Parallel LiFePO4 12V 90Ah pack with 12V 280Ah pack, any issue?

Horsefly

New Member
So, I'm pretty new here (there, that's out of the way ;) ).

I've been reading the board for a quite a while, from way before I ever posted. I'm amazed at the knowledge and sharing of information. It's awesome!

My background: I'm a retired Electrical Engineer, and I designed my own solar power system, but I used Lead-Acid AGM batteries in my system, so I'm mostly new to the idea of LiFePO4 batteries. I admit I don't really understand how charging and discharging of LiFePO4 (or battery in general) really work (there, that's out of the way too 🙂).

A few folks in this thread (and others) have said that when they have packs/batteries of mismatched Ah tied together in parallel, it all works great and they don't see any current / power moving from the bigger pack (say, 280Ah) to the smaller pack (say, 90Ah) during discharge. Further, they report that the current delivered from the two disparate packs is proportional to their overall contribution to the total. I am confused by this, but I have no doubt of the authenticity that it is true. I just don't understand how this works.

Let's walk through it...

A 280Ah pack and a 90Ah pack are wired in parallel. They are both nearly fully charged, and so are quite happy at say 3.5V. Then someone turns on the loads. Both packs seek to fill whatever they can of the current demand of the load. It appears to me that the cells in a 90Ah pack and those in a 280Ah pack are not significantly different in internal resistance (from what I have been able to find), or perhaps it is somewhat unpredictable. Nevertheless, it seems that the 90Ah pack will deliver pretty much (90/(280+90) of the drawn current. Why is that?

As the two packs are drawn down, the voltages obviously are held together. However, it would seem like the smaller pack would be depleted faster. Eventually the smaller pack would not be delivering any current towards the load, and to maintain the same voltage, it would have to be receiving what is effectively a charge from the larger pack, while the larger pack is also serving the demand of the loads. However, it has been reported that there is not current flowing INTO the smaller pack in this instance, so.... Why is that?

Eventually, the loads quit and a source (solar, generator, shore, etc.) start recharging the two packs. Again, the two packs are maintaining the same voltage. Since the larger pack will rise more slowly, this part maybe makes more sense to me: The small gets what it needs to be at 3.3V, but the larger pack needs more current to get it up past that. So, the larger pack takes more current. Do I have that right?

This is probably a little thing, and not worth much discussion. However it is driving me crazy! I just don't get it.

Caveat: I'm coming from the Lead-Acid / AGM world. In that world, there are lots of rules you hear when you are initiated. One is that you don't put batteries that are not of the same Ah and same age - and maybe not even if they are not the same brand and date - together. To do so is asking for unspecified catastrophes! I didn't really understand that much either, but I could some sense of it, but it was mostly just the magic of batteries. Now I'm more prone to question things, I guess...

Any thoughts? Again, I'm just trying to learn here, and you guys know what you are talking about.
 

Bud Martin

Solar Enthusiast
This is what are shown with 20A load, it has about 7A drawn from 90Ah pack, 13A from 280H pack.


90Ah paralleled with 280Ah_20A load.jpgdrawn, the 280Ah is about 13A drawn.
 
Last edited:

Mike Jordan

Solar Enthusiast
So, I'm pretty new here (there, that's out of the way ;) ).

I've been reading the board for a quite a while, from way before I ever posted. I'm amazed at the knowledge and sharing of information. It's awesome!

My background: I'm a retired Electrical Engineer, and I designed my own solar power system, but I used Lead-Acid AGM batteries in my system, so I'm mostly new to the idea of LiFePO4 batteries. I admit I don't really understand how charging and discharging of LiFePO4 (or battery in general) really work (there, that's out of the way too 🙂).

A few folks in this thread (and others) have said that when they have packs/batteries of mismatched Ah tied together in parallel, it all works great and they don't see any current / power moving from the bigger pack (say, 280Ah) to the smaller pack (say, 90Ah) during discharge. Further, they report that the current delivered from the two disparate packs is proportional to their overall contribution to the total. I am confused by this, but I have no doubt of the authenticity that it is true. I just don't understand how this works.

Let's walk through it...

A 280Ah pack and a 90Ah pack are wired in parallel. They are both nearly fully charged, and so are quite happy at say 3.5V. Then someone turns on the loads. Both packs seek to fill whatever they can of the current demand of the load. It appears to me that the cells in a 90Ah pack and those in a 280Ah pack are not significantly different in internal resistance (from what I have been able to find), or perhaps it is somewhat unpredictable. Nevertheless, it seems that the 90Ah pack will deliver pretty much (90/(280+90) of the drawn current. Why is that?

As the two packs are drawn down, the voltages obviously are held together. However, it would seem like the smaller pack would be depleted faster. Eventually the smaller pack would not be delivering any current towards the load, and to maintain the same voltage, it would have to be receiving what is effectively a charge from the larger pack, while the larger pack is also serving the demand of the loads. However, it has been reported that there is not current flowing INTO the smaller pack in this instance, so.... Why is that?

Eventually, the loads quit and a source (solar, generator, shore, etc.) start recharging the two packs. Again, the two packs are maintaining the same voltage. Since the larger pack will rise more slowly, this part maybe makes more sense to me: The small gets what it needs to be at 3.3V, but the larger pack needs more current to get it up past that. So, the larger pack takes more current. Do I have that right?

This is probably a little thing, and not worth much discussion. However it is driving me crazy! I just don't get it.

Caveat: I'm coming from the Lead-Acid / AGM world. In that world, there are lots of rules you hear when you are initiated. One is that you don't put batteries that are not of the same Ah and same age - and maybe not even if they are not the same brand and date - together. To do so is asking for unspecified catastrophes! I didn't really understand that much either, but I could some sense of it, but it was mostly just the magic of batteries. Now I'm more prone to question things, I guess...

Any thoughts? Again, I'm just trying to learn here, and you guys know what you are talking about.

DUUUUUUUUUUUUUUUUDE

Like a brother from another mother. Makes no electrical since to me either. Largely mismatched capacity should cause issues in my mind. Yet a few folks have posted some measured results that show quite the opposite :unsure:
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Welcome to a world of confusion.
I went from Big Heavy Lead to LFP and had to retrain my brain a bit... Basically & simply put, forget all Lead Acid information ! They are batteries but that's it for commonality on many points. It's as bad as thinking in VAC terms and them flipping modes into thinking VDC mode... Certainly has headache potential.

1) The aging between battery packs is not an issue like it is with FLA, where the lowest denominator rules.
2) LiFePo / Lithium battery assemblies have a BMS while Lead does not...
The BMS controls and manages the battery pack and the cells within.
It cannot distribute voltages in/out. It can only connect/disconnect the pack when conditions require it. Low/High Volt disconnect (at cell level) and pack level, temp disconnect (if so equipped) and so on.
Some BMS' can passively Balance (discharge high volts from hi cells) or actively balance (transfer volts from hi to lo cells) but this is low end stuff.

When properly paralleled battery Packs are working correctly within a battery Bank the packs will share Charge/Discharge proportionaly. It has to do with the collective resistance / impedance of the assembled packs, which is directly correlated to the capacity of the collective cells. For all intents & purposes, from low SOC state like 10% (2.950 per cell +/-) to 80/85% (3.350 +/- per cell) is where the power lives. When all packs are within their functional range they stay very level and will distribute power as shown above by Bud Martin's images.

The tricky part where the confusion starts and I likely will not help .... LOL (okay, it's 04:13 atm and I'm edging really tired)
The BMS part...
All BMS' will cutoff for Lo or Hi Volt cell states. Default is 2.50V & 3.65V respectively at cell level.
Hi/Lo volt cutoff is NOT dictated by the pack assembly voltage.

No BMS "distributes power". Meaning that if you are pushing 28V/30A (to a 24V pack) the cells will independently absorb what they can, based on their internal chemistry, resistance & impedance. Generally, within the main power curve, the cells will absorb pretty evenly within the pack itself and not usually deviate too much (< 30mv) but as they start to reach 80/85% and up above 3.400 cells will start to deviate more and can be as much as 1mv per AH capacity of the cells in question. So 100mv for 100AH cells. At this point, all a BMS can do is to run its balancing process if so equipped and it is slow... This is happening within Every Pack inside a battery bank when charging. Once a pack reaches Full Cutoff the remaining charge potential goes to the battery packs which are still taking charge until they all reach full. As all Battery Packs are now "full" and you will be able to observe, that they will "pack balance" as they settle post charging for roughly an hour (if not loaded). The various packs may fluctuate in voltage a bit as the cells settle and these packs will share their charge state across the "bank bus" and level up, usually within 0-20mv which will float a small amount up & down (more notable if there is BMS Balancing going on).

Discharging is again an odd situation... NB there is BMS differences which affect how this works and I am not getting into a long discussion on BMS tech. 04:30 now... Simply put, a BMS will cutoff for Low Volt Disconnect or LVD setting as programmed by user. With independent packs, each with a BMS they are "free agents". Now take the 100AH pack sitting next to a 200AH pack in a bank. Of course the 100AH will not have the same capacity / watt hours as the 200. It will reach FULL before the 200 when charging as will hit Empty before the 200 when discharging.

The confusion and some of this is BMS related....
Lead, AGM have none, they go until they die. Take a 12V Lead battery and you can discharge it to ZERO VOLTS. You can overcharge it to 16.0V and it will have no idea that it is in danger but we know that's not good. With Lithium, the BMS stops "stupid" from wrecking the battery pack and cells within. The BMS will cutoff at damage level points, when it cuts off, it is cutoff, nothing in or out. Some BMS' require a manual reset, others use "test pulses" and open momentarily to see if charge / discharge state exists and act accordingly.

LFP while discharging has no clue about where the juice is going, it does not care. When charging, it can take a low amp charge to full 1C or better (depending on your cells type). When in an LVD state and pulse testing, the BMS won't reopen to charge unless it's significant enough, so it should NOT parasite the other full packs. *** BMS's are different and work differently, I am describing a typical good design. *** The 200 will continue to discharge and service the load until it reaches LVD. Much of the confusion results from the BMS' interactions and the control they have because Lead & AGM do not have this as they are "stupid".

WHAT MAKES IT WORSE ~ Confusion wise ~
We talk about Parallel Battery Packs within a Bank.
Pack (aka battery) = Complete battery assembly with a BMS which can act as an independent entity.
Bank = An assemblage of Battery Packs into one common DC Bank.
~~~ but then we have paralleled cells which does not exist in Lead Land the same way. Take a typical 6V Lead battery, it has 3x 2V cells internally linked and no smarts between the cells. The lead cells are in series to make the 6V and that's it.
With Lithium Based batteries, we can Parallel Cells within the battery pack which increases the packs overall Amp Hour capacity, which is a very different process. This is often a "Mental Trip" especially for people transitioning off Lead Variants.

Something old time Lead Heads will recognize...
When you've tested the electrolyte levels of the cells within your lead battery, ever notice how some cells may present a different gravity compared to their sibling ? When we see that, it's Equalize Time after a watering (if required) to bring the cells within the batteries up to snuff again. With LFP, the BMS (if it has balancing) will level the cells out). With lead, all the batteries in series will collectively go up or down and the parallel set will mimic it (if properly wired) and stay fairly level. They'll also go down to 100% DOD if allowed to should the inverter fail to cutoff for low voltage. They will also go into Overcharge if the chargers fails to cutoff at a specific voltage or End Amps. Lithium BMS prevents that.

OKAY... I gotta call it, 05:00 nap time....
I hope this helps rather than confuses... It is not just a simple sentence to explain and there is a heap more to it... just tried to keep it simple but somehow I figure I may have missed the mark. Ohhh well I tried.
 

Horsefly

New Member
Wow @Steve_S - sorry for having ruined your sleep.

I've read it twice, but I think I need to read it again. I'm guessing I will have more questions based on what you have written. Thanks for spending the time. I really appreciate it!

Steve
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Wow @Steve_S - sorry for having ruined your sleep.

I've read it twice, but I think I need to read it again. I'm guessing I will have more questions based on what you have written. Thanks for spending the time. I really appreciate it!

Steve
I sleep very strange and weird hours and often have to take at least two naps during a day. It's a direct side effect of the cancer and my continued deterioration. Some days it's worse than others and the worse days are becoming ever more frequent. Plugging forward anyhow and doing the good things to do.. like helping folks improve our world by using Solar even a little bit gives my 3 surviving daughters a chance at some sort of future. Sometimes I get a tad OFF when I am really tired or in a lot of pain and what I wrote above actually is not bad considering how I was feeling at the time.
 

Horsefly

New Member
Here is a discussion of some of the issues that are encountered when paralleling strings or packs.
It does not answer all the questions that have been raised but some may find it useful.
That actually is very helpful, and makes me feel better. The paper actually confirms my suspicion that there is current flowing between packs (the paper calls them "Eddy currents") and those currents reduce the total Ah. The example he gives is two 10Ah strings in parallel, will always have less than 20Ah of total capacity.

I still have more to read in the document, but skimming through the rest he never addresses the concept of paralleling disparate capacity packs. Judging from what I've read so far, I think the author would not be recommend it.

Thanks @Ampster !
 

Horsefly

New Member
I sleep very strange and weird hours and often have to take at least two naps during a day. It's a direct side effect of the cancer and my continued deterioration. Some days it's worse than others and the worse days are becoming ever more frequent. Plugging forward anyhow and doing the good things to do.. like helping folks improve our world by using Solar even a little bit gives my 3 surviving daughters a chance at some sort of future. Sometimes I get a tad OFF when I am really tired or in a lot of pain and what I wrote above actually is not bad considering how I was feeling at the time.
I'm very sorry Steve. I wasn't aware.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
I'm very sorry Steve. I wasn't aware.
No worries No problems, it is what it is. Stupid thing is, had I been able to get a Prostate PSA test when I was 40 they may have caught it... But Ohh No, not till after 50 they say... The PSA test was the Big Flag at 2.19 which resulted in more test and well, so much for that, way too late and it had spread and others flourished in the meantime which is why the PSA numbers were so high.

MEN, don't WAIT till your 50 to get a PSA Blood Test ! It is quick, painless and easy, if it comes back with a high number follow up quickly. If your doctor says you do not need it till your 50 and over, Cold Cock the bugger on the spot !
 

Phil Orton

New Member
I sleep very strange and weird hours and often have to take at least two naps during a day. It's a direct side effect of the cancer and my continued deterioration. Some days it's worse than others and the worse days are becoming ever more frequent. Plugging forward anyhow and doing the good things to do.. like helping folks improve our world by using Solar even a little bit gives my 3 surviving daughters a chance at some sort of future. Sometimes I get a tad OFF when I am really tired or in a lot of pain and what I wrote above actually is not bad considering how I was feeling at the time.
Thank you Steve
I've watched other post and always appreciated the comments you've offered up. You've helped us all grow in this forum.
Will put you in my prayers.
I sleep very strange and weird hours and often have to take at least two naps during a day. It's a direct side effect of the cancer and my continued deterioration. Some days it's worse than others and the worse days are becoming ever more frequent. Plugging forward anyhow and doing the good things to do.. like helping folks improve our world by using Solar even a little bit gives my 3 surviving daughters a chance at some sort of future. Sometimes I get a tad OFF when I am really tired or in a lot of pain and what I wrote above actually is not bad considering how I was feeling at the time.
 
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