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can i upgrade a 280A battery bank with a 200A battery

Dijo

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Jul 6, 2021
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Hey guys !

My current battery bank is a 4s 280A lifepo4 single battery with a daly bms 250A .. could i add in parallel an 4s 200A lifepo4 with a 120BMS ? or even just a 100A without any problem or battery damage ?

Arelifepo4 bateries behaving differently than lead acid or agm, where you can't add different intensity in a bank..
 
Mixing any capacity of batteries is generally a no-no regardless of chemistry; however, it is commonly done. Some even parallel LFP with lead-acid.

Connect in accordance with best practices ("Wiring" in link #2 in my signature), and check current flowing through each battery with clamp ammeter to confirm they are operating within spec.
 
There's a pretty thorough discussion here about putting two packs of different capacity in parallel. I think the bottom line is that there are several people who are making such a parallel system work. My gut EE sense would be that it would be best to have the banks somewhat close to each other in capacity, but @Steve_S had what I think were four packs, with two being 280Ah and two being 170Ah, and it was fine. Your 280Ah and 200Ah should be OK.

As for the comparison with AGMs: LiFePO4 cells just seem to be easier to understand and predict. I've had AGMs for years, and it always seems like there are mysterious things going on that make them less consistent and less predictable.

Edit to add: One advantage of having a second parallel pack is that if the BMS on one pack shuts down, your other pack may be able to keep you going.
 
There's a pretty thorough discussion here about putting two packs of different capacity in parallel. I think the bottom line is that there are several people who are making such a parallel system work. My gut EE sense would be that it would be best to have the banks somewhat close to each other in capacity, but @Steve_S had what I think were four packs, with two being 280Ah and two being 170Ah, and it was fine. Your 280Ah and 200Ah should be OK.

As for the comparison with AGMs: LiFePO4 cells just seem to be easier to understand and predict. I've had AGMs for years, and it always seems like there are mysterious things going on that make them less consistent and less predictable.

Edit to add: One advantage of having a second parallel pack is that if the BMS on one pack shuts down, your other pack may be able to keep you going.
Thanks ! couldn't find a good post on it.. appear that i missed this one ..
 
200 & 280 should be fine in parallel, you will lose a little after the cells hit 3.000 because that's then the 200AH will start dropping faster. Less than 100AH difference in this size, not a "big deal" does have quirks to be aware of. More than 100AH and things get "interesting". DO match BMS settings so both packs will have the same thresholds & triggers.

My system finalization is slowly happing (I have building projects underway) 3x 280AH 2x175AH will be the final revision. The 105AH difference was an education in certain quirks which had to be battled. Especially because those 175's came from the Shunbin Disaster. A nasty scam prebuilt with used EV grade LFP cells.
 
I like 2 or more banks for small application. Flick a switch and your on the other bank.
 
There's a pretty thorough discussion here about putting two packs of different capacity in parallel. I think the bottom line is that there are several people who are making such a parallel system work. My gut EE sense would be that it would be best to have the banks somewhat close to each other in capacity, but @Steve_S had what I think were four packs, with two being 280Ah and two being 170Ah, and it was fine. Your 280Ah and 200Ah should be OK.

As for the comparison with AGMs: LiFePO4 cells just seem to be easier to understand and predict. I've had AGMs for years, and it always seems like there are mysterious things going on that make them less consistent and less predictable.

Edit to add: One advantage of having a second parallel pack is that if the BMS on one pack shuts down, your other pack may be able to keep you going.

but what is wrong if we parallel each cell and not the entire pack ?

see my picture below , how I added an infinite number of cells with parallel links on each cell, the endings should be better equalized on each series, I will do that

But I noticed that if I have randomly variations of IR in the same parallel row of cells, overall all cells are extremely well equalized in IR and this is foot too …

8F516D58-8DE1-4FD3-8B57-73415EB35AAD.jpeg
 
but what is wrong if we parallel each cell and not the entire pack ?

see my picture below , how I added an infinite number of cells with parallel links on each cell, the endings should be better equalized on each series, I will do that

But I noticed that if I have randomly variations of IR in the same parallel row of cells, overall all cells are extremely well equalized in IR and this is foot too …
First off: : I may be misinterpreting your photo, but it looks like you have cells of different capacity in each row. That seems to be inherently a bad idea. You intentionally have a mismatch in cells. Things are going to charge very inconsistently. In your case I would definitely break that up into at least two separate packs, with the smaller cells in one pack and the larger ones in the other pack.

Back to your more general point of putting lots of cells in parallel in a single pack: I think the main thing is that if you have one cell that is a problem, that problem will probably be hidden until all the other cells that is in parallel with it are pulled into failure with it. I think that theory is pretty logical.

On the other hand, a BMS managing a pack of individual cells will see a cell going bad pretty quickly, and it will be apparent with an HVD or LVD happening frequently.

I would never be the one to say you shouldn't ever try and run multiple cells in parallel. But the way you have yours is NOT a good idea.
 
Mixed cells with a crazy config oi, facepalm.
This is not lead acid batteries, they will not tolerate this stuff. You be a Brute to Lead but never to ANY Lithium based cells. These are MILLIVOLT Sensitive !
 
Mixed cells with a crazy config oi, facepalm.
This is not lead acid batteries, they will not tolerate this stuff. You be a Brute to Lead but never to ANY Lithium based cells. These are MILLIVOLT Sensitive !

why do you say this ?

Discharge rate is given by the voltage on the entire row of cells, the cells with higher capacity will provide and take more current but voltage stays the same in the row of parallel cells (paralleling cells and not paralleling the entire 16s ).

Let’s think a little, voltage is identical in each row , cells are a lot equal and now every row has almost identical IR, I don’t understand exactly the issue , can you elaborate please ?
 
First off: : I may be misinterpreting your photo, but it looks like you have cells of different capacity in each row. That seems to be inherently a bad idea. You intentionally have a mismatch in cells. Things are going to charge very inconsistently. In your case I would definitely break that up into at least two separate packs, with the smaller cells in one pack and the larger ones in the other pack.

Back to your more general point of putting lots of cells in parallel in a single pack: I think the main thing is that if you have one cell that is a problem, that problem will probably be hidden until all the other cells that is in parallel with it are pulled into failure with it. I think that theory is pretty logical.

On the other hand, a BMS managing a pack of individual cells will see a cell going bad pretty quickly, and it will be apparent with an HVD or LVD happening frequently.

I would never be the one to say you shouldn't ever try and run multiple cells in parallel. But the way you have yours is NOT a good idea.

i did check and voltage is identical with active BMS, and also individual cell checks, think a little , when charging and discharging voltage is identical to the entire row, so larger cells will provide and eat more current because it is dictated by voltage and IR that is much better matched in the row.

please correct me with technical and practical data if I am wrong .

I do agree that if one cell is bad it is harder to find it.

Also can a cell go in short ? What is the usual defect that happens to LiFePo4 cells and can happen here too ?
 
You simply cannot be cavalier with Lithium Based Cells, they are not forgiving like Lead Acid and never will be.
Appreciate that LFP works with Millivolt Sensitivity because the working voltage (the curve) is from 3.000-3.400 . That is 200AH or whatever you have all within 400 Millivolts!
 
You simply cannot be cavalier with Lithium Based Cells, they are not forgiving like Lead Acid and never will be.
Appreciate that LFP works with Millivolt Sensitivity because the working voltage (the curve) is from 3.000-3.400 . That is 200AH or whatever you have all within 400 Millivolts!

Please explain with technical details. Charge current is 0.1C and current flow will be more to larger capacity cells !
What prevents the current flow from your experience, to have the voltage difference ?!
 
Hey guys !

My current battery bank is a 4s 280A lifepo4 single battery with a daly bms 250A .. could i add in parallel an 4s 200A lifepo4 with a 120BMS ? or even just a 100A without any problem or battery damage ?

Arelifepo4 bateries behaving differently than lead acid or agm, where you can't add different intensity in a bank..
You can parallel two batteries together they have to be the same voltage, amp hours does matter as long as the internal resistance of the larger battery is not more than the smaller battery. 1000 amp-hour 24-volt paralleled with a 1 amp-hour 24-volt battery will be no problem you can never discharge the smaller battery faster than the larger battery.
 
You can parallel two batteries together they have to be the same voltage, amp hours does matter as long as the internal resistance of the larger battery is not more than the smaller battery. 1000 amp-hour 24-volt paralleled with a 1 amp-hour 24-volt battery will be no problem you can never discharge the smaller battery faster than the larger battery.
You are totally right !
I want to have in parallel cells not packs of cells.
Only problem I can agree with is that you can’t identify a defective cell from the parallel group.

How possible is for a cell to suddenly go in short and enter in short with the others in the same group and explode ?

Is there any other ethnically and practically proved downside of putting each cell in parallel ?
 
You are totally right !
I want to have in parallel cells not packs of cells.
Only problem I can agree with is that you can’t identify a defective cell from the parallel group.

How possible is for a cell to suddenly go in short and enter in short with the others in the same group and explode ?

Is there any other ethnically and practically proved downside of putting each cell in parallel ?
Two packs in parallel with separate BMS if one pack cell shorts the BMS will over current and disconnect the packs however if the cell is in parallel without a BMS the BMs will not see the overcurrent. The way you can detect if you have a short in a parallel-group is to monitor temperature difference and alarm you of a problem of course the BMS will see a low voltage for that cell group.
 
Two packs in parallel with separate BMS if one pack cell shorts the BMS will over current and disconnect the packs however if the cell is in parallel without a BMS the BMs will not see the overcurrent. The way you can detect if you have a short in a parallel-group is to monitor temperature difference and alarm you of a problem of course the BMS will see a low voltage for that cell group.

so you do not recommend to parallel cells , only parallel groups packs of 16s cells, right ? It is too dangerous to parallel cells , right ?

even if it is equalizing very well the IR of a row, this is not recommended on long therm, right ?
 
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