Mixed Li-Ion and LiFePo4 parallel discharge test

[DThames]

I was just curious how this would work. 8s LiFePo4 and 7s Li-ion can operate as a single pack if you set proper BMS levels and keep your operation between the ditches. The only reason I can think of doing this would be to help with high surge current demands (100C Li-Ion for example) or in a pinch where neither pack was large enough for the duration of a specific discharge demand (keep the freezer on overnight.

Interesting to note that early on and late in the discharge, the Li-ion carried more of the load but in the middle where the LiFePo4 was in its flat voltage area, the LiFePo4 delivered more of the power. Overall the LiFePo4 delivered more of the total energy. However, I didn't allow the test to run until the Li-ion was near its cutoff of 3.3v or so. So it would have been closer to even if I had let it run longer.

See attached.

 

[Supervstech]

This seems a bad idea...
if any cells go weak, having differing chemistries could lead to rapid disassembly...
Math wise, it seems both should play safe...

 

[DThames]

This seems a bad idea...
if any cells go weak, having differing chemistries could lead to rapid disassembly...
<snip>

How is that, as the BMS would shut the pack down if a cell went out of range in voltage?

 

[NPhil]

I found this because I've been planning, learning, and experimenting concurrently, and wound up with a mix of surplus Tesla 2170 cells (intended for under the vehicle in watertight containers) and K2 26650 LiFePO4 cells (intended for the passenger compartment), for various reasons, including that there isn't enough convenient space under my aged minivan for all the battery capacity I suspect I will need. Happily, I had settled on 24V, and doing the math again, I can't see why it wouldn't work to just tie them together (I suppose, I will include a breaker). Of course, I did an internet search to check, and found your thread. I've registered, and plan to read your document. Thanks!

 

[DThames]

I found this because I've been planning, learning, and experimenting concurrently, and wound up with a mix of surplus Tesla 2170 cells (intended for under the vehicle in watertight containers) and K2 26650 LiFePO4 cells (intended for the passenger compartment), for various reasons, including that there isn't enough convenient space under my aged minivan for all the battery capacity I suspect I will need. Happily, I had settled on 24V, and doing the math again, I can't see why it wouldn't work to just tie them together (I suppose, I will include a breaker). Of course, I did an internet search to check, and found your thread. I've registered, and plan to read your document. Thanks!

You might look at this video as well. It is about lead and LiFePo4 but the topic is similar as he is looking at two chemistries with different discharge curves. Li-ion is more linear than LiFePo4 like lead acid is more linear than LiFePo4.

 

[NPhil]

You might look at this video as well. It is about lead and LiFePo4 but the topic is similar as he is looking at two chemistries with different discharge curves. Li-ion is more linear than LiFePo4 like lead acid is more linear than LiFePo4.

Do you mean this video? I think it implies that my plan might work pretty well, especially if I heed the (important, I think) warning to never connect two batteries in parallel without having first carefully charged each to matching voltages, which is even more important if one or more of the batteries is not lead-acid.

 

[DThames]

Do you mean this video? I think it implies that my plan might work pretty well, especially if I heed the (important, I think) warning to never connect two batteries in parallel without having first carefully charged each to matching voltages, which is even more important if one or more of the batteries is not lead-acid.

Yes, I forgot to add the link.

 

[crossy]

Interesting viewing chaps, thanks for that.

I'm currently running 2 x 16s LiFePO4 packs in parallel (separate BMS's), I've been offered a couple of ex-EV 14S Li-ion packs at a very "competitive" price (sorry, we're in Thailand so the deal doesn't apply to most members).

I was hoping to run the lot as a parallel system, should be ok so long as I don't stress it.

Our max charge/discharge power is only about 3kW @ 48V but of course most of that is when the sun is sleeping.

 

[mrzed001]

I am too lazy to do a combined LFP / NMC discharge diagram,
so here is a combined LFP / LEAD discharge diagram:

I think anyone can read it and understand it why it is such a bad idea to parallel different chemistries together

 

[NPhil]

I am too lazy to do a combined LFP / NMC discharge diagram,
so here is a combined LFP / LEAD discharge diagram:

I think anyone can read it and understand it why it is such a bad idea to parallel different chemistries together

You are mistaken.

 

[mrzed001]

You are mistaken.

Please explain

 

[NPhil]

I am testing, currently, 55AH of 1P8S LiFePO4 in parallel with 20AH comprised of 4P of 1P7S 5AH Tesla 2170 packs. I am, as far as I know, a member of the set "anyone", and I saw a chart showing the lead-acid vs LifePO4 discharge curve before I began.

If anyone is curious, it's working tolerably, at least. My test is a use test only, so far, but I would say the result is that it's not obvious that it's a bad idea.

 

[mrzed001]

I am testing, currently, 55AH of 1P8S LiFePO4 in parallel with 20AH comprised of 4P of 1P7S 5AH Tesla 2170 packs. I am, as far as I know, a member of the set "anyone", and I saw a chart showing the lead-acid vs LifePO4 discharge curve before I began.

If anyone is curious, it's working tolerably, at least. My test is a use test only, so far, but I would say the result is that it's not obvious that it's a bad idea.

Not obvious NOW It will be later ... The different charge-discharge state will make a lot of unnecessary charges and discharges between the two pack. Wear and tear will be high.

 

[NPhil]

Not obvious NOW It will be later ... The different charge-discharge state will make a lot of unnecessary charges and discharges between the two pack. Wear and tear will be high.

It's an interesting claim, and possible, as far as I know (I'm still fairly new at this), but, you haven't supported it with compelling evidence.

 

[mrzed001]

It's an interesting claim, and possible, as far as I know (I'm still fairly new at this), but, you haven't supported it with compelling evidence.

The evidence is the diagram if you can read it
Where there is a V difference there will current flow.
As you can see there is a big V difference on any SoC. The NMC curve is a bit like the Lead, not so flat as the LFP.
And if you want to model or imagine how it works with all the SoC, and V differences, and because of that flowing current from one into the other in different C rates in charging and discharging scenario ... prepare for a headache

 

[NPhil]

The evidence is the diagram if you can read it
Where there is a V difference there will current flow.
As you can see there is a big V difference on any SoC. The NMC curve is a bit like the Lead, not so flat as the LFP.
And if you want to model or imagine how it works with all the SoC, and V differences, and because of that flowing current from one into the other in different C rates in charging and discharging scenario ... prepare for a headache

I don't think that diagram means everything you think it means. And I'm already dealing with a headache, arguably the consequence of too many of my co-residents (of the State of California) having made too many hasty assumptions, on matters of consequence, over too many years. That's one reason I'm adding batteries to an old minivan.

BTW, I hope no one will take my experiment as an unqualified recommendation. Some things about it are, I think, obviously less than ideal:
This combination of cell chemistry won't match voltages on 12V systems.
It adds complexity, which is, all else being equal, good to avoid.
The combination of flammable battery and not flammable battery isn't for everyone.
On the other hand...
In principle, at least, it is possible to still have a working battery after the flammable battery has burned up.
Sometimes complexity reveals new information which is useful to know (for example, it might be possible to to make a circuit which compares the two batteries to produce a more accurate simple state of charge indication than the ones I've been looking at).
I wasn't going to do 12V anyway.

 

[NPhil]

Interesting viewing chaps, thanks for that.

I'm currently running 2 x 16s LiFePO4 packs in parallel (separate BMS's), I've been offered a couple of ex-EV 14S Li-ion packs at a very "competitive" price (sorry, we're in Thailand so the deal doesn't apply to most members).

I was hoping to run the lot as a parallel system, should be ok so long as I don't stress it.

Our max charge/discharge power is only about 3kW @ 48V but of course most of that is when the sun is sleeping.

In my opinion, the best location for Li-ion is outside, in a steel box, not too close any flammable structure.

 

[crossy]

In my opinion, the best location for Li-ion is outside, in a steel box, not too close any flammable structure.

Agree 100%.

The battery "box" (more a small shed) is cement board on a steel frame, and it's well away from the house, it's probably a bit close to my truck if it does go fzzzz!

 

[mrzed001]

I don't think that diagram means everything you think it means. And I'm already dealing with a headache, arguably the consequence of too many of my co-residents (of the State of California) having made too many hasty assumptions, on matters of consequence, over too many years. That's one reason I'm adding batteries to an old minivan.

BTW, I hope no one will take my experiment as an unqualified recommendation. Some things about it are, I think, obviously less than ideal:
This combination of cell chemistry won't match up on 12V systems.
It adds complexity, which is, all else being equal, good to avoid.
The combination of flammable battery and not flammable battery isn't for everyone.
On the other hand...
In principle, at least, it is possible to still have a working battery after the flammable battery has burned up.
Sometimes complexity reveals new information which is useful to know (for example, it might be possible to to make a circuit which compares the two batteries to produce a more accurate simple state of charge indication than the ones I've been looking at).
I wasn't going to do 12V anyway.

OK I try to outline what will happen
- charge all up to 29,2V (3,65 x 8 = 4,17 x 7)
- Relaxing cells. LFP drops to 3,4-3,45V/cell = 27,2V
- NMC relax V I am not sure, lets say 4,1V/cell = 28,7V
- There is already 1,5V difference, NMC will overcharge LFP

- start to discharge it.
- LFP drops fast (like 2-5% discharge) to 3,3V/cell = 26,4V
- NMC does not drop, lets say 4V = 28V
- Here is again a 1,6V difference. Both will discharge, but NMC will discharge faster

- 95% NMC 4V -> 28V
- 95% LFP 3,3V -> 26,4V ... NMC discharges faster

- 50% NMC 3,8V -> 26,6V
- 50% LFP 3,25V -> 26V ... they are coming closer

- 10% NMC 3,5V -> 24,5V
- 10% LFP 3,15V -> 25,2V ... now it reversed and LFP discharging faster

- 5% NMC 3,2V -> 22,4V
- 5% LFP 2,9V -> 23,2V ... LFP discharging faster

- If no load and stays here then LFP charging NMC

(I am too tired now to check the charge diagrams too )

I see it working only if you remain in the 20-80% SoC part. That is only 60% usable capacity.
First NMC will discharge faster than LFP will discharge faster ... if constant load. If not they start to charge each other (causing extra wear)

 

[DThames]

In my opinion, the best location for Li-ion is outside, in a steel box, not too close any flammable structure.

Fire is painful and "leaves a mark".