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diy solar

Float LiFePO4 or not??

.........However physics says the same Lithium attachment has to take place in any similar Lithium chemistry. By its shape, a cylindrical cell is in compression automatically and prevents the impact of that expansion just like compressing prismatics does.
To my point, "By its shape, a cylindrical cell is in compression automatically and prevents the impact of that expansion just like compressing prismatics does."

Cylindrical prevents the impact of expansion.
Just like compressing prismatic does.

But prismatic cells do not come compressed. They come as individual cells, no compression. So I have read some DIY builders have not found a compression spec from the cell manufacturers, so they do not compress them thinking that they may do it out of spec and damage the prismatic cell.

It appears as though by reducing the absorb voltage (CC voltage target and CV voltage) target and setting a float voltage at or slightly below a LiFePO4 resting voltage for 99% SOC with no load, the swelling can be controlled in prismatic cells (no need to do this for cylindrical cells). If it can be controlled for prismatic cells, its effect on prismatic cell battery cycles (longevity) can be controlled.

So I surmise that a 12v 100ah LiFePO4 cylindrical cell battery should be able to withstand more charge cycles to 100% SOC (14.4v CC target) than a 12v 100AH LiFePO4 prismatic cell battery.

Could this be true ?
 
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To my point
It seems we agree now. Earlier I disagreed with what I thought you were suggesting about the different charge parameters based on form factor.

So I surmise that a 12v 100ah LiFePO4 cylindrical cell battery should be able to withstand more charge cycles to 100% SOC (14.4v CC target) than a 12v 100AH LiFePO4 prismatic cell battery.

Could this be true ?
Again, I have not seen anything that suggests that form factor has any effect on cycle life. Everything I have read says that depth of discharge has the biggest impact. That is why I rarely cycle my pack more than 60%. With the cost per kWhr of cells being less than $200 per kWh the economics for me favor just buying more capacity.

The Winston and Thundersky prismatics that I used ten years ago never discussed any need for compression. But those cells had much thicker cases with ribs so those cases may have been able to withstand the internal expansion by virtue of their construction.
 
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I have been looking at expensive cell packs and what I see is NOT compression but just a holding system that prevents the cells from expanding. What I see is hard rubber sheets between the cells and the end cells are resting between more rubber and then metal plates that cannot move. This makes a lot of sense to me as you just want to stop expansion not put actual physical force on discharged cells.
 
I have been looking at expensive cell packs and what I see is NOT compression but just a holding system that prevents the cells from expanding. What I see is hard rubber sheets between the cells and the end cells are resting between more rubber and then metal plates that cannot move. This makes a lot of sense to me as you just want to stop expansion not put actual physical force on discharged cells.
Rubber is somewhat of an heat insulator. I think the Prismatic cells have more problems shedding heat that metal cylindrical cells. Have you a link to a manufacturer that suggessts captive Prismatic cells ?
 
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Robby, have you researched the 12v 500ah (5,120Wh usable) , 20 cells_ 4s5p (GBS LFMP battery system) with 7" display monitor.
LFMP Adds a little magnesium to the LFP battery If I remember right. Heres a link to a post showing such a system:
 
Rubber is somewhat of an heat insulator. I think the Prismatic cells have more problems shedding heat that metal cylindrical cells. Have you a link to a manufacturer that suggessts captive Prismatic cells ?
Not a link but I have seen expensive packs torn down on Youtube and during one of my Conversations with a Fortress Battery Tech he told me the same thing. They do not compress the cells with force, they just contain the expansion with a rubber padding.
 
Do you have a link? I do not doubt that it said that. However physics says the same Lithium attachment has to take place in any similar Lithium chemistry. By its shape, a cylindrical cell is in compression automatically and prevents the impact of that expansion just like compressing prismatics does.
"By its shape, a cylindrical cell is in compression automatically and prevents the impact of that expansion just like compressing prismatics does."
The physical difference, cylindrical vs. prismatic LiFePO4 cells is important. Many do not compress their prismatic cells. Cylindrical cells do not require compression because they are bound by a rigid metalcase. Cylindrical cells are metal cased, they shed heat better than non metal cased prismatic cells. These factors change the charging profile choice for the different configurations regarding long term use.
 
. These factors change the charging profile choice for the different configurations regarding long term use.
However nothing you said provides evidence that the form factor makes a difference in the life of these cells if one compares the exact same chemistry. The LFP cylindrical cells from Headway could take a higher charge and discharge rate than the EVE prismatics I am using now. I do not know if they had a shorter life. They possibly they had a subtle difference in the thickness of their anode or cathode material to achieve those C rates.

Do we know anything about the form factor Tesla is using for the LFP cells they are putting in some of their cars?
 
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However nothing you said provides evidence that the form factor makes a difference in the life of these cells if one compares the exact same chemistry. The LFP cylindrical cells from Headway could take a higher charge and discharge rate than the EVE prismatics I am using now. I do not know if they had a shorter life. They possibly they had a subtle difference in the thickness of their anode or cathode material to achieve those C rates.

Do we know anything about the form factor Tesla is using for the LFP cells they are putting in some of their cars?
No.
 
The better stop floating charge, or plug a high power electrical device to your inverter,such as fridge.Because its draw current is faster than the floating charger , the battery will not go to protection
 
The better stop floating charge, or plug a high power electrical device to your inverter,such as fridge.Because its draw current is faster than the floating charger , the battery will not go to protection
Don't entriely understand the comment.

But 'float charging'
has nothing to do with 'battery will not going to protection' as I understand systems.

Float charging will be controlled by the SCC float charge voltage setting. If that float charge is set to a low voltage say for 99-90% SOC voltage point (no load), there is really no float charging going on. 0.0 amps going into the battery, but any small loads on the battery will be suppied by the SCC at that float charge voltage. When the SCC can no longer supply the load demand (cloudy sky, more load on the battery) battery voltage may drops 0.10 volt below the float voltage setpoint and the SCC starts the Bulk/Boost CC /Absorb CV mode once again.Then returns to the float charge set point.

Battery goes into protection mode (BMS function) when any one cell pack exceeds a max voltage level or is less than min voltage level. set in the BMS.

So I don't understand the comment.
 
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So I don't understand the comment.
Same.

I think the intent was to put a load on it so the charger goes back into a bulk mode but depending on the settings this might not necessarily be a good thing.

Somewhat related:

I'll be doing some learning on this matter myself pretty soon if I can ever get my garage cleaned out enough to park the camper inside. For the time being I'm holding the pack at ~13.2 with the campers converter and until a significant load is placed on it, it'll stay there. This means the parasitic load draws the battery down to an acceptable (though not ideal) "storage" voltage as well.

Since I'm still likely to use the thing even in the winter I don't necessarily want to just disconnect the battery and let it sit either.

I still don't fully understand if holding it there at 3.25-3.3v/cell (less ideal) will be an issue vs just letting the thing sit and self discharge (ideal) while disconnected entirely but it's something I intend to look more into later, unless anyone has anything to share on that subject.

Once it's in my garage I can disconnect the shore power converter and use a programmable victron to see what the battery does in various voltage situations in 'storage' and whether I'll be able to hold it at ~50% soc ish then bump it up to full charge in the day or so before a trip.

I'll also then be able to keep the heater running in the battery, if I deem the power consumption worth it.

Otherwise if I don't like things I'll just disconnect the battery and only turn things on as needed manually while using the shore power to maintain my various radio and TV settings.
 
Here is my take. Some things I'm sure about, but there are a few things I am unsure about.

What I am sure about is that a float of 3.40V/cell works just fine, I use this all the time to provide load-support without wasting battery resources. The question comes down to what causes more wear? Floating the battery at 3.40V with almost no current going in or out, or allowing the load to draw the battery down and push it into bulk cycling a few times every day? I think floating (load support) is the better choice.

But now there is the question of load support. At 3.40V/cell, a modest load on the system will draw the battery down and if it goes south of around 3.35V then it will start pulling current from the battery. So the question is, is 0.05V a sufficient differential for the power source to be able to supply load support? In many cases the answer is NO, it is not. Heavy loads will begin to draw down the battery even if there is plenty of external power available. So a better 'float' voltage for load support is 3.45V/cell, not 3.40V/cell. With the float set at 3.45V/cell, a nominal load takes it down to roughly 3.42V/cell and a heavy load takes it down 3.35 to 3.375V/cell... still high enough that it doesn't pull power from the battery.

So in a 'UPS' style application I prefer setting the load support float to 3.45V/cell.

But that is where I become unsure. Is it ok to leave 3.45V/cell on a LiFePO4 battery indefinitely? That is what I don't know the answer to.

At the moment I am doing this on two Bluetti EB55's... I am driving the adapter input with a 24V current-limited power supply (limited to around 7.2A or 175W). The actual supply output is 24.2V (3.457V/cell, the EB55 is 7s I think). But the system has a continuous load of around 100W and under load I measure around 24.02V on the adapter input. So this would really be a cell voltage of 3.43V. I am fairly confident that 3.43V/cell is 'ok' to leave on 24x7x365 days a year. But I am not 100% sure.

Obviously holding higher cell voltages such as 3.50V or 3.55V/cell is a bad idea. Similarly, holding 3.40V/cell should be fine. The question is... what about slightly higher voltages in the 3.41-3.45V/cell range?

There is also the question of the BMS not balancing the cells at those voltages, but LiFePO4 holds its balance pretty well so in my case I just do a full charge every 6 months or so (that's the plan anyway) to keep the cells in balance.

--

There is a similar argument for non-continuous / non-UPS use. If you have a solar-only system and want to take advantage of load support, the solar system will be charging the pack at 0.2C or lower (most likely) so there is no need to set a bulk voltage higher than 3.50V or so as long as the BMS does some balancing. We again want a float voltage that will properly load-support the pack. And again the appropriate range is 3.40V to 3.45V. In this situation due to the daily cycling I don't see any problem with floating at 3.45V from the solar system.

One thing I do take issue with is trying to bulk or float at voltages below 3.40V. If the battery is mostly full, then bulking to 3.35V to 3.40V will do a reasonable job keeping it full. But if the battery becomes depleted then bulking to 3.35V or 3.40V is going to leave a LOT of solar power on the table. The battery just won't get charged up by the solar system! The solar system will limit the current too much. So while a bulk of 3.40V might appear to be fine, it is NOT fine for charging a depleted battery pack. It is not a good target voltage for the Bulk stage.
 
The Full Voltage Range of LFP = 2.50-3.650
The Working Voltage Range - 3.000-3.400 with the nominal voltage being 3.200 (ie 50%)
The 10% from the Bottom & TOP of the Full Range is essentially, that 10%-90%. The Cliff Climb & Fall.
Remember the Voltage curve is Very Flat unlike most other chemistries.
Also note that the "Actual Deliverable Amp Hours" is to be delivered FROM the working voltage range and not the Full Range. This will vary on the grade & condition of the cells used.
Float is the Variable current provided to saturate the cells until IR from SOC reduces it to a trickle.
There is ABSOLUTE no harm to "working" cells are stored @ the top of the Working Range !

Bulk & Absorb to 3.425 and float at 3.420 to saturate cells giving you the full Working Voltage (including the result of the Voltage Drop post charging).

Float can & will ramp up to service the Draw Needs as required but limited to the available solar energy being produced at the time and will only draw from the batteries IF more is required.

FYI per virtually every LFP battery maker: Constant Current can only get cells to 95%. To top & Saturate the cells to 100% at that Voltage the rest is Handled by Variable Current which IS FLOAT.

Hope it helps, Good Luck.
 
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The fully charged _resting_ voltage of LFP is 3.4v per cell. Yes, you charge to 3.65, but after resting a while they come back down to 3.4v Anything over 3.4v will eventually overcharge them if left. So absolutely do not float higher than 3.4v.

Technically, LFP should not have any float stage at all. Float is entered AFTER the battery is FULLY charged. Lead batteries need a low trickle charge to hold them at 100% or they will self discharge. For lead, the float voltage is set slightly *higher* than the fully charged voltage. Because most chargers, even those with an LFP profile, have a float setting, for LFP the float voltage is set *lower* than the fully charged voltage. That effectively disables float, but allows the charger to supply loads instead of the battery.

I float at 3.35v on my solar controller. Yes, the batteries will discharge very slightly before the solar completely powers the load. But it is bad for LFP to hold them at full charge anyway. Even if you are storing them disconnected from charging, they should not be left at full charge. Once the cells discharge to the float voltage (which is still 99% charged) then the solar does completely power the load, and the batteries only helping if there isn't enough sun.
 
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