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opinions needed on my plan for 4P16S lifePo4 pack

Sparky_SC

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In another thread I posted on a bunch (180 on hand now) 170ah surplus lifePo4 prismatic cells I got. I have been testing them in 1P4S 12V configuration and they are testing excellent.

So now its time to move on to the 48V pack(s) that I need for my solar. I found this thread of a build with 4P16S batteries and a lot of good information.

It seems that the 4P16s config is the best choice for me. My present inverter can only charge and discharge at 100 amps although I may add another in parallel in the future. I picked this 200A 16s BMS by JCB to use with the proposed 4P16S battery. https://www.aliexpress.com/snapshot...d=8140593047615742&productId=1005002650532071

For weight reasons, I am planning on splitting the 4P16S battery into 4 modules.

I realize its a LOT of battery for my inverter/solar setup but cost is not a factor and the batteries should stay in the 30-80% SOC range giving a lot of life and reserve for multiple dark days.

Would really appreciate any input before I actually start building a 48V battery. I still have lots of cell testing to do in the meantime.
 
Are you getting four BMS's, one for each module? If not, you'd have to deattach/reattach the BMS leads every time you separate the modules, which might not be ideal. If i were you, i would get four smaller BMS's, to give yourself some more modularity and protection against individual cell failures.
 
Are you getting four BMS's, one for each module? If not, you'd have to deattach/reattach the BMS leads every time you separate the modules, which might not be ideal. If i were you, i would get four smaller BMS's, to give yourself some more modularity and protection against individual cell failures.
That was the original plan, to go 16s 1P and build 4 batteries each with their own BMS, then parallel the 4 batteries. The JCB 200A 16S BMS warns that paralleling packs /BMS modules is not allowed with that model. Thats why I switched to the 4P16S config with a single BMS.
 
I do not see any reason you can't parallel BMS outputs to your charging source and loads. I am not running one pack with a JK-BMS, and 2 more packs with Daly BMS units all running in parallel into my Schneider inverter/charger. It works just fine.

In your case, I would think splitting the difference would be a good option. Make 2 independent 2P16S battery packs. Each having it's own BMS and fusing. My original bank is two strings that I tied into a single BMS. IT works well, but I had a problem with a balance lead failing, and it shut down. I lost the whole battery bank and the inverter was not able to run at all. Had I used 2 separate BM units, the odds are it would have stayed working on one battery string. I later added another pair of battery strings, and was going to use another single large BMS, but I opted for two separate BMS units instead. So this way, I have a bit more redundancy if something does go wrong. I can now easily take one battery offline if I need to service it. In a perfect system, each single string should be able to handle the full load, but if it can't, be sure the fuses are the right value so it can still fail safe. My original 2 string pack is fuses at 225 amps and my two new strings are fused at 125 amps each. My system never really pulls more than 80 amps, and my average current is more like 30 amps. All the extra capacity if for run time. With all 4 strings, I can run all of my backed up loads for nearly 3 days. More normal daily cycles are just 55% to 85% so the cells should last a very long time.
 
I do not see any reason you can't parallel BMS outputs to your charging source and loads. I am not running one pack with a JK-BMS, and 2 more packs with Daly BMS units all running in parallel into my Schneider inverter/charger. It works just fine.

In your case, I would think splitting the difference would be a good option. Make 2 independent 2P16S battery packs. Each having it's own BMS and fusing. My original bank is two strings that I tied into a single BMS. IT works well, but I had a problem with a balance lead failing, and it shut down. I lost the whole battery bank and the inverter was not able to run at all. Had I used 2 separate BM units, the odds are it would have stayed working on one battery string. I later added another pair of battery strings, and was going to use another single large BMS, but I opted for two separate BMS units instead. So this way, I have a bit more redundancy if something does go wrong. I can now easily take one battery offline if I need to service it. In a perfect system, each single string should be able to handle the full load, but if it can't, be sure the fuses are the right value so it can still fail safe. My original 2 string pack is fuses at 225 amps and my two new strings are fused at 125 amps each. My system never really pulls more than 80 amps, and my average current is more like 30 amps. All the extra capacity if for run time. With all 4 strings, I can run all of my backed up loads for nearly 3 days. More normal daily cycles are just 55% to 85% so the cells should last a very long time.
Here is a cut/paste from the JBD instruction doc for the BMS I purchased.

● Supports the use of battery packs in series, but the total number of strings after series connection is less than or equal to 32 strings.


● Parallel use of battery packs is not supported (battery packs are directly connected in parallel, and there is a problem of large current discharge from high-voltage battery packs to low-voltage battery packs).

I have seen other model BMS from other manufacturers made for the purpose of parallel operation. Often they have a BMS in each battery that communicates to a master BMS, they are very expensive. I suppose it all depends on each particular BMS and if the manufacturer allows or prohibits parallel battery operation in their docs.

Also, there is a document link here on the forum on series/parallel configs that goes deeply into the problems of parallel battery BMS issues.
 
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So, you are planning to build four 1P16S banks, with their own BMS, and parallel each of those to gain a set of banks…

That is a lot of wiring. Be methodical wih conductor placement and organize with care.
 
So, you are planning to build four 1P16S banks, with their own BMS, and parallel each of those to gain a set of banks…

That is a lot of wiring. Be methodical wih conductor placement and organize with care.
The document I linked in my prior post details the problems with parallel BMS units in such a config. That document, and the warning against parallel operation in the JBD MNS doc is why I am now leaning towards a 4P16S config.
 
I would think making weight issues easier would be to have four 4P 4S sets, with balance lead connectors so a single BMS can monitor things.
Is the weight issue because you move them around much? Or for shelf mounting strength?
 
The document I linked in my prior post details the problems with parallel BMS units in such a config. That document, and the warning against parallel operation in the JBD MNS doc is why I am now leaning towards a 4P16S config.
I see an excerpt from a document and mention of a link somewhere, but I don’t see you linked any document…
 
I would think making weight issues easier would be to have four 4P 4S sets, with balance lead connectors so a single BMS can monitor things.
Is the weight issue because you move them around much? Or for shelf mounting strength?
YES ! Weight is a consideration for sure !! The original surplus batteries I am using are in 10S config in a real nice stainless steel enclosures. I am considering using those enclosures to house a 4P2S (6V)in each one, bringing out the sense leads. 8 "modules" in that config wouldn't be bad to handle weight wise and not bad wiring either. All plans are preliminary yet, thus why I started the thread.
 
I see an excerpt from a document and mention of a link somewhere, but I don’t see you linked any document…
Yea, its somewhat hidden. That link takes you to a page here on the forum. In the text it references a download icon on the top right of the screen to download the document. Great info !
 
I am having difficulty understanding your answers…

I ask a multiple choice question on WHY weight is important, and you respond yes…

I ask where the link is, and you respond it is hidden… then state how full of info the link is…


Please post the link.

Thanks.
 
I am having difficulty understanding your answers…

I ask a multiple choice question on WHY weight is important, and you respond yes…

I ask where the link is, and you respond it is hidden… then state how full of info the link is…


Please post the link.

Thanks.
Yes, meaning weight is a factor to be considered. Considering various configs for that reason.

This link posted before https://diysolarforum.com/resources/cell-configurations-for-12v-24v-and-48v-lifepo4-batteries.42/

In the text of that link its stated "To get the doc, click on the orange "Download" button in the upper right of this page." The orange download icon is indeed in the upper right part of the screen and works as the author described.

Hope this clears any confusion.
 
Closer.
I wanted to know WHY weight is a consideration in the build…

Are you making it easier to carry, or are the loads being portioned because the location you put them in isn’t stable?
 
Closer.
I wanted to know WHY weight is a consideration in the build…

Are you making it easier to carry, or are the loads being portioned because the location you put them in isn’t stable?
Location is stable enough but moving them to that location, stacking and future maintenance if needed is the consideration. Total weight looks to be well over 1000 lbs. IF a module failed or needed work at some point it would be nice to be able to pull it, replace with a spare without needing hydraulic hoists or such.

The OEM 10 cell modules/batteries are 150lbs each. My config will be 64 cells total plus packaging.
 
Ok, gotcha, thanks.

I have a 1P16S build of 60Ah cells… and it is ALL I can do to move that… I cannot imagine trying to lift a 170Ah set of 16 cells…

If repairs are needed, I am thinking disassembly of the cells would be needed.
 
● Parallel use of battery packs is not supported (battery packs are directly connected in parallel, and there is a problem of large current discharge from high-voltage battery packs to low-voltage battery packs).
I run 5 Packs in Parallel (3x 24V/280AH & 2x 24V/174AH) and maybe I can explain this better.
I have JBD BMS' on my Utility Packs but use Chargery BMS' with Contactors on my Production Battery Bank.

If I have 4 Packs with cells sitting @ 3.500 (28.0V) and I connect another pack with cells at 3.200 (25.6V) the Four Packs (and anything else pushing charge) is connected to THAT Low Pack, it will absorb everything it can and FAST ! This recently happened to me because of a pack fault, and before I realized everything (it was 3AM and I got wakened by alarm) and hit reconnect, the Low Pack ran to 300A Charge instantly before the BMS kicked off. NB: Had that been a FET Based BMS, it would have FRIED ! I fortunately use Chargery BMS with Contactors. In other words a JK or Daly BMS would have POOFED !

Here are the CRITICAL BITS when building a Bank with Parallel Packs (Thousands of people do it - it is quite common).
1) Every Battery Pack gets a FUSE !
2) Every Battery Pack should be configured to similar specs for cutoffs etc....
3) Battery Cables from Each Pack to Common DC Bus should be Equal Length, and kept close together (reduces EMI/RFI noise)

- If properly configured, each battery pack can act as "Last Man Standing" in the even other packs cutoff. This requires that each "Pack" be capable of supporting the Full Load & Charge Potentials as cutoffs can occur under both conditions.
- If properly configured and "Matched Packs" (meaning same ie 48V/200AH with same BMS) they will share/split Charging & Discharging Evenly.
- If Packs in Bank are Mis-Matched (200AH and 100AH) they will divide "proportionately" until at either end of the Working Voltage Curve at which time the Smaller Pack could pull from the other batteries to compensate. There is some BMS trickery here that can be done.

-- !! MisMatched Packs !!-- There should be no more than 20% Capacity difference between packs in a bank,
When charging the smaller packs will reach full first and the BMS will cutoff the packs. The larger packs will continue taking charge and use the extra Amperage if possible until they complete. This is generally not an issue when packs are configured properly.
When discharging, again all the packs will behave properly when within their Working Voltage Curve, BUT as soon as the smaller packs reach lower SOC than the larger counterparts, this is where things get weird.


Now some GRITTY stuff is rarely mentioned.
ALL LFP Cells are capable of Hi Burst Rate Discharge for a short period (like 10 sec or less) That can be up to 5C (some higher). Translating that on a 200AH cell, that means it can dump 1000A in a burst ! You likely just did the quick math in your head and realized Ohhh Shit ! That's HUGE! Luckily we have BMS' and control systems to prevent that but I wanted to highlight this so you understand the "Potential" that can come out of these packs in a Burst. Now the Parallel Packs and one Low Pack being added, you can clearly see the potential Input (even for a few seconds) that can occur.

Do download the 2nd Link in my signature, there is info in that you will want to have handy. Peruse the links and look at the diagrams & schemas on "About My System" page in my sig...

Hope it helps, Good Luck.
 
Ok, gotcha, thanks.

I have a 1P16S build of 60Ah cells… and it is ALL I can do to move that… I cannot imagine trying to lift a 170Ah set of 16 cells…

If repairs are needed, I am thinking disassembly of the cells would be needed.
Yes, dissassembly would be needed for repairs of course. I am thinking a "module" with 8 cells in it 4P2S config would be a reasonable compromise between weight and complexity of the overall setup.

Also... the OEM batteries are in 10S1P config in nice stainless steel enclosures. I am thinking I could reuse those enclosures with 8 cells in each 4P2S config. 8 "modules" for my total battery bank.

Here are some pics of the OEM battery with the top cover removed. Very nice enclosures so it would be nice to reuse if possible.
 

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There are certainly some concerns when you set up a parallel bank, and if the BMS does have to disconnect for any reason, you need to take some care to connect it again. Each bank does need to be fused at a level the BMS units can handle.

When I added my 2 new banks to my original bank, it took 2 days to get the cells all at the same state of charge before they could be connected together. This is certainly a concern. But once the cells are properly balanced, there is no extra stress from the parallel operation. The will likely be a small difference in the amount of current each string carries. With the low resistances and high currents involved, even a few milliohms could cause one bank to carry several amps more than another bank, but this is not a problem as long as the maximum currents are well below what the batteries and BMS's can handle. On my setup, I am seeing about 5-10% more current going in and out of the new pair compared to my old pair. I expected it to be the other way, as the new pair have a longer cable to get over to them, but I figure the newer cells (at least 70 less cycles) and 2 parallel Daly BMS's vs the single JK BMS, just has a tick less total internal resistance.

Now if one of the BMS units goes into a fault mode and opens it's output switch, then there could be a problem. For example, the system is charging. One cell of one pack hits an over charge disconnect. Then the system goes into discharge. The banks that were not in fault run down to 30% charge, but the other pack was shut off near full charge. If the BMS did try to reconnect at this point, the currents between the batteries could be huge. Most likely the fuse would pop. The BMS should also open from an over current situation. But in reality this should not happen. A good common port BMS would reconnect for the discharge current, and the high cell should come down, and the charge side will also then close. The state of charge on all of the banks should still hold pretty close. Anytime the banks are separated for service etc., then the system operator does need to take an extra step to make sure the separate banks are at the same state of charge before they can be safely reconnected in parallel again.
 
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