Parallel modules to reduce BMS expense

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prepared1

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What is the consensus of users regarding putting cells in parallel so a BMS balances more than one cell?

If batteries are closely matched in voltage, and are all healthy, is this a reasonable strategy to save some expense when setting up a BMS such as Batrium? Or for that matter, Orion, Nuvation, etc?

If you have automotive or ESS cells that are often 8-16s (for example) a large bank of those batteries makes for a very significant expense just for things like Blockmons. In my case, it would be 160 cells minimum. That is thousands of dollars just for that component. And then there is the balance of the system. Wow.

Would it be safe, and practical to parallel 2-4 cells together? I see a lot of people building with large prismatic cells, often 200 ah each, where-as my cells are 60ah.

Any comments are appreciated.

Thanks!
 

daromer

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You talk about balancing current per Ah. The older the cells you have the more current/Ah is needed.

And the consensus is somewhere between 1A per 200Ah to 800Ah. Where most recommend not to go above 4-500Ah per 1A actuall balancing power

With that said there are cars and system running alot less than this but thats on new batteries.

But if you cant afford the protection you should reconsider what you are doing :) Personally i went to DIYBms now to be able to afford expanding more.
 

prepared1

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Daromer: thank you for the reply. Before posting my question, I had read some previous posts from others on this topic, but just wasn't sure of limitations versus safety.

My cells are like new. I just checked every cell 2 days ago, sitting on the shelf awaiting assembly, and they were all 31.65-31.68, with the exception of 4 modules from a previous order that were all sitting at 28.91. I will charge all of them up fairly soon, with a laboratory grade charger.
They are from a medical ESS backup system.

In each 8s battery, even If I just paralled every other cell, it would make a huge cost difference. Based on your comment, it seems even 4 60ah cells at total 240ah would be quite safe? Luckily, all of these modules are bolted connections, so no welded buss bars to contend with. And, each module has 4 thermistors.

I'll check out the DIYBMS. Thanks!
 

OffGridInTheCity

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Daromer: thank you for the reply. Before posting my question, I had read some previous posts from others on this topic, but just wasn't sure of limitations versus safety.

My cells are like new. I just checked every cell 2 days ago, sitting on the shelf awaiting assembly, and they were all 31.65-31.68, with the exception of 4 modules from a previous order that were all sitting at 28.91. I will charge all of them up fairly soon, with a laboratory grade charger.
They are from a medical ESS backup system.

In each 8s battery, even If I just paralled every other cell, it would make a huge cost difference. Based on your comment, it seems even 4 60ah cells at total 240ah would be quite safe? Luckily, all of these modules are bolted connections, so no welded buss bars to contend with. And, each module has 4 thermistors.

I'll check out the DIYBMS. Thanks!
Remember that healthy cells don't need balance very often. I go 5 months with no balance at all on 84 packs and still just need touch-ups at that point. Many BMSs don't give you an on/off choice - or only balance as you reach float etc. So it get's confusing.

You can do the initial balance by hooking the packs in parallel and letting them sit for many days - e.g. you don't need a strong BMS to get 'started'. After that....

Batrium WM4 + longmons has enough amps to balance even 500ah individual packs in days rather than weeks/months a smaller BMS might need. A 26ah pack balances easily with Chargery level BMSs and they probably are functional (within weeks) up to 150ah? of healthy packs.

Once a pack drifts severely out balance with others - its not really an issue of BMS / balance but knowing that a pack is drifting too much and facing that the pack needs repair/replace. So pack(cell) level reporting is essential for me so I can monitor/see it before it get's so severe the BMS cuts off.

For me - it was worth it to spend $ on Batrium where I can control either top balance (I don't use it often) or auto-level - balance as the system is running (how I do touch-ups) and that it can do it in days on 260ah packs rather than being patient on lessor BMSs :)
 

daromer

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Lets go into some BMS basics
* When do a BMS balance/when do you want it to balance?

First of all only when you have excess energy
And the voltage/SOC is above a certain level
And the voltage difference between cells could be an issue.
And preferable when charging current is low

Above is the basics and then you also have the fact that you have a cell over a critical level but lets skip that for now but its important to!...



Excess energy that is something you have lets say 4 hours per day when your battery bank is fully charged
Voltace soc is most of time above 85%SOC or lets say 4.0V if your max is 4.1. You dont want to balance in the middle of the voltage curve. For instance on LiFePoe its useless to balance under 3.3V in general.
You also want the difference to be above lets say 30mV or more in general. Cells change so its not always usefull to do it better than that. Note im talking lowest to highest cell.
And when current is low. In end of a charge the current is low and then you often have excess energy.

So lets say you have your 500Ah battery bank and you use a balancer that can burn off 1A continuysly. You then have above scenario 4 hours per day.
You get where im going now and the reason why you shouldnt have to big battery banks? Because above would only be able to balance 4Ah per day :)

BUT as Offgridinthecity said: You dont have to balance that often or much when you have gotten them into synk or if you built your battery properly. But if you have a pack that is constantly discharging.. You need it.

So lets say you only get out of balance with 1Ah per day on a 500Ah pack. Thats not many % at all right.. Something that isnt that uncommon on 2nd hand system if we talk %.

If you also add in the factor that you dont have full sun. Suddenly you will only have above scenario where balancing is usefull 2 days out of 7 in a week :p 2 days = 8Ah balance and 7 days gave you 7Ah offset :p

Of course my numbers are made up and in real life if done properly it doesnt drift as much and on a decent size system you get more than 4 hours of balance time.

And yes im talking about how a decent balancer works and not those ebike ones that only balance when above critical voltage.
 

prepared1

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Balancing packs:

OGITC and Daromer:

Your comments are both highly appreciated. We were ready to install solar a full year ago, but due to storm damage then, we are still rebuilding. While repairs are being made, there is time to get the battery banks built. Until the PV is done, we can use the grid to charge.

At this time, the leading contenders for BMS are Batrium and Nuvation. I looked at the DIYBMS yesterday, but just don't have any desire to assemble a BMS. I didn't see options to buy completed kits, which makes sense for a product with their name! Batrium has a lot of support from this site, which is a positive. Nuvation has exceptionally good documentation, and is basically plug and play. In either case, the cost is going to be thousands of dollars, but with Batrium being much less overall.

I'm not worried about getting the battery banks in sync with each other, and closely balanced. I have a laboratory grade charger I picked up for $60 that will take care of that, using the grid. I will parallel them first anyway, as OGITC stated. At this point, my questions are down to these:

1. The Nuvation system has low voltage limits of 60v, or you can select their high voltage approach. The high voltage equipment can go much lower than 60v, but also goes up to 1250v. There is a high cost for that capability. My inverters can handle a max of 62-63 for charging. I don't know if Batrium has any limits to their hardware?
2. How big is too big for one battery bank? For this first bank, our capacity range would be 36-40 kWh. Should I split that into separate banks? With either BMS system, that means more equipment purchase.
3. These modules are Samsung SDI Li-NMC in 8s; (16s in series per pair) By removing 14 screws, a 60 ah cell can be removed to lower the max voltage. So they could easily become 14s in pairs, and the removed cells can make up another module or be used as spares. That would be much friendlier to a wider range of inverters, and might make the decision process for BMS easier too. Or, we can keep them as is, at 8s, and lower the cell charge to 3.75 to stay within a 60v limit. Is that a stupid idea? It would leave a lot of unused capacity for sure, but cycle life should be really great. Opinions?
4. For like new cells, closely balanced, would it be safe to parallel 2, 3 or 4 of these 60ah cells together to save on cell monitoring cost? It sounds like the consensus is "yes", since people routinely buy large cells at 200 ah rating. Even at a 2 cell pairing, that would save a lot of money.

Thank you for your time and comments!
 

OffGridInTheCity

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Balancing packs:

OGITC and Daromer:

Your comments are both highly appreciated. We were ready to install solar a full year ago, but due to storm damage then, we are still rebuilding. While repairs are being made, there is time to get the battery banks built. Until the PV is done, we can use the grid to charge.

At this time, the leading contenders for BMS are Batrium and Nuvation. I looked at the DIYBMS yesterday, but just don't have any desire to assemble a BMS. I didn't see options to buy completed kits, which makes sense for a product with their name! Batrium has a lot of support from this site, which is a positive. Nuvation has exceptionally good documentation, and is basically plug and play. In either case, the cost is going to be thousands of dollars, but with Batrium being much less overall.
I agree it's significant $ but I'm happy (as I wrote earlier) because Batrium
- Initally met all my criteria - I can parse the stats log to put data into my database every 5 mins (some use custom software to gather status more directly)
- I'm Windows centric - so the app and WiFi make it easy for me to integrate into my overall computers.
- The design of permanent Watchmon - and you just add longmons to expand... + Batrium has remained in business for over 3 years now and as I expand, I'm hopeful to continue to buy longmons over the next few years etc.... and it makes it easy for me to manage the growth / overall operation of my powerwall.

There are many factors and I'm sure I sound like I'm pushing Batrium - but that's not my intent. It's just what I have good experience with and it works for me. Totally respect whatever choices you make. In fact - if you go with something else - it will be interesting to hear about it from you if you post :)

I'm not worried about getting the battery banks in sync with each other, and closely balanced. I have a laboratory grade charger I picked up for $60 that will take care of that, using the grid. I will parallel them first anyway, as OGITC stated. At this point, my questions are down to these:

1. The Nuvation system has low voltage limits of 60v, or you can select their high voltage approach. The high voltage equipment can go much lower than 60v, but also goes up to 1250v. There is a high cost for that capability. My inverters can handle a max of 62-63 for charging. I don't know if Batrium has any limits to their hardware?
The voltage limitation for Batrium w/longmons is the voltage range of an individual pack/cell - https://support.batrium.com/article/123-longmon
  • Battery range 2.2V to 5.4V.
Lithium-ion and LifePo4 (for example) pack/cells easily fit this voltage range for Batrium watchmon to monitor. I guess you could say that since a watchmon can only do up to 250 longmons - https://support.batrium.com/article...l-monitors-can-be-installed-with-one-watchmon - the max battery voltage would be 250 * 5.4v = 1,350v (250s)

2. How big is too big for one battery bank? For this first bank, our capacity range would be 36-40 kWh. Should I split that into separate banks? With either BMS system, that means more equipment purchase.
I do 84 packs/longmons at 260ah each with no problems - that's an 80kwh battery bank.
Batrium could do 500ah packs (per longmon) * 250 (max number of longmons) = 125,000ah. At 3.7v nominal per pack that's a 462kwh battery bank :)


3. These modules are Samsung SDI Li-NMC in 8s; (16s in series per pair) By removing 14 screws, a 60 ah cell can be removed to lower the max voltage. So they could easily become 14s in pairs, and the removed cells can make up another module or be used as spares. That would be much friendlier to a wider range of inverters, and might make the decision process for BMS easier too. Or, we can keep them as is, at 8s, and lower the cell charge to 3.75 to stay within a 60v limit. Is that a stupid idea? It would leave a lot of unused capacity for sure, but cycle life should be really great. Opinions?
To Batrium its just a series of individual packs/longmons that it monitors. You tell Batrium software how many longmons per battery so it can make sense of the overall battery voltage - e.g. 13 or 14or 15 etc. I have 84 packs/longmons but arranged in 6 x 14s batteries - so I tell Batrium the base unit is 14 (e.g. 14s).
1624460421592.png


But personally - I'd go 14s / 48v nominal (voltage range) battery. You'll find that this is where most of the equipment likes to be - and give yourself the widest range of options and lower $.

4. For like new cells, closely balanced, would it be safe to parallel 2, 3 or 4 of these 60ah cells together to save on cell monitoring cost? It sounds like the consensus is "yes", since people routinely buy large cells at 200 ah rating. Even at a 2 cell pairing, that would save a lot of money.
Yes. You can parallel the 'sense leads' for pack monitoring and you don't have to parallel the load wires. My packs are 260ah with is 4.3 * 60ah. I'm confident you could do up to 8 x 60ah = 480ah and still have 'respectable' balance (within days).

Thank you for your time and comments!
 
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prepared1

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OGITC: Your answer to my question is typical of your posts.....very well thought out, well explained, and helpful. Greatly appreciated. And by the way, there is nothing wrong with "pushing" a particular brand. When you have good results with a brand, or bad results, it is valuable info for others to learn your opinions.

I would prefer to build one large bank with these 8s modules. Whether it becomes 16s, or a cell is removed from each to make it 14s is probably more concerned about the hardware. Inverters, chargers, etc. But I do have a large number of 14s modules too, and a crap load of military 7s. So that might be a good strategy, since they all share the same charging voltage limits. (4.2 max per cell)

Your comment about going up to 480ah is pretty interesting too, since that would be one module. Very easy to parallel all the cells in that module. If one cell is removed, it would be 420ah.

One thing I have not mentioned yet regards cabling. The rack trays each have a connection module on the front side with every kind of plug imaginable including ethernet. There are canbus, and modbus posts off to one side. They have battery connection terminals that are like Amphenol, but I don't know the specific brand. And, none of the power cables came with this set up. I'm unsure of what to do with those connectors, or whether or not some of them can be utilized. All of the modules still have the long cabling on them. I doubt any of them other than Ethernet are compatible with Batrium, REC, Nuvation, etc. When EV West was selling their Samsung rack modules that are similar, people were removing the communication ports on the front and just wiring in their own BMS. I think the batteries in those were the same as mine, but theirs were capacity degraded to about 60%, while mine are at 90-95%. The BMS in one tray was completely removed, so I have a nice empty metal box with ports on it, that can hold a lot of gear, and be protected.

I had not thought about using other style cell sensors than Blockmons. I suppose it just comes down to what is easiest to work with. Will look into this some more.

Thanks again!
 

floydR

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Longmons would be very expensive to integrate if you used 1 per cell =208 longmons more than $3000. if you used 1 longmon per cell with the cells paralleled at the cell level you would use 32 longmons. The system 16x 3.6v=57.6v nominal with a capcity of 12x 60Ah=720Ah = ~41KW 2 banks of 16s6p 28/6=4.67Ah of balancing power? per cell. The Cellmate-K9 would have 1Ah in the same configuration. seems that the longmons would be better if not well balanced. Price per Ah of balancing power Longmon =~$40, Cellmate-K9=~$140 you would lose one 16s1p x60Ah 3.4kWh of storage but you would have equal banks of 16s6p to work with.
Batrium WatchMon Core, 500A shunt, 2 Cellmate-k9, testMon, fair use support.= $863.68 USD
Batrium WatchMon Core, 500A shunt, 32 LongMons, IsoMon, fair use support.= $1,126.96 USD
Later floyd
 

daromer

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Floyd perhaps you ment A as in current and not Ah as in capacity when talking about balancing?

With Batrium Longmons i would settle at around 400-500Ah per Longmon. So if you got 800Ah you split it up into 2 batteries with a series of longmon each.

As earlier stated you can have up to 250 longmons per master. Unless you go cellmate but thats different design.
 

floydR

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Just using the same units as Batrium uses to describe the balancing capacity of its LongMons, CellMate- K9.
I broke it down in cost for a 16s6p 320Ah system. I never understood why some companies say balancing at such and such current, yet others say Ah.
Yes it is entirely possible to use 1 LongMon per cell in each module. But the cost benefit is in favor of paralleling the modules at a cell level.
So if you got 800Ah you split it up into 2 batteries with a series of LongMon each.
Which I did when I Placed the 16s13p into 2 16s6p with one LongMon for each 6p. 360Ah of capacity per Longmon with one extra 16s1p module

Just a cost analysis for the above battery.
I too am looking at the Batrium almost bought one a few times all that was left was clicking the buy button.
Later floyd
 

daromer

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Ah Batrium do the same as i calculated above in earlier post. They do an average of what they assume is possible during 24hours. But its important as i wrote to understand that you do not balance 24/7. You only balance when the battery is basically full and you have excess energy :p
 

prepared1

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Daromer: That is a very good point to remember, about when you really need balancing. I'll keep that in mind. And thank you for the reference to balancing limitations per cell. Super helpful. I'll be conservative in how I go about this.

FloydR: Thank you for the calculations. I had not thought of Longmons or Leafmons until OGITC mentioned it. I thought I was stuck with the Blockmon/cellmate approach. To be frank, I still don't completely get that whole set-up. If I may ask, what type of cells are you working with? Just a comment on your one calculation: my cells are li-nmc, with max charge of 4.2. I referenced going to a lower charge in order to keep it inline with specific products made by Nuvation. They can handle some what higher levels than 60, but it is warrantied for 60. The more I think about it, I'm likely to just pull one cell out of each of the 8s modules, and make ithem 7s/14s

On the other hand, after learning that it is okay to parallel at the cell level for monitoring and balancing, I may go with the high voltage approach, if deciding on Nuvation. Based on what you guys are telling me about Batrium, I'll have to re-think that too. They are both really good systems.

I'll have these batteries for a long time, and there does seem to be a trend towards high voltage, so that is something to consider if the same equipment can be re-purposed somewhat.

Thanks, guys!
 

floydR

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My setup is not in the production stage as of yet. But I have 10kw in 6 7s1p modules out of LG RESU and aprox 20kw in LGGM261865 cells also NMC. The LG RESU modules are what has/had me looking at the CellMate they are on a shared balance lead wiring harness. Just trying to get the best bang for the buck. i understand wanting to go lower to increase life of cell but not to fit a particular product. From what I understand much of the energy in a NMC is between 4.15v and 3.6v I am sure someone will correct me if this is wrong.60/16=3.75v per cell I would guess that you would 68% or more more most likely because discharge curve isn't linear.
I am researching the same products as you are. I have been researching BMS's for the last 4 years I know I am slow at completing projects. First it was work interfering, then my health, then covid. So I plan and research, find another project repeat.
later floyd
 

prepared1

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FloydR: I was not aware that the energy of the NMC was near the top portion of the charge profile. I never took the time with discharge graphs to calculate what percentage was at what point. I just assumed I'd over-build and call it good.

The Cellmate is an interesting product. I'm just not up to speed on it yet. Like you, I like to take time and do my research. I'm a hobby guy, and eventually my projects get done. I've learned to accept that life places a lot of limitations on us, so you might as well just go with the flow. Besides, for me, learning is the most fun part.

Have you looked at Autarctech out of Germany? Mattias is very quick to return emails, and his product looks good. Well thought out. I have no idea of pricing, but expect to receive that soon.

In previous posts, I mentioned that I might just remove 1 cell from each of the Samsung modules, and make them 14s. Last night, I was able to buy one more of those original modules, and that put me at the magic number where it allows me to build out additional pairs to arrive at an even number of 14s modules. That seems to be the ideal number, and then I can charge up to 4.1v

The Batrium people said that LithiumSolar was going to be doing another video soon so I'll watch that for sure. Everything being said, based on what I know today, I'd buy Nuvation.
 

Cheap 4-life

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I use EV batteries. Six 60v 16s batteries in parallel. I’m using one BMS. The balance leads from each number cell from each battery are wired together. Each cell of each batteryis actually two 27.5ah cells in parallel. So each cell (two cells) of each battery is 55ah. I have 6 batteries so that’s like a 330ah cell that each of the 16 bms leads are connected to. Since these are EV cells they were already well balanced when I got them. Due to over discharging when I only had one then two batteries they did start to drift to max of 60mv cell voltage difference before I ever started to use the BMS balancing. Before I added 4 more batteries that were perfectly balanced to my 2 existing out of balance batteries I turned balancing on to get the 2 balanced. I set the balance start voltage to 3.85v. They got within 25mv in a few days which is what I have the bms cell voltage difference balance start set to. I then added the additional 4 batteries and have kept balancing turned on. Every once in a while the bms does some small amount of balancing. The cells all stay within 25mv. Love my Chargery BMS.

So each group of 6 cells (one for each battery) is basically 330ah to the bms. With healthy EV cells, If your not charging or discharging with to many amps they will stay very well balanced. So It’s not a problem that the bms has to keep these 330ah cells balanced
 

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