"Floating" LFP batteries

[tanglewood]

I'm interested in having a discussion about floating LFP batteries. Some people say not to float them. Yet many (maybe even most) chargers go into a float mode whether you like it or not. Those charger vendors, and at least CALB as a cell manufacturer say it's fine to float and give the correct voltage.


As best I can tell from these conversations, to float or not to float comes down to what you mean by "float".


For LA batteries, float is holding the batteries at an elevated voltage such that they a receiving a constant trickle charge sufficient to offset self-discharge. With the voltage set correctly, the battery can be left in float indefinitely. There also in a beneficial side effect to floating a LA battery. When there are loads placed on the DC power system, the charger carries some or all of the load rather than cycling the batteries.


With LFP batteries, you don't want to float them the same way as an LA battery. In particular, you don't want to continue to trickle charge them. So in that sense you don't want to float them. But I think there is more to it than that.


Float charging also allows a charger to carry loads rather than cycling the batteries, and I think that is still a desirable feature. Why cycle your batteries if you don't need to? Yes, LFPs have a much greater cycle life than LA, but why throw it away needlessly. Plus, as mentioned earlier, many chargers go into float whether you like it or not. This brings us to the other definition of float, which I think is a desirable thing to do.


With LFP batteries, you just need to set the float voltage equal to the battery's open circuit voltage. This is the recommendation of CALB and Victron, just as examples. When set that way, there will be no on-going charge of the batteries since there is no voltage difference between them and the charger. And if there are DC loads, the charger will help carry those loads rather than cycling the batteries. This strikes me as the preferred way to do things.


Does this make sense to those who have been doing this for a while? What am I missing?

[a64pilot]

Since your on shore power I assume and not Solar?
Why not disconnect the bank and use a good charger as a power supply to run your DC system, as long as a load doesn’t exceed Chargers capacity it will work fine, and you shouldn’t ever have a load exceed say 60 amps anyway.

It’s on a mooring on Solar that becomes the issue, I believe? It has been suggested to have a second lead battery bank for these times.

I am not an LFP user, just someone who is interested and trying to decide the shortcomings of them

[tanglewood]

Originally Posted by a64pilot

Since your on shore power I assume and not Solar?
Why not disconnect the bank and use a good charger as a power supply to run your DC system, as long as a load doesn’t exceed Chargers capacity it will work fine, and you shouldn’t ever have a load exceed say 60 amps anyway.

That would be an option. But I'm hoping to get to the root of the "charge then disconnect" mantra that I hear repeated over and over again. If the battery and charger are at the same voltage, then no current will flow, and it's equivalent to an open circuit (disconnected charger). So I'm trying to find out if I'm missing something, which is entirely possible, or if the "don't float" advice is really "don't float at a voltage above the battery resting voltage".

[a64pilot]

I can only guess that if you were to maintain a voltage that equaled a 50% charge then you would be OK?
However voltage is rather flat for a lithium battery isn’t it?
I’m trying to remember my scooter Lipo pack, it was about 50 V when fully charged, but as SOC dropped so did voltage, so it’s not dead flat.

On edit, surely it can’t be as easy as that, there has to be a reason, and I assume that even at lower voltage the battery continues to charge.

Posting to bump as much as anything

[tanglewood]

Originally Posted by a64pilot

I can only guess that if you were to maintain a voltage that equaled a 50% charge then you would be OK?
However voltage is rather flat for a lithium battery isn’t it?
I’m trying to remember my scooter Lipo pack, it was about 50 V when fully charged, but as SOC dropped so did voltage, so it’s not dead flat.

On edit, surely it can’t be as easy as that, there has to be a reason, and I assume that even at lower voltage the battery continues to charge.

Posting to bump as much as anything

You are right that it's not very precise given the flat voltage curve of LFP. Manufacturer recommended float voltages target about 70-80% SOC. So a little higher than in your example, but same idea. The key is to keep it low enough that you are not continuing to pump current into the battery and over-charge it.

[Maine Sail]

Before using small disposable LFP 18650 cells, for abuse testing, I conducted a very expensive experiment regarding storage at 100% SOC.

The test duration wound up being 12 1/2 months using four 100Ah CALB cells. They cells were charged to 100% SOC and then left to sit idle with no connections to a BMS or other parasitic loads. The low temp recorded over the 12 1/2 months was 46°F and the high temp was 87°F. This was meant to be a representation of real world cells sitting in storage at 100% SOC. I conducted this because I found lots of conflicting data out there in white papers and in spec sheets.

A min/max capture thermometer was used to record the peaks. The cells, prior to letting them sit at 100% SOC, for 12 1/2 months, were regularly testing at 101.2 to 101.3 Ah’s of capacity (previous 6 Ah capacity tests) as a 12V nominal bank. The cells were very well matched to within .1Ah of each other and internal resistance was also spot on.

After 12 1/2 months of sitting idle the cells were discharged to a cut off voltage of 2.9VPC.. After 12 1/2 months of doing nothing but sitting there, at 100% SOC, the cells had lost 11.6% of their previous rigorously confirmed Ah capacity. Every trick in the book was used to try and bring that capacity back but it was not happening.

Imagine if you additionally stressed the cells by continually float charging them even slightly above the natural resting voltage let alone the 13.8V some charger manufacturers recommend..?

Ask yourself how the manufacturers suggest the mere act of storage, at 100% SOC, is bad for the cells, which I have confirmed is, and then tell you float them? How can they say “store at 50-60% SOC" yet then give you a “float” voltage? It's not a trick question at all just contradictory specs..

Today on the test bench I have four LFP prismatic cells that have been charged at 14.4V (with a 1 hour absorption) and floated at 13.4V via solar. The capacity test ended a few hours ago at just 87% of rated capacity. I know from experience that when new these cells routinely test at 105% to 108% of rated capacity. These cells have less than 200 cycles. I strongly suspect that floating and the 1 hour absorption at 3.6VPC had a costly effect on these cells. I can only say this because my 2009 400Ah cells, with nearly 1100 cycles to 80% DOD, still exceed 400Ah in capacity. The disclaimer for my cells is that they were never charged at anywhere near 14.4V nor have they ever had the absorption held for an hour, let alone every charge cycle, they've also never been floated and have always been left at 35-70% +/- SOC when not in active cycling use.

[Maine Sail]

Originally Posted by tanglewood

That would be an option. But I'm hoping to get to the root of the "charge then disconnect" mantra that I hear repeated over and over again. If the battery and charger are at the same voltage, then no current will flow, and it's equivalent to an open circuit (disconnected charger). So I'm trying to find out if I'm missing something, which is entirely possible, or if the "don't float" advice is really "don't float at a voltage above the battery resting voltage".

If your charger allows a float level of 13.00V / 26.00V the battery will simply discharge down to 13.00V (approx 50% SOC) and the charger will then maintain the bank at 50% SOC. Not all chargers can be set that low for dockside storage mode so the simple solution is to take the LFP off line and use a lead buffer bank when at the dock..

[a64pilot]

I just looked at mine
Outback 80 min 12.4V
Magnum MS 2812 min 12.0V
Sterling Pro Charge Ultra 60 amp 12.6 V

Magnum and Sterling are in “custom” mode.

I did not try the Balmar 614, cause it’s a PIA and your not likely running your engine in storage anyway?

[tanglewood]

Originally Posted by Maine Sail

Before using small disposable LFP 18650 cells, for abuse testing, I conducted a very expensive experiment regarding storage at 100% SOC.

The test duration wound up being 12 1/2 months using four 100Ah CALB cells. They cells were charged to 100% SOC and then left to sit idle with no connections to a BMS or other parasitic loads. The low temp recorded over the 12 1/2 months was 46°F and the high temp was 87°F. This was meant to be a representation of real world cells sitting in storage at 100% SOC. I conducted this because I found lots of conflicting data out there in white papers and in spec sheets.

A min/max capture thermometer was used to record the peaks. The cells, prior to letting them sit at 100% SOC, for 12 1/2 months, were regularly testing at 101.2 to 101.3 Ah’s of capacity (previous 6 Ah capacity tests) as a 12V nominal bank. The cells were very well matched to within .1Ah of each other and internal resistance was also spot on.

After 12 1/2 months of sitting idle the cells were discharged to a cut off voltage of 2.9VPC.. After 12 1/2 months of doing nothing but sitting there, at 100% SOC, the cells had lost 11.6% of their previous rigorously confirmed Ah capacity. Every trick in the book was used to try and bring that capacity back but it was not happening.

Imagine if you additionally stressed the cells by continually float charging them even slightly above the natural resting voltage let alone the 13.8V some charger manufacturers recommend..?

Ask yourself how the manufacturers suggest the mere act of storage, at 100% SOC, is bad for the cells, which I have confirmed is, and then tell you float them? How can they say “store at 50-60% SOC" yet then give you a “float” voltage? It's not a trick question at all just contradictory specs..

Today on the test bench I have four LFP prismatic cells that have been charged at 14.4V (with a 1 hour absorption) and floated at 13.4V via solar. The capacity test ended a few hours ago at just 87% of rated capacity. I know from experience that when new these cells routinely test at 105% to 108% of rated capacity. These cells have less than 200 cycles. I strongly suspect that floating and the 1 hour absorption at 3.6VPC had a costly effect on these cells. I can only say this because my 2009 400Ah cells, with nearly 1100 cycles to 80% DOD, still exceed 400Ah in capacity. The disclaimer for my cells is that they were never charged at anywhere near 14.4V nor have they ever had the absorption held for an hour, let alone every charge cycle, they've also never been floated and have always been left at 35-70% +/- SOC when not in active cycling use.

The CALB recommended float voltage is 3.3 vpc. That's right at about 80% SOC, so that would be the equivalent storage SOC. If you are going to leave them like that for an extended time, say while away from the boat, then dialing down to 3.25 or 3.26 vpc would hold them at 50% SOC.


I guess I'm not seeing the contradiction. Is just seems to me that the problem is over charging. And there are lots of ways to do that, including an improperly set float voltage. Excessive absorb is over charging too, like in your example. And floating at too high a voltage is exactly the same thing as too much absorb, just with a different name. But I'm not seeing how floating at the cells 80% SOC open circuit voltage can lead to anything other than 80% SOC. And for long term float, that can be dropped to 40%-60% SOC, if desired.

[Delfin]

Look

Originally Posted by tanglewood

I'm interested in having a discussion about floating LFP batteries. Some people say not to float them. Yet many (maybe even most) chargers go into a float mode whether you like it or not. Those charger vendors, and at least CALB as a cell manufacturer say it's fine to float and give the correct voltage.


As best I can tell from these conversations, to float or not to float comes down to what you mean by "float".


For LA batteries, float is holding the batteries at an elevated voltage such that they a receiving a constant trickle charge sufficient to offset self-discharge. With the voltage set correctly, the battery can be left in float indefinitely. There also in a beneficial side effect to floating a LA battery. When there are loads placed on the DC power system, the charger carries some or all of the load rather than cycling the batteries.


With LFP batteries, you don't want to float them the same way as an LA battery. In particular, you don't want to continue to trickle charge them. So in that sense you don't want to float them. But I think there is more to it than that.


Float charging also allows a charger to carry loads rather than cycling the batteries, and I think that is still a desirable feature. Why cycle your batteries if you don't need to? Yes, LFPs have a much greater cycle life than LA, but why throw it away needlessly. Plus, as mentioned earlier, many chargers go into float whether you like it or not. This brings us to the other definition of float, which I think is a desirable thing to do.


With LFP batteries, you just need to set the float voltage equal to the battery's open circuit voltage. This is the recommendation of CALB and Victron, just as examples. When set that way, there will be no on-going charge of the batteries since there is no voltage difference between them and the charger. And if there are DC loads, the charger will help carry those loads rather than cycling the batteries. This strikes me as the preferred way to do things.


Does this make sense to those who have been doing this for a while? What am I missing?

Once an LFP bank is full, any additional current just goes to heat, which can be damaging. If you have a LA starter bank, disconnect the LFP bank when full, or as soon as it occurs to you, then send that float current to the starter bank. That's what we do, and it works fine.

P.s. this assumes you are using the LFP bank the next day. If not, disconnect at 50% SoC or there about.

[44'cruisingcat]

Originally Posted by tanglewood

I'm interested in having a discussion about floating LFP batteries. Some people say not to float them. Yet many (maybe even most) chargers go into a float mode whether you like it or not. Those charger vendors, and at least CALB as a cell manufacturer say it's fine to float and give the correct voltage.


As best I can tell from these conversations, to float or not to float comes down to what you mean by "float".


For LA batteries, float is holding the batteries at an elevated voltage such that they a receiving a constant trickle charge sufficient to offset self-discharge. With the voltage set correctly, the battery can be left in float indefinitely. There also in a beneficial side effect to floating a LA battery. When there are loads placed on the DC power system, the charger carries some or all of the load rather than cycling the batteries.


With LFP batteries, you don't want to float them the same way as an LA battery. In particular, you don't want to continue to trickle charge them. So in that sense you don't want to float them. But I think there is more to it than that.


Float charging also allows a charger to carry loads rather than cycling the batteries, and I think that is still a desirable feature. Why cycle your batteries if you don't need to? Yes, LFPs have a much greater cycle life than LA, but why throw it away needlessly. Plus, as mentioned earlier, many chargers go into float whether you like it or not. This brings us to the other definition of float, which I think is a desirable thing to do.


With LFP batteries, you just need to set the float voltage equal to the battery's open circuit voltage. This is the recommendation of CALB and Victron, just as examples. When set that way, there will be no on-going charge of the batteries since there is no voltage difference between them and the charger. And if there are DC loads, the charger will help carry those loads rather than cycling the batteries. This strikes me as the preferred way to do things.


Does this make sense to those who have been doing this for a while? What am I missing?

Yep , this is exactly what I've been doing since installing the lithiums.

[a64pilot]

Originally Posted by 44'cruisingcat

Yep , this is exactly what I've been doing since installing the lithiums.

What voltage? What are your voltage set points for absorption, and float? What trips float, time?

[tanglewood]

Originally Posted by Delfin

Look

Once an LFP bank is full, any additional current just goes to heat, which can be damaging. If you have a LA starter bank, disconnect the LFP bank when full, or as soon as it occurs to you, then send that float current to the starter bank. That's what we do, and it works fine.

P.s. this assumes you are using the LFP bank the next day. If not, disconnect at 50% SoC or there about.

I think this is at the heart of the debate. Our LA mind-set equates float with an on-going charge current, because that's what happens with LA. On-going charge current is harmful to LFP, therefore float must be bad for LFP, right?


But there is only a charge current if the float voltage is greater than the battery's resting voltage. In our past LA world, this is always the case, but in our new LFP world lowering the float voltage to the resting voltage of the battery results in ZERO charge current. It's the same as being disconnected. But the charger or alternator is floating along ready to carry any DC loads, and there is no need to switch out the LFP, and switch in some other battery.


Now I'm not saying there is anything wrong with other battery management approaches. You have a setup that works for you, and that's what matters. But I do think there is more to the whole float question, and it's not all bad. In fact, I think there are some attractive aspects to it if done correctly. I'm hoping to get all that out on the table, and better understood.

[44'cruisingcat]

Originally Posted by a64pilot

What voltage? What are your voltage set points for absorption, and float? What trips float, time?

13.8 trips absorb, timed for 1 minute, then float at 13.2.

Outback flexmax 80.

[a64pilot]

OK so it wakes up in the morning, then slowly climbs to 13.8 then to float?
I’m assuming 13.8 equates to a pretty high SOC 80 ish?
BMS do any balancing?
Cause if not, then all you really need is a high voltage disconnect?

I hear that besides an initial full charge that they don’t need balancing from then on? I don’t understand that myself, I would think they would drift apart in voltage.