Float of LFP

[Don L]

You read it all time that it is bad to float a LFP battery.

Anyone have an actural link to paper on an actural test to support this? Not just a statement from "X" (even if a supplier), but an actural test.

I couldn't.

 

[twistedtree]

I think it comes down to your definition of "float".


The the extent "float" means an ongoing charge current, then it's definitely damaging and there is lots of research to back this up. If you search for research on overcharging of LFP you might be able to find it.


But that doesn't mean you can't used the "float" mode of a charger to act as a power supply for loads without providing ongoing charge current to the batteries.

 

[Don L]

You know of an actual study

 

[twistedtree]

You know of an actual study

I have collected a whole pipe of research papers over the years, and it's a constant theme that if you keep applying charge current, even a very small one, you will ruin the batteries. Once the voltage kicks up, you need to stop applying current. I don't recall anything that looked exclusively at that, but it's fundamental to how you charge them.


I just did a quick search and here are some references that might be helpful. I haven't read any of them.

1. Liu, J.; Duan, Q.; Ma, M.; Zhao, C.; Sun, J.; Wang, Q. Aging mechanisms and thermal stability of aged commercial 18650 lithium ion battery induced by slight overcharging cycling. J. Power Source 2020, 445, 227263. [Google Scholar] [CrossRef]
2. Ouyang, D.; Chen, M.; Weng, J.; Wang, J. A comparative study on the degradation behaviors of overcharged lithium-ion batteries under different ambient temperatures. Int. J. Energy Res. 2019, 44, 1078–1088. [Google Scholar] [CrossRef]
3. Liu, Y.; Xie, J. Failure study of commercial LiFePO4 cells in overcharge conditions using electrochemical impedance spectroscopy. J. Electrochem. Soc. 2015, 162, A2208–A2217. [Google Scholar] [CrossRef]

 

[Don L]

2 of those don't to have anything to do with LFP. The third I don't what it is saying.

But none have any info about conditions to be able to relate how we would use a battery.

 

[folivier]

Do a search for dendrites on Lifepo cells. Dendrites are the key to understanding when or not to float.

 

[Don L]

Do a search for dendrites on Lifepo cells. Dendrites are the key to understanding when or not to float.

Well I searched that and couldn't find anything about float. Most talk about "high" voltage. Also it seems that the problem with the dendrites is shorting the cells and causing a thermal runaway, which I feel has been shown to not be a problem with LFP.

So now I have read more dark alley paths on the question (because I have the same thread going in 3 places) and have not seen anything far real world operation of LFP as to the topic.

 

[SteveK]

Don, I did not keep links to float topics. When I searched for LFP battery info there were several that said no float period and then there were some that said some float is OK as long as the volts are fixed but the amps are low.
My inverter charger Magnasine 2012 when LFP battery type is chosen, the float is controlled by the BMS of the battery, so you would have to do custom setting to force a steady current float.

 

[Don L]

Someone on another forum posted a link that is close so I am going to add it here also:

https://iopscience.iop.org/article/10.1149/2.0411609jes/pdf

The study was about aging capacity loss due to storage at various state of charge (SOC). I feel one can argue that a float voltage corresponding to a given SOC is reasonable to assume.

So from page A1874 of the page and the chart for the LFP battery stored for 9 months, and using the 25C

stored SOC between 80-100% the capacity was 94% relative (a 6% loss of capacity)

stored SOC between 40-80% the capacity was 97% relative (a 3% loss of capacity)

To me this suggests the "don't float at full" is almost meaningless loss compared to other SOC.

So overall to me this says float doesn't mean anything for LFP. Assuming of course you aren't holding a voltage above 100% SOC, which is a different subject.

 

[mvweebles]

My batteries are Lion Energy. They have a much lower charge recommendation than others such as Battle Born (13.9v). I queried them to make sure I understood correctly. They confirmed, but I must add that the tech didn't seem particularly knowledgeable. They also seem a bit ambivalent about charging.

My point being is not all LFP batteries will carry the same charge recommendations. That said, I take TT at his word about no-float as a guardrail for LFPs due to their chemistry. I don't understand why but will accept the guidance. There's a point where discussions like this induce sleep-like behavior in me. I didn't get the right brain cells I suppose.

Peter View attachment lion-safari-config-for-victron-bmv-battery-monitor.pdf

 

[Delfin]

2 of those don't to have anything to do with LFP. The third I don't what it is saying.

But none have any info about conditions to be able to relate how we would use a battery.

Perhaps you can treat your LFP just like LA, then report back in a year or two.

 

[Delfin]

My batteries are Lion Energy. They have a much lower charge recommendation than others such as Battle Born (13.9v). I queried them to make sure I understood correctly. They confirmed, but I must add that the tech didn't seem particularly knowledgeable. They also seem a bit ambivalent about charging.

My point being is not all LFP batteries will carry the same charge recommendations. That said, I take TT at his word about no-float as a guardrail for LFPs due to their chemistry. I don't understand why but will accept the guidance. There's a point where discussions like this induce sleep-like behavior in me. I didn't get the right brain cells I suppose.

Peter View attachment 142792

Peter, other than internal wiring between cells that might effect max charge/ discharge current, I wonder why there would be any differences between assemblers other than differences in opinion. Seems like the chemistry should be the same regardless of who wires the cells together, but I'm likely missing something.

TT, thoughts?

 

[Don L]

Perhaps you can treat your LFP just like LA, then report back in a year or two.

What xxxx statement! I have had my LFP for 10 months and treat them as the manufacturer says to.

 

[DDW]

You need to define what you mean by "float".

Usually if means to hold the terminals at a well regulated voltage, and maybe also with a current limit. The voltage makes a difference even on LA, but especially on LFP.

 

[Delfin]

What xxxx statement! I have had my LFP for 10 months and treat them as the manufacturer says to.

So what does your manufacturer say about floating?

And FWIW, I believe the reason floating LFP is not recommended except below resting voltage as TT suggested is because they are nearly 100% efficient, so once charged, further charge current just goes to produce heat.

 

[SteveK]

Peter, other than internal wiring between cells that might effect max charge/ discharge current, I wonder why there would be any differences between assemblers other than differences in opinion. Seems like the chemistry should be the same regardless of who wires the cells together, but I'm likely missing something.

TT, thoughts?

You would need to watch a few Will Prowse Youtube as I do not remember the correct terms. He points out that same/similar cells are controlled by a variety of different BMS and associated hardware, circuit boards. Seeing it while he is explaining is priceless.

 

[Don L]

we are talking LFP, "float" is not more than 100% SOC voltage

I found my info and am moving on

 

[Delfin]

You would need to watch a few Will Prowse Youtube as I do not remember the correct terms. He points out that same/similar cells are controlled by a variety of different BMS and associated hardware, circuit boards. Seeing it while he is explaining is priceless.

Thanks.

 

[Larry M]

we are talking LFP, "float" is not more than 100% SOC voltage

I found my info and am moving on

Do you mind sharing?

 

[Don L]

Do you mind sharing?

I don't mind, see post #9

 

[Don L]

So I asked the same question on other forums looking for separating myth from fact.

Being clear here that when we say "float" on a LFP battery we are saying to hold no more that 100% SOC voltage.

Have now seen 2 actural test papers about storing Lithium batteries and the effect that SOC has on the capacity.

The answer from the tests is YES storing at 100% does reduce capacity. It in fact DOUBLES the loss over storing at 30% SOC. But that only means a 2% loss and far as I am concerned that is meaningless in the practical user world of the battery.

 

[Delfin]

So I asked the same question on other forums looking for separating myth from fact.

Being clear here that when we say "float" on a LFP battery we are saying to hold no more that 100% SOC voltage.

Have now seen 2 actural test papers about storing Lithium batteries and the effect that SOC has on the capacity.

The answer from the tests is YES storing at 100% does reduce capacity. It in fact DOUBLES the loss over storing at 30% SOC. But that only means a 2% loss and far as I am concerned that is meaningless in the practical user world of the battery.

I think the question of what SOC LFP should be "stored at" is different from whether they will lose capacity if floated. I don't think there is much debate on the disadvantage of letting them sit off a charge source at 100% SOC. Floating at a voltage low enough not to increase SOC but that can supply transient power needs could be applied to a battery at 100%, 75% or 30% SOC. Constant float voltage above that level that increases SOC being bad is also not controversial.

 

[SteveK]

So I asked the same question on other forums looking for separating myth from fact.

Being clear here that when we say "float" on a LFP battery we are saying to hold no more that 100% SOC voltage.

Have now seen 2 actural test papers about storing Lithium batteries and the effect that SOC has on the capacity.

The answer from the tests is YES storing at 100% does reduce capacity. It in fact DOUBLES the loss over storing at 30% SOC. But that only means a 2% loss and far as I am concerned that is meaningless in the practical user world of the battery.

Don. What is the definition of storage per the papers. I mean to me storage is putting the fully charged battery on a shelf.
But if they mean fully charged and not used for a period of time due to lack of demand then time to rethink leaving boat unused and battery on float and at 100% SOC

BTW I monitored SOC when in float mode. Voltage 13.5 current 0A until the fridge turned on and then current rose to the need leaving battery at 100% SOC

 

[Don L]

I think the question of what SOC LFP should be "stored at" is different from whether they will lose capacity if floated. I don't think there is much debate on the disadvantage of letting them sit off a charge source at 100% SOC. Floating at a voltage low enough not to increase SOC but that can supply transient power needs could be applied to a battery at 100%, 75% or 30% SOC. Constant float voltage above that level that increases SOC being bad is also not controversial.

The thing is no one has posted a study of the "floated" LFP (float being no more than 100%). But it is reasonable to me to assume a "floated" battery at given SOC voltage would be about the same as a "stored" battery at same voltage.

So it looks that a float or storage at anything above 30% results in a 2-3% loss of capacity. That would appear to be what would happen if you left you battery charger on all the time.

When I started the topic I was more interested in whether it was ok to get to "float" everyday of 13.6V, hold it all afternoon on solar, discharge with use at night, cycle the battery back up the next, repeat over and over. Still have not found a study of that.

 

[guy with a boat]

…
When I started the topic I was more interested in whether it was ok to get to "float" everyday of 13.6V, hold it all afternoon on solar, discharge with use at night, cycle the battery back up the next, repeat over and over. Still have not found a study of that.

That sounds like normal battery use to me. Charge them up and run them down. The float debate really comes into play when you store the boat. I use ours as you describe when on the boat. When away from the boat, the charger settings hold the LFP bank at about 60% SOC.

Even if you could stretch the life of the batteries further by not charging to 100% daily, you wouldn’t be using the fully capacity and would have effectively a smaller bank than you purchased. Use them fully, store at reduced SOC, and they last how long they last, IMO.

 

[SteveK]

What is storage in this context?

With FLA float maintained the batteries while not in use.
With LFP and no need for float on fully charged is that storage. I do not see disconnecting an LFP bank for true storage, but wonder if I should leave them 50% charged between boat usage.

 

[twistedtree]

I think the reason you aren't finding exactly what you are looking for is because the fundamental issue is storage at various SOCs, and that has been studied extensively. It's then a simple relationship between how you set a float voltage and the resulting SOC that gets maintained. It's not worthy of a study. If you set the float voltage above the 100% SOC open circuit voltage of the cells they will continue to charge and be ruined. If you set the float voltage at the 100% SOC open circuit voltage of the cells, the batteries will stay at 100%. If you set the float voltage at the 70% SOC open circuit voltage, it will hold the batteries at 70% SOC. The whole goal of float with LFP is to act as a power supply to power loads without raising or lowering the battery SOC.

 

[Don L]

I think the reason you aren't finding exactly what you are looking for is because the fundamental issue is storage at various SOCs, and that has been studied extensively. I

must be secret studies then as I couldn't find and with the question posted on 3 boat forums only studies have been found

 

[twistedtree]

Here are some that explore storage at different SOC in one way or another. One attached, and the other can be found by searching for LFP Ageing Catton_John.pdf (it was too large to upload)

And each references other studies.


I guess I'm unclear on what you are trying to figure out in terms of battery management. Whether it's OK to leave them at 100% SOC for extended time, vs other SOC levels? Or is it a study that shows the relationship between float voltage and SOC? Or something else?

 

[Delfin]

I think the reason you aren't finding exactly what you are looking for is because the fundamental issue is storage at various SOCs, and that has been studied extensively. It's then a simple relationship between how you set a float voltage and the resulting SOC that gets maintained. It's not worthy of a study. If you set the float voltage above the 100% SOC open circuit voltage of the cells they will continue to charge and be ruined. If you set the float voltage at the 100% SOC open circuit voltage of the cells, the batteries will stay at 100%. If you set the float voltage at the 70% SOC open circuit voltage, it will hold the batteries at 70% SOC. The whole goal of float with LFP is to act as a power supply to power loads without raising or lowering the battery SOC.

Bingo.