Parallel and series connection of dissimilar amp hour LiFePO4 - best practices

derekisastro

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As the title says ...

I have 8 LiFePO4 batteries of 271Ah and I have 8 LiFePO4 batteries of 150Ah. I wish to connect them into a single 8S 24V battery bank.

Should I make 2 separate banks of 8S from each group and then parallel the 2 banks together or should I combine 1 cell from each group into a parallel cell of 1 x 271Ah and 1 x 150Ah cells, then series them into a single battery bank?

I guess, it partially depends upon how many BMS's I have? Ignoring that, is there any reason why 1 configuration is better/significantly better than the other?

What are best practices in this case (ignoring the obvious, don't mix the 2 groups of cells)?

Cheers,

D.
 

Supervstech

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As long as you have excellent low loss bussbars connecting the choices, it won’t matter.

Not wires, bussbars…
 

HRTKD

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Two separate batteries, each with a distinct set of like cells. So one 150 Ah battery and one 271 Ah battery.

I would never mix such dissimilar cells together within the same battery.
 

racmaster

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however you will connect such a different batteries, it will result in much higher aging of the bigger cells, as they will be discharged much more under both - load and charge states. and you will lose control of balancing, as the bigger pack will charge the smaller and hold it on its actual voltage. but if your load is small enough and will not reach limits of the battery, it could be technically done bmo...

lifepo is safe technology, you can destroy them, but no fire or explosion should be possible. at worst, you will get an expensive experience :)

however, i would better split the load for both separate circuits - one with 150ah battery and one with 270ah battery. for example connect one of your appliances to the smaller batt circuit and 2 other appliances to the bigger one.. of course, adequate wattage...
 

derekisastro

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Two separate batteries, each with a distinct set of like cells. So one 150 Ah battery and one 271 Ah battery.

I would never mix such dissimilar cells together within the same battery.
Why not? In "theory", there should be no problem doing it right? Especially at rates of charge and discharge 0.2C?
 

derekisastro

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however you will connect such a different batteries, it will result in much higher aging of the bigger cells, as they will be discharged much more under both - load and charge states. and you will lose control of balancing, as the bigger pack will charge the smaller and hold it on its actual voltage. but if your load is small enough and will not reach limits of the battery, it could be technically done bmo...

lifepo is safe technology, you can destroy them, but no fire or explosion should be possible. at worst, you will get an expensive experience :)

however, i would better split the load for both separate circuits - one with 150ah battery and one with 270ah battery. for example connect one of your appliances to the smaller batt circuit and 2 other appliances to the bigger one.. of course, adequate wattage...
Do you have data showing this degradation that you mentioned? I'd be interested in understanding if it is happening and what charge and discharge parameters may help with any possible degradation.

My understanding has been that even if it's not the most ideal scenario, at charge and discharge rates of 0.2C and the like, it has essentially no negative effects on the battery.

I do understand the way batteries behave in series and in parallel and how when charging and discharging the 150Ah and 271Ah will do so at different rates, even though they are connected in parallel ... I just don't understand how this is will be bad for the batteries, assuming I'm not charging and discharging at 100's of amps? Can you help me understand what I should be wary of as I am pretty committed to utilizing the cells together in one battery bank?
 

racmaster

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Can you help me understand what I should be wary of as I am pretty committed to utilizing the cells together in one battery bank?
problem will be balancing, cell destroying and capacity loss.

1- you will start discharging fully charged battery
2- until about 120 Ah discharged will nothing wrong happen, just the 150ah cell will be discharged on lets say 20% depth of discharge, while the 271Ah cell only at 50% DOD
3 - if you will continue discharging, 150ah cell will go down and down with its voltage until it will reach the low voltage limit and it will be destroyed, while the 271ah cell will be fine, just discharged on 40% DOD

in real life, you will not be able to find when this limit will be reached, until you will stand near the battery and attend. or you can use BMS, to do it for you and it will probably disconnect battery before this happen. but as soon as you will be charging the battery, the same problem will be on higher end, max charging voltage will be reached. again cell failure.

with every charge/discharge cycle this difference will be higher and higher and the probability of failing will be higher too. later, you will only get about 20ah from this battery, because it will be so much unballanced. and you will not be able to ballance the battery, because different cells have different internal resistances and therefore it will be charging at different speeds...
 

derekisastro

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problem will be balancing, cell destroying and capacity loss.

1- you will start discharging fully charged battery
2- until about 120 Ah discharged will nothing wrong happen, just the 150ah cell will be discharged on lets say 20% depth of discharge, while the 271Ah cell only at 50% DOD
3 - if you will continue discharging, 150ah cell will go down and down with its voltage until it will reach the low voltage limit and it will be destroyed, while the 271ah cell will be fine, just discharged on 40% DOD

in real life, you will not be able to find when this limit will be reached, until you will stand near the battery and attend. or you can use BMS, to do it for you and it will probably disconnect battery before this happen. but as soon as you will be charging the battery, the same problem will be on higher end, max charging voltage will be reached. again cell failure.

with every charge/discharge cycle this difference will be higher and higher and the probability of failing will be higher too. later, you will only get about 20ah from this battery, because it will be so much unballanced. and you will not be able to ballance the battery, because different cells have different internal resistances and therefore it will be charging at different speeds...
Won't the 150ah and 271ah batteries in parallel self-balance? Such that at your example of 120ah used ... it'll be that the majority of the 120ah (approximately 150/271 as a fraction) comes from the 271ah and the remainder from the 150ah so that they are at a close state of charge? One won't be at 50% DOD and the other at 20% DOD but that both would be around 35% DOD? That's my understanding of how cells behave in parallel.

I appreciate that if that 120ah is discharged extremely quickly that maybe the parallel balancing may not happen as ... efficiently? But if it's 120ah over 3 or 4 hours ... what am I missing?
 

Supervstech

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however you will connect such a different batteries, it will result in much higher aging of the bigger cells, as they will be discharged much more under both - load and charge states. and you will lose control of balancing, as the bigger pack will charge the smaller and hold it on its actual voltage. but if your load is small enough and will not reach limits of the battery, it could be technically done bmo...

lifepo is safe technology, you can destroy them, but no fire or explosion should be possible. at worst, you will get an expensive experience :)

however, i would better split the load for both separate circuits - one with 150ah battery and one with 270ah battery. for example connect one of your appliances to the smaller batt circuit and 2 other appliances to the bigger one.. of course, adequate wattage...
I'm not sure I agree.

If you built a bank, and paired a 150 and 270 cell on the same bussbar, then built the pack from the pairs,the battery should act like a 420Ah pack...
 

racmaster

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sorry i described serial example. in paralel they will act as you wrote. if there will be small currents/loads of charge/discharge cycling, it may perform without problem.

before connecting them to paralel u should make sure they are charged to the very same cell voltage.
 

Ellcon123

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Apr 10, 2021
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As the title says ...

I have 8 LiFePO4 batteries of 271Ah and I have 8 LiFePO4 batteries of 150Ah. I wish to connect them into a single 8S 24V battery bank.

Should I make 2 separate banks of 8S from each group and then parallel the 2 banks together or should I combine 1 cell from each group into a parallel cell of 1 x 271Ah and 1 x 150Ah cells, then series them into a single battery bank?

I guess, it partially depends upon how many BMS's I have? Ignoring that, is there any reason why 1 configuration is better/significantly better than the other?

What are best practices in this case (ignoring the obvious, don't mix the 2 groups of cells)?

Cheers,

D.
Create 2 separate batteries and parallel them as long as the voltages are close. They will sort themselves out after that. I have a 120ah pack which I then added a 202ah pack to. They have been working fine for about 8 months.

I recently did a full discharge/charge test and basically pulled the full 322ah out and back in. In the flat part of the voltage curve not much happens. In the upper and lower knees the higher voltage pack will feed the lower so they will stay in sync.

There are many others in forum's reporting similar, ie not a problem.
 

rickst29

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(I complained about a withdrawn post, I have no argument with racemaster)
Let me describe how it will actually work with 2 different-sized parallel battery packs. This is unfortunately very long, because I'm going to be VERY detailed in this response.

The 2 battery packs are attached on a parallel bus. The current draw will generally favor the less resistive battery pack (that resistance includes both the cells, AND the BMS resistance during 'discharge' operation). This may tend to pull on both battery packs equally, as you describe - and it may even tend to pull somewhat more from the "smaller" battery pack. In that latter situation, it might be good to use longer lead wires from the Busses (grounding and positive) to the smaller battery terminals, making it somewhat less attractive. That could prevent over-current with the less capable battery pack.

But when the "150Ah" battery pack becomes significantly discharged, its internal voltage in discharge state will begin to fall off from a nominal 2.8V value. The less discharged bigger battery will not yet have that voltage roll-off, and it will become a more attractive source for providing the load current. It's changing "attractiveness" will vary, according to the different SOC and discharge Voltage of the two battery packs.

In real life, neither battery pack should be driven into that low SOC on a frequent basis. But when it occurs, the imbalance of SOC between the two packs will be largely self-correcting, with the bigger pack being PULLED DOWN towards the SOC of the smaller pack.
- - -
There can be a similar discrepancy in charging unbalanced packs, until one of the packs reaches a high SOC and Voltage begins to increase significantly. At first, the larger pack (now accepting a bit more charge current, with a lower internal Voltage) is only slightly more attractive. But, as you approach and then reach balancing voltage on the smaller pack, its BMS will switch to balancing mode, and only accept very small "balancing current". ALL of the remaining charger current will be available to the larger and less charged battery pack. This situation is also self-correcting, with just one limitation: The BMS within the "larger" of two packs, or the "largest" of 3 packs, must be able to accept the full current of the charger.
- - -
As Elicon123 describe within the preceding post, this kind of configuration is NOT otherwise a problem in real life. In discharge, some small "tuning" changes to the resistance of the terminal connection wires might be desirable, If I ever do that to my own batteries, I will simply lengthen the wires from the "grounding bus" to the individual terminals of the smaller battery packs.

Up to this date, in my own small RV configuration (with slightly dissimilar battery packs in parallel), no such "tuning" has been necessary. But, if I add a 3rd pack with MUCH larger capacity than the others, and then try to run the Air Conditioner from batteries exclusively, a significant reconfiguration may be helpful. (I would probably want to lengthen grounding wires on the two "small" battery packs, causing more current to be drawn from the "big" battery pack).
 
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