Non-identical batteries in series?

[Treva31]

I know they say if you want two batteries in parallel they should be identical brands and specs.
But what about if they are in series?

I currently have a 100ah LiFePO and am considering adding another 100ah one but it will be a different brand and will be newer.
So I am thinking why not just switch to a 24v system and put them in series.

 

[rmaddy]

It's far more important that batteries in series are identical, even down to the exact same SOC before you connect them in series.

There are plenty of threads here about different batteries in parallel being doable but I don't think anyone has suggested you can do it with batteries in series.

It's similar to all of the threads about building a battery from cells and how the cells in series should be matched. No one is building a battery by putting different brand cells in series.

 

[Bud Martin]

Dose the battery spec show that the battery can be connected in series even when using the same model?

 

[Treva31]

It's far more important that batteries in series are identical, even down to the exact same SOC before you connect them in series.

There are plenty of threads here about different batteries in parallel being doable but I don't think anyone has suggested you can do it with batteries in series.

It's similar to all of the threads about building a battery from cells and how the cells in series should be matched. No one is building a battery by putting different brand cells in series.

Oh ok then. Thanks.

 

[Blackhat]

It's far more important that batteries in series are identical, even down to the exact same SOC before you connect them in series.

There are plenty of threads here about different batteries in parallel being doable but I don't think anyone has suggested you can do it with batteries in series.

It's similar to all of the threads about building a battery from cells and how the cells in series should be matched. No one is building a battery by putting different brand cells in series.

That's a bit absolutist. There are quite a few threads about building banks from different brand cells in series:

Search results for query: mixing eve lishen

diysolarforum.com

And all of those that I've opened have people saying it's OK to do so, even when discussing different rated capacities. So it's certainly not "no one".

To OP: while it's true that you should try to match cells in series it's just for performance maximization. The bank is limited by the worst cell, so it's best to minimize variation to take full advantage of the cells capabilities. So before connecting batteries into a series config definitely balance them and remember that the worst one will be your throttle, but it's extremely unlikely to cause any damage.

Paralleling majorly different batteries on the other hand could definitely cause permeant damage. If they both settle at 13.8 it's fine regardless of the brand. But if one settles at 13.8 while the other settles at 13.5, the 13.5 will bleed the other down while constantly experiencing mild over charging. This can be especially bad when paralleling two different chemistries.

So one hurts performance, the other longevity. Ideally we all have identical cells throughout the system, but very few people get all the capacity they ever need in one go. For those of us learning and growing along the way some mix and match is inevitable. It's a shame that sub-optimal practices are dismissed as "bad idea, don't do it" instead of explaining the tradeoffs. In this particular case it scares people away from experimenting and growing their system and there really isn't a good reason for it.

 

[HRTKD]

I'm with @rmaddy on this. Cells and batteries put in series should be identical.

Experimenting is fine, but for a production system, we're going to recommend best practices.

 

[Blackhat]

I'm with @rmaddy on this. Cells and batteries put in series should be identical.

Experimenting is fine, but for a production system, we're going to recommend best practices.

It seems like this guy is just getting started and still feeling out what he needs. Nothing approaching a production system @ 2.4kWh. He wants to know if his experiment is dangerous, which it is not.

rmaddy went a bit further than "recommending" best practice. He was quite unequivocal.

I also recommend matching cells as best practice. It's just that instead of treating best practice like an ironclad rule that must never be broken I discussed the tradeoff that it actually is. This hobby has enough hard and fast rules to follow lest someone get hurt or property destroyed. If we don't make it clear what's a must and what's a recommendation we lose sensitivity to the really important bits.

 

[HRTKD]

I would rather err on the conservative side than have someone come back and ask why we didn't warn them or ask why we suggest the best practice strongly enough.

It's like putting together a PV array out of mismatched panels. I'm not going to explain the pro's and con's. I'll recommend against it, but then I'll also provide a link to a series of videos explaining what's going to happen in that situation.

 

[A.Justice]

I vote no. Don't mix them. Mixing parallel banks at similar state of charge is fine, but serializing different brands could definitely cause problems.

Cells in series should be pretty much exactly the same specs, a difference in internal resistance or resting voltage could end up over or under charging your cells. Even a few millimeters in cell height difference could cause a massive amount of strain on the terminals as well.

If you are going to do this you MUST MUST MUST MUST use a BMS. Technically speaking, a BMS should prevent any kind of real problems on a mix and match pack, but I definitely wouldn't want that pack. It CAN be done, but I would not recommend it.

 

[Blackhat]

Can somebody verbalize these problems we’re being warned about? I keep hearing about them, but it’s like they’re a mythical creature that will haunt you forever if you speak their name and we have to guess what they are from context.

One such guess based on the BMS comment is the risk of cell under or over voltage. But it seems clear from the OP that he is talking about off the shelf 12V battery’s not raw cells.

This is a DIY forum, so hopefully we can all handle wading through a little nuance.

 

[A.Justice]

Can somebody verbalize these problems we’re being warned about? I keep hearing about them, but it’s like they’re a mythical creature that will haunt you forever if you speak their name and we have to guess what they are from context.

Pack one and pack two to start to stray apart due to the differing IR, eventually one pack will have a different state of charge than the other, then the BMS then cuts the other pack due to HVD or LVD, and you loose power, even though one pack still has a charge. Not really dangerous (because of two BMS's) but definitely not the best way.

One such guess based on the BMS comment is the risk of cell under or over voltage. But it seems clear from the OP that he is talking about off the shelf 12V battery’s not raw cells.

I missed that he was talking about prefabricated batteries, and my comment wasn't a guess. Other than me last paragraph about the BMS (which is irrelevant because the packs have a BMS anyway), the prefab packs could still have the same problems. Whether it's individual cells or packs, the internal resistance will eventually cause a voltage deviation, and in the case of two pre-built packs I think it would be worse because the BMS wouldn't be able to balance current across all all 8 cells, only blocks of 4. In some ways I would feel better using an 8s BMS and 8 different cells, than 2 separate, serialized, uncommunicative units.

Edit: Just to be clear, I'm not saying that it can't be done, and I'm not saying that it's particularly dangerous, I'm just saying that it's not "best practice" or even good practice.

 

[Blackhat]

Those are good points. I didn't mean to suggest that your comment was a guess - I was referring to the fact that I had to guess what the concern was from the mention of a BMS.

He could mitigate the imbalance issue by occasionally recharging the batteries separately with a 12V charger. Kind of a pain, but if it saves hundreds of dollars while he experiments for a bit it would probably be a good use of time. Obviously if 100 Ah 24V is the final evolution of the system a proper setup would be a lot less trouble.

I certainly agree that 8S with unmatched cells and a single BMS would be better than two 12V systems in series. But OP has two 12V batteries right now and I'm guessing he does not have a bunch of cells and an 8s BMS lying around.

Do you have any suggestions for how he should use the equipment he has to further develop the system without having to retire part of his investment?

 

[HRTKD]

Do you have any suggestions for how he should use the equipment he has to further develop the system without having to retire part of his investment?

Rereading the post that started the thread, I don't think he would be out any money. He hasn't bought any 24 volt equipment yet.

 

[Blackhat]

Rereading the post that started the thread, I don't think he would be out any money. He hasn't bought any 24 volt equipment yet.

I don’t think he has either, but he does have two batteries and a desire for more capacity. How should he use them? Parallel? He was hesitant to do that, are we advising that parallel is fine?

 

[A.Justice]

I don’t think he has either, but he does have two batteries and a desire for more capacity. How should he use them? Parallel? He was hesitant to do that, are we advising that parallel is fine?

I would parallel them, yes. I also run a 12v system.

 

[GLC]

Not only is "branding" important but age of battery is also important. Just as one off brand cell will be your throttle in your battery bank, an aged cell will cause the same issue, even with the same brand. Most battery makers have an age limit as to adding new batteries with older batteries due to balancing the old with the new. I know Battleborn is pretty strict as to increasing size of a battery bank even if you are using just their brand of Batteries.

 

[RCinFLA]

Most likely outcome is one of the two different series connected batteries will trip for overvoltage during charging. It may be time consuming to top balance them so they can be fully charged, requiring very low charge rate. It will be more difficult to keep them in top end balance so you don't continuously have problems with tripping BMS overvoltage during recharging.

You will get flustrated trying to recharge battery array, resulting in lack of proper charging so batteries can't balance fully. Results will be continued degradation in internal cells' state of charge matching, with less and less extractable capacity before one of the battery's BMS has a low cell trip.

 

[HRTKD]

I don’t think he has either, but he does have two batteries and a desire for more capacity. How should he use them? Parallel? He was hesitant to do that, are we advising that parallel is fine?

Yes, put the two batteries in parallel, with equal access to the common bus bar.