12v lithium batteries can't be paralleled?

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Re: 12v lithium batteries can't be paralleled?

Post by jonescg »

Are these RV failures lead gel batteries? Or LFP?

If it's LFP, the best option is to parallel them at the cell level, and that means having all the battery in one place. Which is not always the easiest option.
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Re: 12v lithium batteries can't be paralleled?

Post by TyTower »

@T1 Terry

Well its a lot of words atm . Hearsay that's called without proof , so I certainly look forward to seeing it demonstrated in any way.
Having made the statements I hope you can find the time and resources to make a video . I am interested .

Certainly you have my word that the next time I take my battery pack back to 24 volts I will put current meters in each string just to satisfy myself . If I am able to buy the next 16 alloy 100Ah prismatics soon I will set them up that way.. Definitely I will video it when I do eiither.
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Re: 12v lithium batteries can be paralleled?

Post by TyTower »

jonescg wrote: Wed, 23 Dec 2020, 14:54 If it's LFP, the best option is to parallel them at the cell level, and that means having all the battery in one place. Which is not always the easiest option.
If one cell lets go and shorts out the other cells will hold a value of voltage well above it and mask the fact that its gone. If you are there it will probably be noticed and the rest of the cells saved. If the cell is left shorted then the other cells will discharge into it and the BMS switching off will not stop the discharge right through the pack .

So if you are not there at the time or look in only once a day or so the whole pack may be flat and many more cells damaged .

It makes sense to parallel with 18650's and the like but expensive alloy cell prismatics need a BMS on each string watching each individual cell.
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Re: 12v lithium batteries can't be paralleled?

Post by jonescg »

What do you mean by 'lets go'?
If it forms an internal short, its voltage will drop and the cells in parallel with it will dump current until the buslink melts. If it goes open circuit the capacity is reduced, the DCIR increases and will result in voltage sag across that one cell group.

Parallel first and then series. Has worked for me.
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Re: 12v lithium batteries can be paralleled?

Post by TyTower »

Internal short .
What do you mean by "buslink" ?
If you have a 5 amp fuse wire connecting individual cells to a solid busbar on each group that would make sense with 18650's etc
Anything prismatic or larger cylindricals can't normally be fused between themselves so current continues to flow.
They could be fused individually but its not normally done .

My point here is that prismatics are expensive . Typically $100 to $500 each . You don't want to go paralleling them.
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Re: 12v lithium batteries can't be paralleled?

Post by jonescg »

It depends - when I build LiPo cell packs, the series buslink is a piece of 2 mm thick copper, and it serves as the parallel link too. I have never seen an internal short fail that way.
I have seen cells with a small self-discharge bring the entire parallel group down to zero though. Which sucks, but there's not much you can do about it except better QC.

If I were to parallel 400 Ah LFP cells I would just use the same stacked copper buslinks. Keep them short and of equal length / resistance.
Series first or parallel.png
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In the diagram above, you'd need a BMS capable of monitoring 12 cells. In the one below, you'd only need three.
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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

jonescg wrote: Wed, 23 Dec 2020, 14:54 Are these RV failures lead gel batteries? Or LFP?

If it's LFP, the best option is to parallel them at the cell level, and that means having all the battery in one place. Which is not always the easiest option.
Yes, drop in lithium batteries, built in a case to look just like the AGM batteries they removed. The way to build a house power lithium battery is most definitely multiple smaller capacity cells in parallel to build the capacity required, then sets of these in series to build the voltage required. One BMS that looks after everything from cell voltage when charging to battery isolation if a cell goes well outside the parameters required for long cycle life, both high and low cell voltage protection.
Better to spend the $$ on a quality single BMS rather than multiple half arsed attempts designed to cost as little as possible yet give a false sense of security.

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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

Seems to me there is a misunderstanding regarding the failure mode with quality LFP and LYP cells. They do not go sudden short circuit where massive current would flow into the affected cell, it is more a very slow drain that heats the electrolyte to the point it boils and gasses out through the relief valve ... generally very slowly over a long period, not the cloud of white vapour seen in the dramatic You tube videos.
Once the cell looses its electrolyte, the heat will increase until the very thin plate material that connects the affected plate oxidised and simply fails, eliminating the short completely. The capacity of that cell is lost once the electrolyte is lost and this will show up on a quality BMS system that monitors cell voltage. Even if the pack is not attended, the latest BMS systems can upload the information along with a running graph of each cell groups performance ... the problem will show up very quickly and be obvious that attention is required.
Now compare this to the multiple batteries built from single cells and connected in parallel. A cell in one battery develops this same slow discharge short and fails in the same manner as the single cell in the parallel cell battery pack. The terminal voltage in the battery drops compared to the other batteries in the parallel array. The current will flow from the other batteries in an attempt to equalise the voltage across all the batteries in parallel. The battery with the compromised cell has all the cells in series, so the current flowing to try and raise that batteries voltage is flowing through all 4 cells, 3 cells that don't need the added current and one cell that does and will continue to do so until the plate link oxidises and fails.
The other 3 cells in that battery are now seriously over charged and will also boil off their electrolyte. The whole battery is now a drain on the other batteries in the parallel pack, the whole battery pack is now lost rather than a single cell .....

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Re: 12v lithium batteries can be paralleled?

Post by TyTower »

I can see from both the above posts there is a bit of posting without experience going on.
I would suggest that before you post on LFP prismatics you buy some and get some experience.
You may get a shock .
Applying other technology experiences to them simply does not work.The jelly roll sits in a bath of electrolyte ,ethylene carbonate. Heat can gas it off but new flows in .
Dendrites cause dead shorts at any point in the jelly roll and they are common which may leave you with 1 volt or 0 volts .
The prismatics you mostly buy on Ali-express are used cells or rejects that have dropped to 80% of original capacity . These are refurbished by resellers in China with new stick ons and are sold as new with a lower capacity.
They are very prone to failure because the plates are often burnt.
They are still bloody expensive per cell so you want to protect each cell . A good BMS can be had for $20 AUD so why not protect each individual cell?
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Re: 12v lithium batteries can't be paralleled?

Post by jonescg »

I have experience with both circuit diagrams drawn. The parallel first, then series arrangement is more reliable, in my experience.

Of those two diagrams, are you suggesting the upper one is better?

By the way, if you open up a large prismatic LFP cell, you will find 3 jelly-rolls wound up and paralleled anyway. Do they need a BMS too? ;)
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Re: 12v lithium batteries can't be paralleled?

Post by nuggetgalore »

coulomb wrote: Tue, 22 Dec 2020, 16:35
T1 Terry wrote: Tue, 22 Dec 2020, 13:18 ... the 100 plus people who witness the demo projected up on the big screen saw the first battery provided more than 75% of the load, the second battery started out providing around 20% and the third battery around 5%.
Well, that just shows that it's not a good idea to parallel battery modules improperly.

It's quite easy to parallel two battery modules properly, using "diagonal takeoff". Four modules or cells can also be done, it's a little more involved.

This thread has me a bit worried. I have 16 Avass Lifpo cells in a 2P8S configuration.The links connecting the cells are plates with 4 holes, the cells are packed tightly close together but each has it's own top balance shunt.Obviously having cells on parallel and series is quit common in traction batteries (by the thousands in a Tesla for instance). I just wonder if I need to watch out for something I may have missed ?
With hindsight yes I probably should have changed my original 24V system to 48 V, but that meant junking a lot of gear ( I hate waste!).
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Re: 12v lithium batteries can't be paralleled?

Post by coulomb »

nuggetgalore wrote: Sun, 27 Dec 2020, 04:32 This thread has me a bit worried. I have 16 Avass Lifpo cells in a 2P8S configuration.The links connecting the cells are plates with 4 holes....
I just wonder if I need to watch out for something I may have missed ?
I think the four-hole plates will be OK for paralleling, though I don't like the idea of rigid cell connections.

So then the things to look out for would be the main cables, and the interconnection if you have two strings of eight cells (2P4S) beside each other. Maybe you used another 4-hole plate, just a different size.

With the main cables, the temptation is to connect both cables at the same end, which usually looks slightly neater and usually results in slightly shorter cables. But you should use the other cell, so that no cell has a shorter path to the load or charger than any other.

Strictly speaking, your plates should really be U-shaped, inverting with each alternate cell connection. But as I said, my feeling (without doing any tests or even thinking about it very much) is that it's not worth changing now.

As an aside, my own 2S16P pack, (not using Avass cells) is all done with crimped lugs and the CALB interconnects, paying strict attention to diagonal takeoff. Two of the paralleled sets are with different sized cells, and some "buddy pairs" are split onto different shelves to fit all this on 3 shelves. Thanks to Weber for most of that interconnection design.
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Re: 12v lithium batteries can't be paralleled?

Post by nuggetgalore »

coulomb wrote: Sun, 27 Dec 2020, 06:54
I think the four-hole plates will be OK for paralleling, though I don't like the idea of rigid cell connections.
I should have been more specific re the 4 hole plates.
The two rows of cells in series are clamped together as suggested by Chris (to reduce swelling ?).
So the plates connect two cells in parallel and also two cells in series.
coulomb wrote: Sun, 27 Dec 2020, 06:54 So then the things to look out for would be the main cables, and the interconnection if you have two strings of eight cells (2P4S) beside each other. Maybe you used another 4-hole plate, just a different size.
No cables connecting any cells, both end cell pairs are connected with the same type of (aluminium) plates where the connection cables to ground and fuse etc are bolted on.
Where a possible problem could occur, is that if a single cell starts to fail, his buddy takes up the slack and will suffer. The shunts start bleeding off at 3.55 V and the charging stops at 28.6 give or take the inaccuracies of the PLC that I use as a BMS substitute. The shunts have coloured LEDs, I occasionally check the cell (pair)voltages when they light up like a Xmas tree.
coulomb wrote: Sun, 27 Dec 2020, 06:54
With the main cables, the temptation is to connect both cables at the same end, which usually looks slightly neater and usually results in slightly shorter cables. But you should use the other cell, so that no cell has a shorter path to the load or charger than any other.

Strictly speaking, your plates should really be U-shaped, inverting with each alternate cell connection. But as I said, my feeling (without doing any tests or even thinking about it very much) is that it's not worth changing now.

As an aside, my own 2S16P pack, (not using Avass cells) is all done with crimped lugs and the CALB interconnects, paying strict attention to diagonal takeoff. Two of the paralleled sets are with different sized cells, and some "buddy pairs" are split onto different shelves to fit all this on 3 shelves. Thanks to Weber for most of that interconnection design.
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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

TyTower wrote: Fri, 25 Dec 2020, 05:21 I can see from both the above posts there is a bit of posting without experience going on.
I would suggest that before you post on LFP prismatics you buy some and get some experience.
You may get a shock .
Applying other technology experiences to them simply does not work.The jelly roll sits in a bath of electrolyte ,ethylene carbonate. Heat can gas it off but new flows in .
Dendrites cause dead shorts at any point in the jelly roll and they are common which may leave you with 1 volt or 0 volts .
The prismatics you mostly buy on Ali-express are used cells or rejects that have dropped to 80% of original capacity . These are refurbished by resellers in China with new stick ons and are sold as new with a lower capacity.
They are very prone to failure because the plates are often burnt.
They are still bloody expensive per cell so you want to protect each cell . A good BMS can be had for $20 AUD so why not protect each individual cell?
:lol: Do you want photos of what a cell looks like when stripped out so you follow just how prismatic cells are built? Being doing this for 10 yrs now, I have a lot of system out there travelling the highways, all built in the parallel/series form as a single battery to the capacity and voltage required. Do you think I'd still be doing this after 10 yrs relying on word of mouth only for new customers if the systems didn't work?

Happy New yr to you all

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Re: 12v lithium batteries can be paralleled?

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What like these you mean? https://www.facebook.com/media/set/?set ... 519&type=3
You might have to join first but there aare 47 pictures in that album and many more in others that I have pulled apart and BMS teardowns too.
This is the facebook page https://www.facebook.com/groups/271980786862023
Do you doubt my experience? I was working on development at JCU back in 1992.
BTW this was intentionally not aimed at anyone . I do not mean to offend . I just find where the information posted is such that I disagree with it I post a correction so its my opinion and others can judge as they wish .
Yes a happy new year to all.

[ Moderator: fixed URL tags. Link still doesn't work for me. ] Funny I checked it after posting and it was fine . Its still fine

I've got a few others Ill attach. It may be you have to join first.
PulledDown100Ah.jpg
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Overheated Plates.jpg
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Re: 12v lithium batteries can't be paralleled?

Post by jonescg »

So you're highlighting poor quality cells? 100% that is a massive problem in the DIY market and it's always worth buying known good brands. No amount of good planninf can compensate for junk cells.
But that's also a separate matter to building a battery of high capacity and potential, where series and parallel arrangements are well known and battery management systems are essential.
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Re: 12v lithium batteries can be paralleled and are.

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jonescg wrote: Fri, 01 Jan 2021, 09:12 So you're highlighting poor quality cells? 100% that is a massive problem in the DIY market and it's always worth buying known good brands. No amount of good planninf can compensate for junk cells.
But that's also a separate matter to building a battery of high capacity and potential, where series and parallel arrangements are well known and battery management systems are essential.
No I don't know what you are on about ,do you?

I'm responding to the title."12v lithium batteries can't be paralleled?

I am saying of course they can , and are, and will continue to be . We parralel them in balancing , in usage and even in the sealed packs.
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Re: 12v lithium batteries can't be paralleled?

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OK so now I am confused, but I am also a bit tired and I might be missing something. You appear to have taken issue with something that Terry had done, or observed, but I'm not sure what it is now :?
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Re: 12v lithium batteries can of course be paralleled?

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jonescg wrote: Fri, 01 Jan 2021, 18:21 OK so now I am confused, but I am also a bit tired and I might be missing something. You appear to have taken issue with something that Terry had done, or observed, but I'm not sure what it is now :?
So forgive me for wondering then ,why are you posting?
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Re: 12v lithium batteries can't be paralleled?

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I'm sharing my experience with paralleling large format LiFePO4 cells for the benefit of other readers. How about you?
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Re: 12v lithium batteries can't be paralleled?

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jonescg wrote: Sat, 02 Jan 2021, 09:59 I'm sharing my experience with paralleling large format LiFePO4 cells for the benefit of other readers. How about you?
I have a feeling this is the same person who has chased me across many forums, basically to dispute anything I post.
If anyone is interested in me posting a Winston prismatic cell with the top off and another actually out of its plastic case, please let me know and I'll get Margaret to take some photos and post them for me ...... I shake so bad at the moment I seem to be able to outpace the auto focus :roll:

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Re: 12v lithium batteries can't be paralleled?

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T1 Terry are you talking about 12V units all with there own BMS's and then Parallel the outputs of the BMS's

OR

just 4 sets of cells and the outputs of them then paralleled with the next set

because as i said earler i would do it with naked cells but NOWAY would i do it after the BMS's because no 2 BMS's are exactly the same and a mismatch can cause big trouble if your unlucky

yes i would have a BMS on the charging side of things on each set but NOT on the output
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Re: 12v lithium batteries can be paralleled?

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T1 Terry wrote: Wed, 13 Jan 2021, 12:19 I have a feeling this is the same person who has chased me across many forums, basically to dispute anything I post.
If anyone is interested in me posting a Winston prismatic cell with the top off and another actually out of its plastic case, please let me know and I'll get Margaret to take some photos and post them for me ...... I shake so bad at the moment I seem to be able to outpace the auto focus :roll:
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Re: 12v lithium batteries can't be paralleled?

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Adverse Effects wrote: Fri, 15 Jan 2021, 06:08 T1 Terry are you talking about 12V units all with there own BMS's and then Parallel the outputs of the BMS's

OR

just 4 sets of cells and the outputs of them then paralleled with the next set

because as i said earler i would do it with naked cells but NOWAY would i do it after the BMS's because no 2 BMS's are exactly the same and a mismatch can cause big trouble if your unlucky

yes i would have a BMS on the charging side of things on each set but NOT on the output
I guess it depends on the quality of the BMS and just what it is monitoring and acting on. Just about all drop in lithium batteries rely on terminal voltage rather than individual paralleled cell group voltages. If the individual 12v battery shut down its charging at 3.6v in any cell and load at 2.8v or even 2.5v and did not reconnect the load until all cells read above 3.2v, then the 12v batteries could be paralleled in theory. The problem is, the maximum discharge capacity from all individual 12v batteries could not be added together, it would be limited to the maximum capacity of the load disconnect method used. If ech battery used say a Gigavac GX14, then maybe the capacity could be added together to say 300 amps current, then it would be well within the disconnect load rating of 350 amps minimising the possibility of the contacts welding together. The smaller GV 200 range are also supposed to be rated at 350 amps but actually can't actually handle a lot of 200 amp load disconnects, proven by the number of these things that have failed in my Prius range extender unit ...... the max current transfer I have ever seen was 175 amps, but they still end up welding closed after a while. The GX11 is rated at 150 amps but every one we used welded closed at a 100 amp current disconnect, so we had to replace every one of them .....
The cost of multiple Gigavac GX14 units would make the idea ridiculously expensive so I can't see why anyone would do that just so they could run multiple 12v batteries with all the down sides and nothing that I can see to be gained .....

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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

Adverse Effects wrote: Sat, 16 May 2020, 01:20
Just watched this video. What is really scary is Will doesn't realise the risk involved with aluminium cased cells with no proper isolation between the cells ..... a fire just waiting to happen? There is nothing preventing these cells from bulging during cycling, there is just sticky tape holding the 4 cells together.
A fellow member of the SA branch lost his Prius because he used similar cells and not enough bracing to stop them bulging. The end result was the case of one of the cells came into contact with the aluminium battery tray and the resulting arc blew a hole through the metal floor of the wheel well. This created a path for the air to feed the fire and that was the end of the Prius ......
Imagine that under the bed in a caravan or motorhome?

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