later to an LFP bank if needed for extra capacity with out issue?
  • Would my existing Array of 2565w 285w panels in 3S3P (95.7 vmp / 89.1 charging amps) be large enough to recharge the 8s2p 564ah bank in a day in NE FL USA? (4.3 average hours of solar sun daily)​
  • Based on all above, does it just make sense to go ahead and order 16 cells for the 8s2p bank from the start?

    Thanks in advance for your help. And thank you for letting me post in this new to me forum.​
Mike
I am planning to make the exact same setup you made. 16 cells in a 8s2p config with 2 individual bms's feeding a lv2424 or similar.
How is the build going? Any pics of yet? I actually saved the first picture you put up to my desktop for future reference with wiring.
 

MrM1

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I have everything ~ 2 Overkill solar bms units, cables, lugs, fuses, etc. Just waiting on cells.

As far as I know right now this will be my configuration. (except I'll use 175 amp fuses instead of 200)

Battery confit 8s2p.jpg
 

Jimmynik1

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Awesome. That's exactly what I plan to make. Eager to see your progress.
 

MrM1

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I am actually going to build a Mach up of the cells and start pulling together the cell compression and all the cables this week. Then when the cells arrive all I'll have to do is top balance and place them into service
 

Screwball

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MrM1, based on my (lack 🙄) of understanding of electric/electronics I'm going to follow your setup with lots of interest (in a monkey see, monkey do fashion 😆)

I have a 46" x 11" x 14" tall battery area over my wheelwell , so like you im eyeing a possible 24v 16 cell setup in 8p2s with two overkill bms (thinking 200ah cells ?!?)
 

Jimmynik1

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MrM1, based on my (lack 🙄) of understanding of electric/electronics I'm going to follow your setup with lots of interest (in a monkey see, monkey do fashion 😆)

I have a 46" x 11" x 14" tall battery area over my wheelwell , so like you im eyeing a possible 24v 16 cell setup in 8p2s with two overkill bms (thinking 200ah cells ?!?)
If you're going to go through the work of getting a supplier and stuff arranged there's not that much price difference between 200 and 280 from what I found. I think the best price I found shipped to me was 16 280 cells for $1600 and then they offered to put 2 bms's with them for $1750
 

MrM1

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If you've not ordered cells yet, start a conversation (this forums method of a PM) with @Michael B Caro at the LFP Buy Group. I think he can get a verity of sizes and becomes Ur middle man state side to China. He would help with any issues and had the best price I could find.
 

MrM1

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Discussion starts here

 

Jimmynik1

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@Michael B Caro is also going to have a line of already made batteries in 12v & 24v with bms and meter (?) in a case for about half the price +/- u have found a ready made any where else yet
A pre made 24v 8 cell from 3.2s wired with bms for 1/2 the price from the manufacturer or half the price of a typical person that imports and builds them and sells on amazon?
 

MrM1

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The biggest benefit in dealing with Michael for individual cells is him being the middleman importer from China. Prices are going to be extremely similar to what you will find on AliExpress and Alibaba. But it will not be questionable and he will deal with any problems
 

Jimmynik1

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The biggest benefit in dealing with Michael for individual cells is him being the middleman importer from China. Prices are going to be extremely similar to what you will find on AliExpress and Alibaba. But it will not be questionable and he will deal with any problems
Yea im not opposed at all. Shot him a message. Also saved the picture you shared a bit ago as i plan to make the exact 16c 24v system.
 
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