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Cell capacity choice

NMNeil

Solar Enthusiast
Joined
Mar 14, 2020
Messages
443
Looking at the available capacities of LiFePo4 cells, there is an overwhelming choice; 50ah; 100ah; 150ah; 272ah; 280ah; 310ah etc.
As each new higher capacity cell is available; that becomes the cell to have, and the lesser capacity cells become slightly cheaper and more readily available.
My question is about the possible future expansion of a battery bank.
Would it be better to standardize the capacity of cell you use, and what would be the ideal capacity?
My group order with Michael is for 150ah cells, which means that I can mix and match 50ah cells x 3; 100ah cells x 3 would be the same as 150ah cells x 2, and so on.
But if I used 272ah cells there is no way the capacity could be matched with other cells; OK maybe 280ah or multiples of 90ah, but you understand my point.
If for example you had a bank of 16 Lishen 272ah cells for a 48V system and wanted to double the capacity by running another 16 batteries in parallel you can only use 272ah cells.
I know that for an RV application, due to space constraints the higher the capacity the better, but for home backup which cell capacity would the members choose as a standard size for their future battery banks, or does it matter and I'm finding problems where none exist?
 
Is weight or size a consideration for you? I wanted to hang mine on the wall, so I went with 200ah cells so things don't stick out too far. Also, 200ah was the minumum battery size recommended for my setup. I intend to double that eventually.
 
Well Yes, No and Not Quite. Gosh I so dislike Lead-isms which only apply to Lead.

You can add & remove packs from an LFP bank without issues.
You CANNOT intermix Lithium Chemistries, so no LFP Pack + LTO pack + NMC Pack !
Unlike LEAD, they can be of different capacities. IE 200AH & 280AH sitting side by side, without issues *.
Also, they are NOT age dependent like LEAD. 1, 2 or 5 years apart does not matter, provided the existing packs have NOT been abused & degraded as a result of such abuse.
* Capacities can be different BUT not too far apart, less than 100AH difference is suggested/ The closer the Capacities the better as it is more "balanced".

A FEW PROVISIONS
The Wiring/cabling should be as close to exact same length from Batt Terminals to DC Bus Bars and of all the same gauge.
Each battery pack must be fused obviously.
Each Pack should have similarly configured BMS' (adjusted of course for AH/kWh capacities) for SOC calcs.
Do try to keep the AH Rating of the packs closer and it is best NOT to be too far apart, general 100AH difference is fine but more you will see the smaller packs disconnect sooner during Charge & Discharge and the bigger packs "may" attempt to charge them in a balancing mode. Some BMS' can prevent that. **

** I have been running 2x 175AH & 2x 280AH and with 105AH difference there are issues at the Top & Bottom of the voltage curve. My BMS' are set to prevent cross charging from the bigger packs. I am now reconfiguring the 2X 175's into a single 350AH which will only have a 70AH difference and pretty be invisible for differences. I also have a 22kWh Lead Acid (Rolls Surette S-550's) Battery Bank that runs on the side as my secondary storage. These can be used "together as LFP + Lead" but there are serious limits. The Lead is what I started with years ago, still serviceable but just not good enough anymore. As I live Offgrid and am quite remote/rural, no time to furtle around with stuff.

Paralleling Cells.
EV's use Cells in Parallel BUT those are all Matched, Batched & Binned to perform identically with the same Internal Resistance & Impedances throughout their operating range. ONLY Matched cells should be set in parallel to ensure that all the cells are working within their spec and that not a single one is too high or low which result in failures (sometimes really bad fails too).

Most Vendors on Ali* sell Bulk Commodity cells, Grade A or otherwise. Please read the link in my signature regarding ESS Cells. These are NOT suitable for paralleling together. They WILL deviate and unbalance resulting in issues. That's a Virtual Guarantee.

Few Vendors are selling properly Matched, Batched and Binned LFP cells. These all perform very closely together through their voltage & discharge ranges. They do cost a bit more (IE bulk 280's = $92, Matched = $122 USD) BUT you do get what you pay for. Luyuan Tech is ONE of the few who carry Matched & Batched cells.

BTW: You likely have not run across them in your perusals. LFP cells are available in capacities up to 1,000AH per cell. You don't wanna know the $$ it is scary ! MORE LFP production is coming online and on scale too. Some of it is starting to FINALLY happen in North America, it is underway & growing in EU and China just can't keep up with demand.

Hope it Helps, Good Luck
Steve
 
Well Yes, No and Not Quite. Gosh I so dislike Lead-isms which only apply to Lead.

You can add & remove packs from an LFP bank without issues.
You CANNOT intermix Lithium Chemistries, so no LFP Pack + LTO pack + NMC Pack !
Unlike LEAD, they can be of different capacities. IE 200AH & 280AH sitting side by side, without issues *.
Also, they are NOT age dependent like LEAD. 1, 2 or 5 years apart does not matter, provided the existing packs have NOT been abused & degraded as a result of such abuse.
* Capacities can be different BUT not too far apart, less than 100AH difference is suggested/ The closer the Capacities the better as it is more "balanced".

A FEW PROVISIONS
The Wiring/cabling should be as close to exact same length from Batt Terminals to DC Bus Bars and of all the same gauge.
Each battery pack must be fused obviously.
Each Pack should have similarly configured BMS' (adjusted of course for AH/kWh capacities) for SOC calcs.
Do try to keep the AH Rating of the packs closer and it is best NOT to be too far apart, general 100AH difference is fine but more you will see the smaller packs disconnect sooner during Charge & Discharge and the bigger packs "may" attempt to charge them in a balancing mode. Some BMS' can prevent that. **

** I have been running 2x 175AH & 2x 280AH and with 105AH difference there are issues at the Top & Bottom of the voltage curve. My BMS' are set to prevent cross charging from the bigger packs. I am now reconfiguring the 2X 175's into a single 350AH which will only have a 70AH difference and pretty be invisible for differences. I also have a 22kWh Lead Acid (Rolls Surette S-550's) Battery Bank that runs on the side as my secondary storage. These can be used "together as LFP + Lead" but there are serious limits. The Lead is what I started with years ago, still serviceable but just not good enough anymore. As I live Offgrid and am quite remote/rural, no time to furtle around with stuff.

Paralleling Cells.
EV's use Cells in Parallel BUT those are all Matched, Batched & Binned to perform identically with the same Internal Resistance & Impedances throughout their operating range. ONLY Matched cells should be set in parallel to ensure that all the cells are working within their spec and that not a single one is too high or low which result in failures (sometimes really bad fails too).

Most Vendors on Ali* sell Bulk Commodity cells, Grade A or otherwise. Please read the link in my signature regarding ESS Cells. These are NOT suitable for paralleling together. They WILL deviate and unbalance resulting in issues. That's a Virtual Guarantee.

Few Vendors are selling properly Matched, Batched and Binned LFP cells. These all perform very closely together through their voltage & discharge ranges. They do cost a bit more (IE bulk 280's = $92, Matched = $122 USD) BUT you do get what you pay for. Luyuan Tech is ONE of the few who carry Matched & Batched cells.

BTW: You likely have not run across them in your perusals. LFP cells are available in capacities up to 1,000AH per cell. You don't wanna know the $$ it is scary ! MORE LFP production is coming online and on scale too. Some of it is starting to FINALLY happen in North America, it is underway & growing in EU and China just can't keep up with demand.

Hope it Helps, Good Luck
Steve
Steve, thanks for this great info! Especially the insight about not having Ah capacities from bank to bank being too far apart in parallel. I've been struggling/researching how to add an 8S 24V 280Ah LFP bank into my existing 8S 24V 100Ah LFP bank. I need more capacity! Seems like that's a stretch from your comments. Hell of a lot of BMS and CC settings to keep them both balanced and in an equal SOC during charging and discharging. Re-thinking my strategy now.
Thanks again..
 
Parallel capacity doesn't matter that much.
Going parallel will increase the completability of the setup, with more possible points of failure.
especially when a few mouts / remounts will cost you the tapped thread inside the terminals... (never use the bolts they give you, always use studs / grub screw / headless bolts and preferably with loctite RED and primer (aluminium, needs primer for loctite)
Not an issue with laser welded studs :)

for series... having 2 x 50, 1 x 150 and one 200Ah cell to make up a 12v pack, you have 50Ah..
In other words, the smallest capacity is leading. That you add higher capacity on a few spots doesn't hurt, but won't help either.
 
how to add an 8S 24V 280Ah LFP bank into my existing 8S 24V 100Ah LFP bank.
Don't get me wrong, you can add them together BUT the charging & discharging limits will have to be worked around as the 100AH will charge & discharge faster than the 280AH and the BMS fudging to protect the 100AH pack has to be right to prevent Over/Under charging the pack.
 
Here's my plan.
Two semi independent systems; each consisting of 12 * 250W panels, feeding into an MPP 3Kw all in one unit, then into a 48V battery bank. All fused with breakers of course.
I say semi independent because the MPP units will have the kit to allow them to run in parallel for the 120V AC output to the house.
So 2 solar panel arrays, 2 MPP all in one units and 2 battery banks.
One battery bank will be 32 150ah cells in a 16S2P configuration and the other, 16 200ah cells in a 16S configuration. Not my first choice but dictated by cell availability.
The idea was that if one system went down I could isolate it completely and run the house on the remaining system. The great thing about grid power is that if it goes down it's fixed very quickly; solar, it can take a very long time to get replacement parts, which is a major fear for going completely off grid.
The question is; can I connect both battery banks in parallel with each other; 150ah 16S2P (15.3Kwh) with 200ah 16S (10.2Kwh), or is there a better way to mix & match?
 
Here's my plan.
Two semi independent systems; each consisting of 12 * 250W panels, feeding into an MPP 3Kw all in one unit, then into a 48V battery bank. All fused with breakers of course.
I say semi independent because the MPP units will have the kit to allow them to run in parallel for the 120V AC output to the house.
So 2 solar panel arrays, 2 MPP all in one units and 2 battery banks.
One battery bank will be 32 150ah cells in a 16S2P configuration and the other, 16 200ah cells in a 16S configuration. Not my first choice but dictated by cell availability.
The idea was that if one system went down I could isolate it completely and run the house on the remaining system. The great thing about grid power is that if it goes down it's fixed very quickly; solar, it can take a very long time to get replacement parts, which is a major fear for going completely off grid.
The question is; can I connect both battery banks in parallel with each other; 150ah 16S2P (15.3Kwh) with 200ah 16S (10.2Kwh), or is there a better way to mix & match?
If you are going to parallel the inverters, they must share the batteries. ( at least thats what the MPP manual states).
That is why I am still waiting on the group buy for more 272ah cells. I already have 16 cells, and do NOT want to make things more complex by adding unlike cells.
I am adding a second LV6548, and want to keep it simple with one bank consisting of 2 seperate 48V 16S cells, each with its own BMS.
 
Don't get me wrong, you can add them together BUT the charging & discharging limits will have to be worked around as the 100AH will charge & discharge faster than the 280AH and the BMS fudging to protect the 100AH pack has to be right to prevent Over/Under charging the pack.
Just spitballing..on the load side, if I set the LVD on the 100Ah bank bms higher so it cuts out at say 40 or 30% capacity, leaving the rest of the load drawing off the 280Ah bank? Might help? My current bms is a split port, (overkill 8S-100A) and I'm guessing that's what I'd need for my future 280Ah bank. (Split port) But what about 1 bank feeding into the other...at night for instance, when theres no load or charging going on, and all charge and discharge switches are on, and they are at different soc. Is that necessarily a bad thing?
Hmm..still thinking this thru..
 
If you are going to parallel the inverters, they must share the batteries. ( at least thats what the MPP manual states).
That is why I am still waiting on the group buy for more 272ah cells. I already have 16 cells, and do NOT want to make things more complex by adding unlike cells.
I am adding a second LV6548, and want to keep it simple with one bank consisting of 2 seperate 48V 16S cells, each with its own BMS.
I understand, but my plan is to have both battery banks in parallel feeding both MPP units, but if maintenance is needed to be able to isolate one battery bank and on MPP unit completely.
 
I understand, but my plan is to have both battery banks in parallel feeding both MPP units, but if maintenance is needed to be able to isolate one battery bank and on MPP unit completely.
You can still isolate if using split phase, and sharing the batteries. Each inverter is supplying a 120V leg of the 240V. Watch one of DanF's videos or DMI INC. Thats how I am setting mine up, based on their solutions.
 
One bank feeding into the other has its drawbacks.

You NEED something in between to reduce the speed.
If one set is 2.9v and the other 3 25 you will kill both sets in a few minutes, if you just put them together.

In theory you can use one inverter to power DC charger and charge the other cells by it.
You lose about 25% due double conversion.

Why not parallel them?
And use both inverters from the 250-100 pack?

Parallel or not, it will get the most out of both inverters.
Hybrid inverter have its own solar charger...
I haven't seen non-hybrid inverter able of parallel.

You probably have if will have 2 solar arrays.

This way each array can give its max to the joined cell.

As you have hybrid, split port BMS doesn't make much sense as the charge and discharge is from the same line.

If it's contactors operated by "split port", it does make sense.
You can cut of the solar line into the hybrid with Contactor.
And stop the DC line if battery gets too low with other Contactor.

I haven't played with the overkill BMS, so please educate me here!!
 
Thanks for all the replies.
It looks like the most practical way will be to have both battery banks made up of the same capacity cells, then parallel them both together.
I'll wait and see if the AliExpress order for the 200ah cells works out, and if so, order some more and sell the 150ah cells from the group buy when they arrive.
 
Thanks for all the replies.
It looks like the most practical way will be to have both battery banks made up of the same capacity cells, then parallel them both together.
I'll wait and see if the AliExpress order for the 200ah cells works out, and if so, order some more and sell the 150ah cells from the group buy when they arrive.
I think that is the best approach. Keep it simple. I created a diagram of how my system is put together. As is apparent, both banks of cells are shared. I am adding another Inverter to this config, so both inverters will be attached to both batteries.
 

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My question on the different Ah is more around cells suitable for use in mobile solutions that are subject to high shockload. That giant pothole or wash out section that jars your tailbone also jars the battery. Every EV rated battery I see is lower Ah, high ( 3-6) C rate and weighs much more per Ah than the .5/1C rated cells. Will the conductors experience failures in the comodity cells? I did try emailing a couple Manufacturers with this question and did not get a reply, not that I expected to. So parallel them, but then you need matched cells? Lions, tigers and Lithium oh my :)
 
My question on the different Ah is more around cells suitable for use in mobile solutions that are subject to high shockload. That giant pothole or wash out section that jars your tailbone also jars the battery. Every EV rated battery I see is lower Ah, high ( 3-6) C rate and weighs much more per Ah than the .5/1C rated cells. Will the conductors experience failures in the comodity cells? I did try emailing a couple Manufacturers with this question and did not get a reply, not that I expected to. So parallel them, but then you need matched cells? Lions, tigers and Lithium oh my :)
EV Cells such as those in any EV Drive pack are not usually LiFePo4, rather pouch or cylindrical cell and NMC or similar chemistry.
Vehicle Drivetrain LFP (current tech) comes in the form of Cell-To-Pack formats which are also pouch based but now LFP Large Capacity cylindrical is also coming into play.

The prismatic cells, the blue square ones used for ESS have been used in EV packs as well but that is a different grade as well. If these cells are properly "Bound" with slight compression and fitted into a sufficient casing that can endure the environment, they should be fine. The BMS is delicate electronics, so appropriate precautions will be required. All nuts, screws etc will have be "fixed" so they do not loosen with vibrations, this is often done with serrated or split washers as well as other methods. Note, that good binding of the cells together makes the entire "package" more solid, it prevents any rubbing or chaffing which over time could cause damage to the casings and result in failures.

If paralleling cells of ANY type, it is always best to have Matched & Batched cells. EV Packs for example, are rigorously tested & matched before being assembled into a pack. At present, Luyuan Tech has 280AH Matched Grade-a with welded studs for $122USD or Bulk cells for $92USD and I don't know if the bulk cells get welded studs or not. I have packs that are Bulk Cells and these Matched ones, there IS a difference and very little deviation between cells through their operation cycle. Well worth it too ! but that's just my opinion.

Hope it helps, Good Luck
 
EV Cells such as those in any EV Drive pack are not usually LiFePo4, rather pouch or cylindrical cell and NMC or similar chemistry.
Vehicle Drivetrain LFP (current tech) comes in the form of Cell-To-Pack formats which are also pouch based but now LFP Large Capacity cylindrical is also coming into play.

The prismatic cells, the blue square ones used for ESS have been used in EV packs as well but that is a different grade as well. If these cells are properly "Bound" with slight compression and fitted into a sufficient casing that can endure the environment, they should be fine. The BMS is delicate electronics, so appropriate precautions will be required. All nuts, screws etc will have be "fixed" so they do not loosen with vibrations, this is often done with serrated or split washers as well as other methods. Note, that good binding of the cells together makes the entire "package" more solid, it prevents any rubbing or chaffing which over time could cause damage to the casings and result in failures.

If paralleling cells of ANY type, it is always best to have Matched & Batched cells. EV Packs for example, are rigorously tested & matched before being assembled into a pack. At present, Luyuan Tech has 280AH Matched Grade-a with welded studs for $122USD or Bulk cells for $92USD and I don't know if the bulk cells get welded studs or not. I have packs that are Bulk Cells and these Matched ones, there IS a difference and very little deviation between cells through their operation cycle. Well worth it too ! but that's just my opinion.

Hope it helps, Good Luck
Thank you, great response,
Yes I have only been looking at LFP cells vs ones with cobalt, etc. The largest “EV” cell I have seen is 150 with most being 100 and under. I was looking at some of the old info allegedly from Sinoply of under 200 in mobile and 100 being much better. A lot could have changed since that info was reported years ago, but.... I tend to be overly cautious with things that can burn a long way from assistance.
Many mobile have much higher draw rates than solar, motors, winches, thrusters oh my :) and then shock loads...then there are the off-road karts... now throw in high moisture and salt air environments.... damn desert people :)

That is one reason I find this site great is the broad prospectives and general good nature
 
My question on the different Ah is more around cells suitable for use in mobile solutions that are subject to high shockload. That giant pothole or wash out section that jars your tailbone also jars the battery. Every EV rated battery I see is lower Ah, high ( 3-6) C rate and weighs much more per Ah than the .5/1C rated cells. Will the conductors experience failures in the comodity cells? I did try emailing a couple Manufacturers with this question and did not get a reply, not that I expected to. So parallel them, but then you need matched cells? Lions, tigers and Lithium oh my :)
Folks have been using the SOK and Battleborn Lifepo4 batteries in RV's and on the road in mobile applications for some time with great results. The battery just needs to be built right.
I took my SOK's apart, and they are made like a tank. Very solidly built. I would have zero concerns about them.
As for DIY Lishen or EVE cells with a BMS, I would build them just as solid as 12V batteries and then use flexible battery cables to put them in parallel, and then strap them down like in a sailboat.
 
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