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Do LiFePo4 batteries need to be matched? (1 Viewer)

Jun 10, 2010
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With Sla, AGM, etc we've always been told that 2 batteries in parallel have to be same age and capacity or the weaker/smaller one will pull the stronger/larger one down to its level. Just wondering it that's still true with lithium? Especially those with their own BMS not being an old dumb thing?

Any thoughts?
 
Apr 14, 2022
292
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Providing each bank has its own BMS then there is no need to match them. There is you tube video I watched where they had connected a 280ah Lifepo4 12v bank with a 5ah 12v bank and it worked just fine (bit extreme though)

 

AshVanBitz

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Aug 7, 2018
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Lithiums work differently to your standard Agm’s, lead acid etc so they do not need to be the same age/size. As long they have their own BMS as DavidCH has said you will be fine. Please give me a call or send me a PM if you want to discuss this more 😁👍🏻

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bigtwin

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Oct 29, 2009
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It’s my understanding that the biggest issue with paralleling LiFePo4 batteries is ensuring that all of the interconnections are well made. Any variations in the connection resistance has a bigger impact as a result of the low internal resistance of LiFePo4s and can result in the batteries not all making an equal contribution to the demanded load.

Ian
 

bigtwin

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Providing each bank has its own BMS then there is no need to match them. There is you tube video I watched where they had connected a 280ah Lifepo4 12v bank with a 5ah 12v bank and it worked just fine (bit extreme though)



Very interesting video but no great surprises there, particularly when he confirmed the measured resistance of each battery bank. The charge/discharge currents being driven by the ratio of the internal resistances.

It’d be interesting to see the same test undertaken for lead acid batteries of different capacities.

Ian
 
Sep 29, 2007
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Motorhoming since 2006, 30 years tent camping in Africa
They can be reasonably mismatched but the most important thing is that the BMSs use MOSFETs instead of conventional relays. MOSFETs allow current flow only one direction. Relays allow flow in any direction.
The system can be a little forgiving if the smaller battery has MOSFETs and the larger has relays, but better if both use MOSFETs.
 

AshVanBitz

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Aug 7, 2018
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Or, alternatively, post the discussion here so that we can all learn from it. 😎

Ian
As long as the BMS’s are configured to match you can mix the size of the batteries. I wouldn’t recommend having two different makes of battery but mixing between the same manufacture is fine. A lot of motorhomes as we know have very little real estate to work with so having two batteries the same size can be difficult. This is why it is good to have lithiums as you have more flexibility.

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Apr 14, 2022
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As long as the BMS’s are configured to match you can mix the size of the batteries. I wouldn’t recommend having two different makes of battery but mixing between the same manufacture is fine. A lot of motorhomes as we know have very little real estate to work with so having two batteries the same size can be difficult. This is why it is good to have lithiums as you have more flexibility.
I don’t think it makes any difference, the individual bms units will look after their own battery and the internal resistance will handle the respective charge/discharge rates. Worst that’s going to happen is minor differences in the settings may mean a difference in the detection of fully charged or discharged
 

AshVanBitz

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Aug 7, 2018
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I don’t think it makes any difference, the individual bms units will look after their own battery and the internal resistance will handle the respective charge/discharge rates. Worst that’s going to happen is minor differences in the settings may mean a difference in the detection of fully charged or discharged
Agreed but personally I wouldn’t want any differences regardless of how minor they are.
 

bigtwin

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I don’t think it makes any difference, the individual bms units will look after their own battery and the internal resistance will handle the respective charge/discharge rates. Worst that’s going to happen is minor differences in the settings may mean a difference in the detection of fully charged or discharged

The video you posted used not only different capacities but also different types of cells and all was fine. He made a point though of ensuring that each BMS was set up according to the cells attached. They behaved as I expected.

I’m still sceptical about the traditional advice relating to mixing lead acid batteries (of differing capacities) so it’d be interesting to see a similar test for those. Fundamentally, it’s about internal resistances and dc theory should take care of it all. However whenever it’s raised as a topic you get the standard reply that one will drag the other down. 🤷‍♂️

Ian
 
Jan 27, 2018
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The video you posted used not only different capacities but also different types of cells and all was fine. He made a point though of ensuring that each BMS was set up according to the cells attached. They behaved as I expected.
Now I'll admit to not viewing all the video i could not cope..
My position is i have a 3 year old ultramax 100Ah with no axcess to the BMS set up (so must assume its acceptable for the cells) 2 questions
1 can i add another 100Ah Ultramax no problems
2 can i add another 100-200Ah of another manufacturer.
3 sell off existing battery and buy somat else.
I am sufficiently solared at 550w

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bigtwin

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My position is i have a 3 year old ultramax 100Ah with no axcess to the BMS set up (so must assume its acceptable for the cells)

It’d be fair to assume that the integral BMS was suitable for the battery which is is monitoring/managing.👍

It’s a shame that you don't have access to the BMS settings but it is what it is.

2 questions

Ahem, there were three questions! 😎

1 can i add another 100Ah Ultramax no problems

Undoubtedly. There’s no reason why you shouldn’t be able to. 👍

2 can i add another 100-200Ah of another manufacturer.

Based on what we saw in the video, I should think so but your option 1 would be preferable.

3 sell off existing battery and buy somat else.

Always an option but probably a bit extreme if you can implement option 1.

Ian
 

AshVanBitz

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Aug 7, 2018
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Now I'll admit to not viewing all the video i could not cope..
My position is i have a 3 year old ultramax 100Ah with no axcess to the BMS set up (so must assume its acceptable for the cells) 2 questions
1 can i add another 100Ah Ultramax no problems
2 can i add another 100-200Ah of another manufacturer.
3 sell off existing battery and buy somat else.
I am sufficiently solared at 550w
Speak with the manufacturer to make sure that the profile hasn’t changed and if all ok buy another one to suit and install that. The is no point replacing a perfectly good battery if you do not need to
 
Dec 2, 2019
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What Andy tested in the video above posted, it’s pure for learning how it behaves. For long term cycling it will drift. One think to bear in mind: by coupling two different packs with different settings in the bms, you remove the first line of defence and rely solely on bms to protect from charge-discharge. Parameters in bms are always higher than your charger or load limits. So you don’t stress the bms in normal cycling, charger cuts out before the bms intervenes, and so is the load if designed and set properly. Having two bms’es with different parameters, in the long run, one will always run the risk of overlapping the first line of defence settings.

If you look at Andy’s battery shelf, all tree bms’es are dialled in exactly the same for a reason. Same as his two paralleled seplos, both are set identical to each other, and on those ones, it does have some imbalance, due to different sizes. But acceptable.

I will have no problem in paralleling packs with the same settings, even if I don’t know the values set.
 

Hoovie

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May 16, 2021
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Whilst it is not essential, they DO need to be matched for the optimum results, similar to the way that individual cells in a lithium battery are matched and grouped to go into a battery pack.
If you speak to an actual battery manufacturer and supplier, you would find that - again like they match cells to go into a battery pack - when the batteries go into QA and post-production testing, they are batched again and batteries from the same batch should be selected when a buyer wants a multi-battery bank.

Now if that happens does depend on both the quality of the supplier and the quality of the distributor, but if the batteries are not matched and pretty equivalent, you will end up with different discharge/charge levels and not an optimum setup. I doubt that most battery sellers in the UK either know or care about that.

Now just like many things discussed on this forum, doing things in a sub-optimal way will still work (like the discussion on why bother to cable up pairs of batteries right if it is too much like hard work - yes, it will work but no it is still not the right thing to do).

The better Lithium batteries also have BMSes that operate interactively with each other when they are combined and not just standalone to try and make sure they are running equally. For example the pair of Lithium Batteries I run do, of course, have their own BMSes, but they are also fitted with a data interface and I have an RJ45 cable between them so they communicate with each other and the result is the SOC of each is almost always within 1% of each other.

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AshVanBitz

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Aug 7, 2018
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BMS - Battery management system.
Looks after the battery as a whole.
MOSFET - Metal oxide semiconductor field effect transistor.
Controls the electricity flow from one source to another.
 
Aug 25, 2020
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THANK you very much Ash. I would never have worked that out! Also one question. Does the on board mains charger fully chance have battery as after mine has been on for a few days the readout still does not show full?
Sorry for bothering you.

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AshVanBitz

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Aug 7, 2018
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THANK you very much Ash. I would never have worked that out! Also one question. Does the on board mains charger fully chance have battery as after mine has been on for a few days the readout still does not show full?
Sorry for bothering you.
it depends entirely on the charger itself and it’s charging profile. Also it can sometimes be down to something a simple the battery monitor as these need to be set to suit each individual setup. Changing a setting can show you the correct readings.
Kindest regards
Ash
 

merl

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Feb 10, 2023
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It won't be perfect as far as extracting the absolute maximum Kilojoule capacity from the batts but remember that for maximum life span you shouldn't be pushing lifepo4 to it's absolute limit (either charge or discharge) anyway.
Ultimately current is driven by potential difference so sooner or later you'll extract the available power from both batts. The BMSs will be monitoring both battery status and individual cell status so they'll prevent a destructive cell state so I really can't see an issue for the average Joe camper.
 
Apr 14, 2022
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On the subject of longevity of LifePO4 I think mine are rated for 3000 cycles, on that basis they are going to last an awful lot longer then I am!
 

merl

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Obviously manufacturers will quote their life expectancies at usage that suits their figures, this probably won't be typical in the field usage.
I wouldn't mind betting that the 3000 cycles is something like discharged to 20% and charged to only 85% and at discharge currents of say 1/5 C. Cycle them hard 100% - 5% and drive heavy inverter loads and that 3000 cycles figure will be a pipedream!

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