Can I parallel 14s Li-Ion and 16s LFP in the same powerwall/system?

[harrisonpatm]

I got a great deal on a bunch of nearly new prismatic LFP cells, too good to pass up, but I'm already in place with building my 14s Li-Ion powerwall. Apologies if this has been discussed earlier, but I think it could work, if....

1. Separate BMS for each bank
2. Monitor charge/discharge levels

If i keep the LFP bank between 3.0-3.5V per cell, which I believe is a conservative range, I get 48-56V at 16s. For 14s Li-Ion, that range gives me about 3.4-4V per cell, which I also think is a good conservative range. More conservative than I had originally planned for my 14s build, actually, but that hopefully just means better cycle life.

If my total system discharge drops below 48V, then the LFP BMS will/should cutoff, and the Li_ion will powerwall will bear the brunt. So, I should set the min-max voltage limits on both BMS's to parallel each other.

I think it will work, but I also haven't done it before. Any advice? Like I said, too good of a local deal to pass up, at about $55 USD per kwh, so if I can't parallel the two chemistries, I'll just stash the new purchases until I can develop a separate system.

 

[Korishan]

You could, but you probably shouldn't. They are still different chemistries with different charge/discharge curves. Ideally they should be connected to different chargers/inverters. Some Inverters can accept different chemistry battery hook ups, as long as they have at least 2 battery ports that are truly isolated.

 

[harrisonpatm]

You could, but you probably shouldn't. They are still different chemistries with different charge/discharge curves. Ideally they should be connected to different chargers/inverters. Some Inverters can accept different chemistry battery hook ups, as long as they have at least 2 battery ports that are truly isolated.

What if I'm not using an inverter? Everything staying in DC, in and out?

What if I set the voltage parameters in my charge controller to 48-56VDC?

 

[OffGridInTheCity]

@DG98 just posted about this - https://secondlifestorage.com/index.php?threads/paralleling-packs-size.11954/page-2#post-86400
I don't have any hands-on but I can see where there would be some (unnecessary) stress between Lithium-ion and LifePo4 since LifePo4 will basically be 1 voltage over the operating range whereas Lithium-ion won't - causing current flow.

And in my case it wouldn't work since I use voltage as my means of controlling DOD this may not work to have these paralleled - https://secondlifestorage.com/index.php?threads/paralleling-packs-size.11954/page-2#post-86401

I guess the question is - what is the current flow between the fixed voltage LifePo4 and the lower and higher Lithium-ion batteries.

 

[harrisonpatm]

LifePo4 will basically be 1 voltage over the operating range

Can you explain what you mean by that?

 

[Korishan]

Can you explain what you mean by that?

LiFePO4 has almost a flat discharge curve, but the other Lithium Ion chemistries have more of a slowly changing curve from about 90% to 40%.

So as the Li-Ion discharges, it's voltage will slowly go down. Whereas as the LiFePO4's discharge, they'll drop about 0.2V, then the voltage will basically stay the same until it reaches about 40% discharged, then the voltage will start to drop off

 

[OffGridInTheCity]

Can you explain what you mean by that?

Sure - LifePo4 is basically a single voltage over most of the discharge curve.

For example let's say you want to operate between 30% SoC and 80% SoC - the voltage difference is so small you can't (effectively) use voltage as the decision of when to turn an inverter on/off as an example. You also have voltage variances due to voltage sag because of load and higher due to charging, Bottom line is 'I've read consistently' that you can't effectively use voltage. You *must* use a shunt coulomb counting to effectively know the SoC except at the very top and very bottom.

BUT this is what I've read - would be interested if it's actually true

I'm lithium-ion and it's pretty easy to control the range of SoC to within 10%? just by voltage - close enough for voltage alone to be effective. The trouble with shunts/coulomb counting is how do you keep it 'correct' unless you to to full charge once in a while to do a reset. In my system I *rarely* get to full charge and as a consequence my Batrium SoC is way off most of the time.

Here's Batrium at 6:00am for the last 8 days showing voltage and SoC %. All over the place from 50.0v = 38.8% to 50.0v = 32.4% SoC.

 

[harrisonpatm]

LiFePO4 has almost a flat discharge curve, but the other Lithium Ion chemistries have more of a slowly changing curve from about 90% to 40%.

So as the Li-Ion discharges, it's voltage will slowly go down. Whereas as the LiFePO4's discharge, they'll drop about 0.2V, then the voltage will basically stay the same until it reaches about 40% discharged, then the voltage will start to drop off

Sure, I understand the different discharge curves, but at 16s, even a shallow discharge curve still has a curve. Li-ion will still curve more sharply regardless, so, what I imagine...

Li-ion and LFP in parallel, same voltage. Load applied. Voltage starts to drop. Voltage attempts to drop faster on Li-Ion because of its steeper curve. Can't drop too far though, because it's in parallel with LFP. LFP's more stable voltage shares current,, and brings Li-Ion back up. Li-Ion tries to drop, but essentially gets "charged" by its parallel LFP buddy, so goes back up.

I realize that I've done a terrible job explaining it, with non specific terms and oversimplifications. How about this: picture a set of descending stairs, next to a slope, both with the same downward curve, but the stairs are broken up. Li-Ion is the stairs, LFP is the curve.

Bottom line is 'I've read consistently' that you can't effectively use voltage. You *must* use a shunt coulomb counting to effectively know the SoC except at the very top and very bottom.

True, but that's why I would select DoD range for both packs conservatively. The individual BMS's still use voltage to determine LVCs. It shouldn't matter what SOC is for them to share a load, as long as they are sharing the same voltage, right? The load will be unevenly shared regardless, unless both packs are perfectly matched in capacity, which is not possible.

Even if you take 2 of the exact same rated 18650s, put them in parallel, and apply a load, the load will be uneven because of minute differences between IR, actual capacity, whatever. The load still gets its power, and you could argue that despite them both being identical 18650s, their real time discharge curves aren't identical, and it doesn't matter.

I've used a 24s LFP pack on my bike, I have first hand experience watching the voltage while riding for hours, so I understand that LFP has a shallow curve. I guess im wondering, so what? If they're in parallel, sharing a load, shouldn't it not matter, as long as each BMS is doing it's job, and each pack is appropriately monitored and fused?

 

[OffGridInTheCity]

Even if you take 2 of the exact same rated 18650s, put them in parallel, and apply a load, the load will be uneven because of minute differences between IR, actual capacity, whatever. The load still gets its power, and you could argue that despite them both being identical 18650s, their real time discharge curves aren't identical, and it doesn't matter.

I wouldn't agree with this being anywhere near the same level as I describe below.... When 18650(s) are in parallel, they are effectively 'identical' in their voltage all the time. And they are usually in close proximity - so there is virtually no cell to cell (lateral) stress.

I've used a 24s LFP pack on my bike, I have first hand experience watching the voltage while riding for hours, so I understand that LFP has a shallow curve. I guess im wondering, so what? If they're in parallel, sharing a load, shouldn't it not matter, as long as each BMS is doing it's job, and each pack is appropriately monitored and fused?

The BMS is not the issue as far as load sharing.

It's a matter of the LifePo4 battery being 52.8v @ 80% capacity and Lithium-ion being 56v at 80%. The voltage difference between the 2 batteries will cause current to try to flow from the 56v battery to the 52.8v battery. As Lithium-ion discharges down to 30% DOD (3.5v * 14 = 49v) then it will reverse and then the 30% DOD 52v LifePo4 battery will try to equalize voltage with the 49v Lithium-ion battery.

This mismatched voltage will case current flow - e.g. unnecessary stress thru out the 90% of the operational range. How harmful this is - it depends on several factors - but it's can't be ideal.

If you decide to do this - I look forward to seeing how it works for you and hope you'll report on the details.

 

[harrisonpatm]

I wouldn't agree with this being anywhere near the same level as I describe below.... When 18650(s) are in parallel, they are effectively 'identical' in their voltage all the time. And they are usually in close proximity - so there is virtually no cell to cell (lateral) stress.

If they're not in close proximity, it still happens. Say a 3000mah cell in parallel with a 2000mah cell. Unequal load sharing, still parallel.

It's a matter of the LifePo4 battery being 52.8v @ 80% capacity and Lithium-ion being 56v at 80%.

That's not what would happen if they're in parallel, though; they wouldn't be at 2 different voltages. Why would they ever be at different voltages ever if they're in parallel? Rather, for example, both at 52v, LFP at 50% capacity, Li-Ion at 80% capacity. Is there something inherently wrong with that?

Edit: i'm hopefully not coming off as rude or obtuse, I understand different voltage curves. I'm just wondering, if you put two batteries in parallel, won't they always just be at the same voltage, and sharing a load unequally?

This mismatched voltage will case current flow - e.g. unnecessary stress

This is what I'm wondering about. Im imagining each pack trading current with the other constantly. Perhaps mitigated by thicker cables?

 

[J_Mack58]

I got a great deal on a bunch of nearly new prismatic LFP cells, too good to pass up, but I'm already in place with building my 14s Li-Ion powerwall. Apologies if this has been discussed earlier, but I think it could work, if....

1. Separate BMS for each bank
2. Monitor charge/discharge levels

If i keep the LFP bank between 3.0-3.5V per cell, which I believe is a conservative range, I get 48-56V at 16s. For 14s Li-Ion, that range gives me about 3.4-4V per cell, which I also think is a good conservative range. More conservative than I had originally planned for my 14s build, actually, but that hopefully just means better cycle life.

If my total system discharge drops below 48V, then the LFP BMS will/should cutoff, and the Li_ion will powerwall will bear the brunt. So, I should set the min-max voltage limits on both BMS's to parallel each other.

I think it will work, but I also haven't done it before. Any advice? Like I said, too good of a local deal to pass up, at about $55 USD per kwh, so if I can't parallel the two chemistries, I'll just stash the new purchases until I can develop a separate system.

I have a 16s50p LiFePo battery paralleled with a 14s100p Lithium battery. I charge both to 55 volts, and my lowest discharge voltage is 45.5 volts. Less then 45.5 volts and my inverter will kick off. My system powers my house and with just two working people the load is lite. Seems to take forever to discharge below 51 volts. I have not experienced any dangerous events ”knock on wood”. I‘m protecting my system 3 ways, BMS, Programmable voltage relay ($35) and a PLC (Allen Bradley). I‘m so called trying to develop a smart house.

 

[OffGridInTheCity]

Edit: i'm hopefully not coming off as rude or obtuse, I understand different voltage curves. I'm just wondering, if you put two batteries in parallel, won't they always just be at the same voltage, and sharing a load unequally?

I don't take it as rude - and don't take my comments that way either. I know it's not good per every post I've ever read over the last 4 years but not clear as to the in-depth core of the issues from hands-on experience. But I do understand voltage ranges / Soc % and you're right - parallel MEANS equal voltage. Based on this and thinking about it - I suspect the mis-matched voltage curves mean that significant capacity is lost operationally.

I'm hoping one of our deeply technical members will comment on how this works electrically.

If they're not in close proximity, it still happens. Say a 3000mah cell in parallel with a 2000mah cell. Unequal load sharing, still parallel.

Not the same thing. These cells are the same chemistry and share the same voltage curve so they are able to maintain the same voltage all thru the charge/discharge - so you get full capacity out of the parallel cells..

That's not what would happen if they're in parallel, though; they wouldn't be at 2 different voltages. Why would they ever be at different voltages ever if they're in parallel? Rather, for example, both at 52v, LFP at 50% capacity, Li-Ion at 80% capacity. Is there something inherently wrong with that?

Li-ion cannot be 52v and 80% capacity at the same time.

This is what I'm wondering about. Im imagining each pack trading current with the other constantly. Perhaps mitigated by thicker cables?

Let's say the Lithium-ion equalizes down to 52v when initially paralleled.... in addition to the stress on the cells, you've lost access to significant capacity as 52v is around 40% charge for Li-ion from the git go. And you cannot charge up to 80% (~56v) for a Li-ion battery as this exceeds LifePo4 53.2'ish volt for 80% capacity. In fact 56v Li-ion would be really stressing (damaging) the top charge of LifePo4 at ~54.5v.

 

[harrisonpatm]

Li-ion cannot be at 80% capacity and 52v at the same time.

You are of course correct, I typed a response with hypothetical numbers, without thinking of realistic capacities. So, roughly, 50% capacity at 52v. at 52v for LFP, hard to say, its right in the middle at 3.25 per cell. Im just wondering whether SoC matters.

I don't see how Lithium-ion can 52v thru the entire discharge curve of the LifePo4 and be able to use the full capacity of the lithium-ion battery.

I think that maybe the answer is, it doesn't. The load will be shared unequally.

Let's say the Lithium-ion equalizes down to 52v when initially paralleled.... then you've lost a great deal of capacity from the beginning. And if the Li-ion pullup the LifePo4, maybe it damages the cells.

LFP in my experience (and hopefully others' as well) is rather robust. Regardless, i would never connect them in parallel without making sure they're already equal in voltage. All of my queries are regarding what happens after they're connected, during charge and discharge, not even with heavy loads in either direction.

And i will not be using an inverter, I understand that various voltage requirements from equipment to equipment could cause issues. So that's not a concern in this case, at least.

 

[OffGridInTheCity]

You are of course correct, I typed a response with hypothetical numbers, without thinking of realistic capacities. So, roughly, 50% capacity at 52v. at 52v for LFP, hard to say, its right in the middle at 3.25 per cell. Im just wondering whether SoC matters.

I think that maybe the answer is, it doesn't. The load will be shared unequally.

Parallel = same voltage. This is fundamental. A LifePo4 battery in parallel with a Li-Ion *must* have the same voltage.

Final thought - I think that they will maintain the same voltage when in parallel and so you can't use the entire capacity range of Li-Ion when yoked to LifePo4. And you can't do simple voltage SoC decisions.

OK - I'll stop here. I'm happy to be wrong but principle of parallel = same voltage is a bedrock principle that has been drilled into me on many posts and I've observed this myself.

It's an interesting discussion for sure and I look forward to additional comments

 

[harrisonpatm]

Final thought - I think that they will maintain the same voltage when in parallel and so you can't use the entire capacity range of Li-Ion when yoked to LifePo4.

I think we're more in agreement then we realize, because this was my first conclusion as well.

And you can't do simple voltage SoC decisions.

Maybe not as important to me, but certainly true. I originally purchased a voltage-based SoC meter for my LFP bike. Ditched it and replaced it within less than a couple weeks of use, it was so frusturating and inaccurate.

 

[harrisonpatm]

Thanks for the info,, do you have any specifics on how the two chemistries behave together?

Seems to take forever to discharge below 51 volts.

This seems to imply that the flat discharge curve of LFP is kind of "pausing" the curve of Li-Ion at the point where the two curves are closest: 3.65v for Li-ion and 3.18v for LFP

 

[italianuser]

But won't the higher voltage battery continuously pass current to the lower voltage battery? If voltage in the LFP stays higher then the Li-on will continue to receive current from it; and during activity, still continue to lower it's own voltage quicker than the LFP.

In a balanced system (or almost balanced) the current flowing will be as small as possible. On an unbalanced system it will be higher.

Is there's any scientific paper showing that a bigger/longer/heavier current transit does actually shorten cells life? Then that's the answer. I can suppose that's the truth, but I'm, only supposing.

[Disclaimer -> Could be missing something here, I'm no good with this particular stuff! ]

 

[floydR]

iirc daromer had a hybrid powerwall used separate charge controller for each system not sure how he managed the switch over from one system to the other or if he used both concurrently. https://secondlifestorage.com/index.php?threads/esperyd-solar-and-powerwall-system.486/
Later floyd

 

[harrisonpatm]

But won't the higher voltage battery continuously pass current to the lower voltage battery

Why would they be higher/lower voltage than each other? This situation won't happen. If they're in parallel, they're same voltage, that's it. OGITC and i certainly agree on that point:

Parallel = same voltage. This is fundamental. A LifePo4 battery in parallel with a Li-Ion *must* have the same voltage.

Agreed. If I'm an idiot and I connect the two different packs in parallel the first time, when the voltage is significantly different, then yes, there will be a current surge from the higher voltage pack to the lower one. That will happen if I'm an idiot, hopefully im not. Once they're connected in parallel, they will always be at the same voltage.

What I'm predicting is what I think is happening in J_Mack's situation:

Seems to take forever to discharge below 51 volts.

This seems to imply that the flat discharge curve of LFP is kind of "pausing" the curve of Li-Ion at the point where the two curves are closest: 3.65v for Li-ion and 3.18v for LFP

But frankly, I'm not sure. I do fully intend to build both systems and parallel them, I can do it safely with fuses and monitoring, and I will certainly report what happens at that time. It'll just take a few months to get to that point.

 

[italianuser]

Why would they be higher/lower voltage than each other? This situation won't happen. If they're in parallel, they're same voltage, that's it. OGITC and i certainly agree on that point:

Wait a sec, don't look at the static view in a given moment. Of course they'll balance all the time.

I mean when there's a load. Look at what happens to keep that voltage the same. If the discharge curve is different than the Li-ion battery there will be a constant flow of current between the two batteries. And if the discharge curve is different, always under load, there will be more flow than if the discharge curve is equal (or very similar).

So, ok, the voltage will be the same, also with two same chemistry batteries the voltage will be the same. But, to keep the same voltage, seen as the curve is different, it means that the batteries are working more, constantly moving current to keep that balance.

My question is: if what I said is correct will the batteries have a shorter life? (I'm asking, I don't have the answer)