Parallel cells - what are the pitfalls as the number of cells in parallel gets deeper ?

[Ohms_Cousin]

No, I have had this $800 Orion BMS for four years and did not want get rid of it.

Orion look good. how do you find it ? Strengths/weaknesses ? If I did go 96v its one of the BMS's that can handle it

 

[Steve_S]

Here, virtually everyone is working with the rectangular prismatic cells from varied suppliers providing them. More often than not they are NOT properly Matched or Batched, which means they will collectively perform identically, IE: Same IR @ 3.000 & 3.100 & 3.200 etc. In EV Land like Tesla, their cells are perfectly matched and each cell is actually fused as well... Apples to Grapefruit !

Only a FEW Vendors can supply properly Matched/Batched Prismatics and they do cost more. IF Paralleling cells they cannot be mix-n-match, hope for the best, but the results will never be what is desired. Even if you took 200AH Cells and paralleled them in a pack to come up with a 400AH Pack, you would still be limited to 200A Output & 100A Charge.

Given the nature of cells available from assorted vendors and people squeezing their wallets we generally do not recommend paralleling cells, within a battery pack. Flip of the coin risk with minimal benefit.

Paralleling Battery Packs, builds in redundancy & backup because if 1 Pack fails for any reason the others continue to operate without issue. Technically there is little limit on how many Packs can be put in parallel. A Battery Pack of course being the Cells & BMS for those cells. I run 5 Packs in Parallel and there are a few here running even more in multiple racks of battery packs.

Do remember that when Paralleling Packs they will divide all Discharge & Charge between themselves. The bigger the Battery Bank is the more complicated it is to charge because you always have to keep in mind the Lowest Common Denominator which is the "Single Pack" with the lowest AH Capacity. As above, if you have 6x 200AH Packs in parallel for 1200AH you are still limited to a MAX of 100A Charge to a pack, which translates to 16A per pack for charging.

 

[Ohms_Cousin]

Redundancy is important on a boat and several small packs may be easier to service and place in various spots for good weight balance. If my installation was on a boat, I would go with redundant packs. Today good BMSs are reasonable in cost and Bluetooth monitoring is adequate.

Yep - everything you say is true and a big issue. I am going to tackle those issues in another thread in order to keep it all focused.

 

[Ampster]

Orion look good. how do you find it ?

I have been using Orion's since an EV conversion in 2012. It is an automotive grade BMS that has more features than needed on a boat or stationary use case. I have a 20S JBD that works fine for a chipper shredder conversion. It can handle 200 Amps with contactor and is flexible to only do 8S, which after experimentation is all I needed.

 

[740GLE]

First time I connected my 2p4s I notice “cells” 1 and 4 were picking up charge much faster than 2 and 3. I then redid the cell to cell interconnections such that the diagonal terminals were tied to adjacent cell connections.

My initial thought was that it didn’t matter but low and behold it made huge difference. After redoing the inter cell connections, delta between cells remained around .001-.005 after a charge/discharge cycle.

 

[Ohms_Cousin]

Here, virtually everyone is working with the rectangular prismatic cells from varied suppliers providing them. More often than not they are NOT properly Matched or Batched, which means they will collectively perform identically, IE: Same IR @ 3.000 & 3.100 & 3.200 etc. In EV Land like Tesla, their cells are perfectly matched and each cell is actually fused as well... Apples to Grapefruit !

Only a FEW Vendors can supply properly Matched/Batched Prismatics and they do cost more. IF Paralleling cells they cannot be mix-n-match, hope for the best, but the results will never be what is desired. Even if you took 200AH Cells and paralleled them in a pack to come up with a 400AH Pack, you would still be limited to 200A Output & 100A Charge.

Given the nature of cells available from assorted vendors and people squeezing their wallets we generally do not recommend paralleling cells, within a battery pack. Flip of the coin risk with minimal benefit.

Paralleling Battery Packs, builds in redundancy & backup because if 1 Pack fails for any reason the others continue to operate without issue. Technically there is little limit on how many Packs can be put in parallel. A Battery Pack of course being the Cells & BMS for those cells. I run 5 Packs in Parallel and there are a few here running even more in multiple racks of battery packs.

Do remember that when Paralleling Packs they will divide all Discharge & Charge between themselves. The bigger the Battery Bank is the more complicated it is to charge because you always have to keep in mind the Lowest Common Denominator which is the "Single Pack" with the lowest AH Capacity. As above, if you have 6x 200AH Packs in parallel for 1200AH you are still limited to a MAX of 100A Charge to a pack, which translates to 16A per pack for charging.

Steve, glad you joined this thread. I have read a metric TON of your posts in the last 2 years or so. Part of what put me off the prismatic cells is what you have to say about them. I dont feel confident that I can get REAL EV grade prismatic cells - too many stories of people not getting what they thought they were getting. Of course some of this is on the buyer - you get what you pay for so if you think you are getting the best cells for a song - well the many experiences regaled here tell the story on that approach. I dont mind paying for quality EV cells but where to get a REAL GENUINE NEW FRESH EV prismatic cell? I have seen it written many times - China is a crap shoot!

Then there is the argument about not using prismatic cells on a boat. This article https://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/ very clearly states that the author was given what he considers trustworthy advice from a manufacturer - I quote

"A sales manager at Sinopoly I was talking to was adamant about using 100Ah or 200Ah cells only for assembling marine battery banks, with 100Ah being preferred and 200Ah acceptable. Large cells simply don’t have the structural strength-to-weight ratio required to be taken to sea on board small crafts and would exhibit shortened life due to internal mechanical damage arising from on-going vessel motion. It is common sense: as a cell becomes larger, its internal weight increases much faster than the rigidity and surface area of the casing and the casing is all what holds the plates together in a prismatic cell.

Failures have been reported on vessels equipped with 700Ah cells following ocean passages: some cells were suddenly found to be losing charge inexplicably, rendering the battery bank completely unmanageable and the matter ended in a complete write-off. All big-brand commercial marine lithium battery packs on the market today are built from cells no larger than 200Ah."

So for a boat it seems to be safe, if using prismatic cells, to stick with 100Ah or smaller cells. Cylindrical cells may be even better in this respect. The cylindrical cells I have mentioned are from the factory direct - no resellers involved. The cells are Gushen GODSEND 22Ah 3.2v LiFePo4 cells. I came across them in a car audio face book group I am in and they say these cells are very very good (experienced admin who runs the group and who by all accounts is reliable). I contacted the factory direct and they will sell direct - at great pricing to boot. Just today they contacted me to say they are about to start production of another batch of these cells if I want to place an order with them. So brand new fresh cells rated at 5C discharge - catch is only 22Ah per cell and that means either a lot of batteries or a good number of parallel cells in a battery to get the capacity I need. I am targeting around 50kWh x 2 - that is I want to build out 2 battery systems - each with thier own BMS and inverter. Out here we live by the saying "2 is 1 and 1 is none". 50kWh hours gives me the range i need on the electric propulsion and leaves about 15 kWh for the house duties - before needing to recharge.

Your statement .....

"Paralleling Battery Packs, builds in redundancy & backup because if 1 Pack fails for any reason the others continue to operate without issue"

would depend on the BMS chosen. I was looking at Battrium - a lot. I need to build a distributed battery bank to balance out the weight in the boat. Battrium is good for this but, at least to me, has the undesirable action of dropping the WHOLE battery bank if there is a critical fault in one of the parallel battery packs in the battery bank. This defeats your preferred action of having redundancy if 1 pack fails - with Battrium the whoe BANK is taken off line. The ony way around this is to build out 2 battery banks - i think.

I am still reading on the approach of a JK BMS on every paralleled battery - not quite sure how that all works out just yet. Lots of reading to do on this.

It seems I am missing something quite important here - or under a false belief anyway...... I dont quite get this part that you wrote....

"Even if you took 200AH Cells and paralleled them in a pack to come up with a 400AH Pack, you would still be limited to 200A Output & 100A Charge."

I, for some reason was under the belief that if you took a 3.2v 22Ah 5C discharge /0.5c Charge cell and paralleled that cell with another 3.2v 22Ah 5C discharge / 0.5C cell that you ended up with a 3.2v 44Ah 5C discharge /0.5C charge cell pair - meaning that you can now discharge a 44Ah cell at 5C and charge a 44Ah cell pair at 0.5C.

To put this in a clearer way : a 22Ah cell 5C discharge cell could be discharged at 110amps and the 0.5C charge rate for that 22Ah cell was 11amps. Now if that cell was paralleled with another cell of the exact same type then you would have a 44Ah cell pair that can be discharged at 5C - ie, 5C of the 44Ah pair meaning it can be discharged at 220amps and charged at 0.5C of 44Ah - ie, charged at 22amps.

Seems I have this very wrong. Can you please clarify this point to make sure it is clear for those of us who must have been sleeping in class when this bit of the lesson was given.

 

[cs1234]

"Even if you took 200AH Cells and paralleled them in a pack to come up with a 400AH Pack, you would still be limited to 200A Output & 100A Charge."

I, for some reason was under the belief that if you took a 3.2v 22Ah 5C discharge /0.5c Charge cell and paralleled that cell with another 3.2v 22Ah 5C discharge / 0.5C cell that you ended up with a 3.2v 44Ah 5C discharge /0.5C charge cell pair - meaning that you can now discharge a 44Ah cell at 5C and charge a 44Ah cell pair at 0.5C.

To put this in a clearer way : a 22Ah cell 5C discharge cell could be discharged at 110amps and the 0.5C charge rate for that 22Ah cell was 11amps. Now if that cell was paralleled with another cell of the exact same type then you would have a 44Ah cell pair that can be discharged at 5C - ie, 5C of the 44Ah pair meaning it can be discharged at 220amps and charged at 0.5C of 44Ah - ie, charged at 22amps.

Seems I have this very wrong. Can you please clarify this point to make sure it is clear for those of us who must have been sleeping in class when this bit of the lecture was given.

Maybe it has something to do with varying battery state of charges. The full ones will stop charging, leaving too much charging amperage going into the now smaller total pack amperage charging allowance?

 

[Ohms_Cousin]

deleted post.

Its been a long long day of reading. Need to rest before going any further with all this. It is critical to get this right.

 

[cs1234]

any really good reasons why 96v should be avoided? - other than shock risk, which can be managed.

Not as many available products for inverting and charging them. Puts you in that "high voltage" over 80vdc range that seems to be a concern for ARC fault detection/prevention, rapid shut down requirements and metallic conduit requirements in some places.

Would certainly reduce wire size requirements or reduce voltage drop.

 

[Steve_S]

"Even if you took 200AH Cells and paralleled them in a pack to come up with a 400AH Pack, you would still be limited to 200A Output & 100A Charge."

That is related to Standard Prismatic Cells which have a 1C Discharge & 0.5C Charge rate.
I am quite aware of Nordkin and posted their links numerous times over the years.

Lithium battery systems | Nordkyn Design

nordkyndesign.com

With prismatic's you absolutely need to Bind Them, especially for anything that is mobile & subject to vibration, bouncing etc... They should be bound when the cells are at 3.200V each. A True Grade-A Matched & Batched EVE 230AH is likely as far as I would go for such an application as yours.

Cylindrical cells have their pro's & con's BUT must be dealt with as LFP and not like NCA/NCM etc chemistries. Fusing each cell is not necessary.

Not to toss a rock in the pond... Look at BYD LFP Blade Cells which are Long, Thin & Narrow by design and have a huge amount of kick in them. There are some ALI* vendors selling them (138AH models) but it's unlikely that BYD itself will retail them in any case... BYD is now rolling those out into Busses & Heavy Trucks (EU) and already is using them in their standard EV's, so supplies will be limited at best and what grading they have I dunno. The one thing for sure, noone can "copy" them as such due to the format. The BYD SVolt 138AH Cells appear to be available but not their latest 200AH & up cells. They ain't cheap !

BYD Blade Battery: Everything you should know

The BYD Blade battery was planned for select cars, but BYD has fitted the battery in multiple models, including the Qin Plus, Song Plus, Tang EV, Yuan Plus and the E2

topelectricsuv.com

 

[A.Justice]

any really good reasons why 96v should be avoided? - other than shock risk, which can be managed.

It sounds like there's more reasons to go 96 volt than not.

 

[kuranaga]

with more cells you have better probability that the cells balance each other. so more small cells and GOOD connection between them (like welded) = BETTER. worse are few big cells in parallel e.g. 2 x 280 AH because then one defective cell will take down the whole pack

 

[toms]

so what happens if a cell in one of 3P strings short circuits?

The same thing as if one section of a single cell short circuits - the current surge blows apart the short circuit and that section of the cell goes open circuit. (ie you lose capacity).

Other than the risk of incorrectly assembling the interconnect - there is zero difference a single 200ah cell vs 2x100ah cells in parallel.

 

[Ohms_Cousin]

Not as many available products for inverting and charging them. Puts you in that "high voltage" over 80vdc range that seems to be a concern for ARC fault detection/prevention, rapid shut down requirements and metallic conduit requirements in some places.

Would certainly reduce wire size requirements or reduce voltage drop.

Yes that is true - finding a quality Inverter at that 96v is difficult _ i have found only one, but its a good one

SP PRO Models - Selectronic Australia

There are some very nice features in this inverter. I am interested in the 7.5kW version - note it is 7.5 KILOWATT - > not VA as per Victron. It can sustain an 18kW load for 30 seconds - thats pretty impressive. So would easily start a 2/3kW motor on a star delta or VFD starter.

I think in the EU it is 72v that is the limit. Its certainly much easier to stay 48v. I am not sure which way to go on this.

 

[Ohms_Cousin]

It sounds like there's more reasons to go 96 volt than not.

Yes technically there are some good reasons to go 96v ;

1/ less amps being taken out of the batteries for the same workload at the motor
- I am guessing this can only be good for the cells ?

2/ Using the same cable used at 48v would result in less voltage drop and not as much heat in the cabling.

3/ Able to get more capacity into a 32 series string without paralleling cells

4/ If one still needs to parallel cells to get the desired capacity then it would be less paralleled cells which can only be a good thing. It would be ideal to stay at 2P32S. (ie, the capacity at 2P32S would be the same as 4P16S if using the same cells)

5/ The electric motor itself runs cooler at the higher voltage as it spins faster thereby drawing more air through the windings. It was the factory who told me this and why I started considering 96v - my choice of electric motor is a DC brushed air cooled motor. The cooler it runs the better, so 96v is a win for that. I looked at and considered a liquid cooled electric motor setup - they use heat exchangers. Its a good way to get rid of the heat out of the motor bay but it introduces complexity and more points of failure and lots of maintenance issues. I decided air cooled was the way to go for simplicity's sake.

6/ The specs of contactors and shunt trips allow use of these devices at up to 600VDC - so it seems contactors and and shunt trips are no problem at this voltage.

Cons of going 96v

1/ Not much gear out there for a 96v nominal system.

2/ What is out there is more expensive.

3/ Safety concerns - DC arching at that voltage seems to be an issue that needs to be carefully considered.

4/ Loss of solar on the boat - this is a big one. Cant get enough panels in a series string to get the voltage high enough to charge a 96v bank.

5/BMS's become harder to find for this voltage.

I am not tied to 96v at all. I would prefer to stay at 48v.

There are 2 things that are critical in this changeover to electric propulsion - the motor running as cool as possible and a tight battery setup. 96v seems to tick both boxes resoundingly.

I am intending to install a DC genset. The manufacturer can supply the DC genset as 48v or 96v. It is an extra $500 to wind the alternator as 96v compared to the 48v. Its a wash as far as the genset goes.

https://eniquest.com.au/wp-content/uploads/2020/01/PowerMakerCruiseEmailSize.pdf

96v sounds all well and good but it just feels uncomfortable going in that direction. At 48v there is a lot of information and resources out there to figure things out. 96v not so much.

 

[Ohms_Cousin]

with more cells you have better probability that the cells balance each other. so more small cells and GOOD connection between them (like welded) = BETTER. worse are few big cells in parallel e.g. 2 x 280 AH because then one defective cell will take down the whole pack

I guess its a "is the glass half full or half empty" type thing. I follow your logic on more cells having a better chance to even out glitches in the battery overall. But then again more cells = more chances of a cell going bad.

I had not considered that if a big cell goes bad it takes the battery down. Seems to be an argument for more small cells in parallel to counter this.

And besides, Battle Born do lots of smaller cylindrical cells in a battery, so it cant be a poor practice.

 

[Ohms_Cousin]

so thinking all this through, is there a case to be made for 2P setups no matter the voltage or ampacity of the battery?

As pointed out, if a large 280Ah cell goes bad it will take out the entire battery. If that same 280Ah cell is in a 2P pair and one cell goes bad, the battery still maintains its voltage overall, but with less amps. Or does the one cell going bad take both cells out?

 

[sunshine_eggo]

so thinking all this through, is there a case to be made for 2P setups no matter the voltage or ampacity of the battery?

I've kept my mouth shut so I don't seem like hypocrite... My first battery is a 21P14S 21Ah Panasonic NMC-LMO PHEV cells. These cells are internally fused. I'm working on the second of the same configuration. Batrium WM5 on the first, and I have a MM8 for the second one, so in the end, I will have 21P14S 2P. I'm contemplating putting a JBD on each as a means of shutting down one of the batteries BEFORE it trips the Batrium. I'm operating well inside voltage limits (3.50-3.92V, 20-80%) using about 60% of rated capacity.

As pointed out, if a large 280Ah cell goes bad it will take out the entire battery. If that same 280Ah cell is in a 2P pair and one cell goes bad, the battery still maintains its voltage overall, but with less amps.

This is optimistic. Depending on the nature of the failure, the cell may fail in short with a high resistance. This will cause the failed cell to drain the good cell.

Or does the one cell going bad take both cells out?

It's definitely a risk. It may not destroy the good cell in the pair provided you break them apart ASAP.

 

[Ohms_Cousin]

Than

I've kept my mouth shut so I don't seem like hypocrite... My first battery is a 21P14S 21Ah Panasonic NMC-LMO PHEV cells. These cells are internally fused. I'm working on the second of the same configuration. Batrium WM5 on the first, and I have a MM8 for the second one, so in the end, I will have 21P14S 2P. I'm contemplating putting a JBD on each as a means of shutting down one of the batteries BEFORE it trips the Batrium. I'm operating well inside voltage limits (3.50-3.92V, 20-80%) using about 60% of rated capacity.



This is optimistic. Depending on the nature of the failure, the cell may fail in short with a high resistance. This will cause the failed cell to drain the good cell.



It's definitely a risk. It may not destroy the good cell in the pair provided you break them apart ASAP.

Thanks for the info. Definitely getting a much better understanding of the issues surrounding the paralleling of cells.

 

[toms]

Connect your cylindricals like this if you are worried about short circuit failure. There are a lot of EV cylindricals using this method. The fusible link is sized to carry maximum discharge current, then fuse if that is exceeded.

Parallel batteries introduce more problems than they solve. They invariably go out of balance, and introduce more failure points to the system.

If you need redundancy, parallel batteries won’t give you that - you need two independent systems.