Lithium experts out there (?) ... Battery storage charging question/concerns

[twvette]

Well, they day has finally come where was just a little more (actually cheaper per useable battery capacity) to go lithium/LiFePO4 than replace my old oversized Trojan 6V's. Its been a journey with a residential electric refrigerator doing 95% boondocking and I have an old long post on this. So glad to be done with 400lbs of batteries!!!

Anyways, I have got my Magnum Charger/Inverter setup per the proper recommendations from various sources including Magnum support. Looked into this extensively but in the end it was very simple. Here is the settings Magnum said were the only critical ones ...

Battery type: Custom (I don't have the newer model that has a lithium setup pre defined but Magnum said does same thing ... don't use CCV type as know to have some issues with lithium)

AmpHour Size: Set to battery bank size at 12V (I have 2x300AH batteries = 600AH)
Max charge: 80ADC (I have a 100A charger but prefer to not max it out and my batteries can handle 200A each for 400A total so well within)
Absorb: 14.4V
Absorb time: .1 hr (minimum setting it will do)
Float: 13.6V
EQ: 14.4V (as low as it will go as minimum is tied to the Absorb setting .. EQ is generally never done on lithium so this is just protection incase accidentally done)
Final Charge Stage: Silent

My concern is the general info I have learned about lithium is that it will hold a charge for a very long time and that not ideal to hold it close to 100% all the time. Better to charge it up, let it drift down and then recharge rather than float/absorb per my understanding. Frankly, still confused on what exactly Float/Absorb settings are really doing. I have the battery monitor kit and looking at the voltages over a 24 hour period my minimum is about 13.25V, maximum is about 14.85, and its typically sitting in Silent charge mode at around 13.4 to 13.2. The 14.85 is also beyond the 14.6V maximum I see in literature from the battery maker so concerned (although its apparently +- .2V per battery literature so perhaps ok). I just don't understand how it is doing this with the above settings and would'nt it make more sense to let it drift down to say something like 12.0V (battery literature says minimum discharge voltage is 10.8 and protects itself at 10V)?

I have read about charging profiles on some other Lithium chargers with a dedicated lithium mode and seems all might recharge every 24 hours regardless which appears the Magnum is doing also and seems strange to me for storage purposes. Seems more like what you would want with say your solar charger to do while it is in use use to keep it topped off for the night time. Any knowledgeable input appreciated. Just don't want to burn these up in storage as worked super good my first trip out.

I store in my garage so I keep it plugged into 120V and have about a 2A VDC draw. Don't really want to shut my 12V off or use a separate charger but perhaps an option.

[snoobler]

No value in "micro cycling" the battery vs. storage at a constant SoC.

Key factor is heat. If you're storing in a mild/cool/cold environment, very little degradation occurs at high states of charge.

Most cell manufacturers recommend a storage SoC between 30-70%, and they also indicate a full charge/discharge cycle every X months (3-6 typically).

I have personally stored individual LFP cells for a year in my home (70-80°F) at full charge. They retained 95%+ of the prior charge and re-charged to the same level with >1% degradation in capacity. These were premium CALB 40Ah cells.

Simple option is to reduce float to 13.1-13.2V and disable any periodic equalizations. With a DC load, the SoC will drift down into the mid-range and hold there.

For normal operation, if most of your charging is done on shore power or solar, 13.8V absorption can notably reduce stress on the cells while still achieving 98%+ SoC, but this will require a substantially longer absorption time (5-6 hours). With generator charging 14.4 is optimal for the minimized run-time.

[twvette]

Thanks much, will try a bit lower float when store it.

I have just under 1000W solar and usually do get long periods of solar so might also bump the solar charger absorb down to the 13.8V.

Is the "mid-range" considered like 13.4V? Have been a bit confused here too as I know they are 12V batteries and when I received them they were at 12.8V. Not sure if the 14.4-14.6V is just considered a "charge" voltage and they then drop down to the mid 13's when disconnected from an active charger. Never got a chance to look at this before connecting my load but I assume they don't ship fully charged.

[snoobler]

Originally Posted by twvette

Thanks much, will try a bit lower float when store it.

I have just under 1000W solar and usually do get long periods of solar so might also bump the solar charger absorb down to the 13.8V.

Is the "mid-range" considered like 13.4V? Have been a bit confused here too as I know they are 12V batteries and when I received them they were at 12.8V. Not sure if the 14.4-14.6V is just considered a "charge" voltage and they then drop down to the mid 13's when disconnected from an active charger. Never got a chance to look at this before connecting my load but I assume they don't ship fully charged.

With good solar, the longer absorption time is rarely an issue as the solar will still provide any needed loads it can while the battery is charging. If you find the 13.8V doesn't get you adequately charged daily (not due to excessive use), then increment it by 0.1V until you get there.

You will want to keep an eye on your absorption time. You'll want your absorption time to allow the charging current to drop to around 20A at termination.

LFP voltage is "mushy" when trying to equate voltage to state of charge. The aforementioned CALB cells were all at 3.300V after sitting a year, and they all yielded 95% of their charge. That would be 13.2V on a 12V battery.

Difference cell size/types/construction/manufacturers all behave a little differently. Some will settle to lower voltage very quickly, and others will take their sweet time and hold 13.6V for months. This makes it's hard to generalize.

The tiny current drawn by the BMS can also pull the SoC down over a long periods of time, and it's why many manufacturer's recommend, "charge every X months" to provide CYA margin against excessive discharge.

Mid-range is even more of an issue due to the crazy flat discharge voltage of the batteries where a 0.1V difference can represent a 30% difference in the state of charge.

NEW large LFP cells ship at about 3.290V and at 30-50% SoC.

My goal with 13.1-13.2 puts the cells at 3.275 - 3.300V, which gives you confidence that they're somewhere in the mid-range.

[twvette]

Thanks again, I will try and get some further info from the battery manufacturer to see if can better nail down at what voltages likely have what State of Charge.

It dawned on me there is also a ReBulk setting that I played with before and recalled from setting up my solar charger, but interestingly this did not come up in my conversation with Magnum support. Dove a bit deeper into user manual and its looking like the Magnum recommended Silent mode really does not have a Float stage and that seems to make more sense with what I have seen. Here is the info on it:


Thinking perhaps ReBulk might be good to set it it at something between 12.8 to 13.1 for storage. Had 13.2 previously. Its kinda a guess that 12.8 might be the battery manufacturer low voltage they like to store them at since both mine were at this when received. I changed it to 12.8 and will see what it does overnight but not sure they will go down that low for a while as the lowest in use I have seen so far is 13.0V in use with a lot of draw between any charging.

[snoobler]

Is there a different option for the final stage? Can you link me the manual?

[twvette]

yes, there are different final stage modes but Magnum support said Silent is best suited for lithium. Here is the manual:
https://www.magnum-dimensions.com/si...ME-ARC_Web.pdf

[snoobler]

Originally Posted by twvette

yes, there are different final stage modes but Magnum support said Silent is best suited for lithium. Here is the manual:
https://www.magnum-dimensions.com/si...ME-ARC_Web.pdf

They are just wrong. There are many misconceptions about LFP based on "they don't need to be floated." When this is taken as gospel, it can cause problems. Most manufacturers of hardware apply this across the board, and it's the official line almost every time. Few know enough to ask, "what's your application and usage profile."

The reality is that, "it depends."

If you're using it in a cyclic power system (off-grid solar), and you don't set a float voltage, once charged, the solar will not power any of your loads, and the battery will discharge until a re-bulk voltage is hit. This may cause multiple cycles in a day and finish the solar day with a lower state of charge than you would have if the solar had been able to float the battery and power the loads all day.

In your situation, you need a float since you will have some 12V loads.

Change to float and set to 13.1-13.2V.

[xrated]

Originally Posted by snoobler

They are just wrong. There are many misconceptions about LFP based on "they don't need to be floated." When this is taken as gospel, it can cause problems. Most manufacturers of hardware apply this across the board, and it's the official line almost every time. Few know enough to ask, "what's your application and usage profile."

The reality is that, "it depends."

If you're using it in a cyclic power system (off-grid solar), and you don't set a float voltage, once charged, the solar will not power any of your loads, and the battery will discharge until a re-bulk voltage is hit. This may cause multiple cycles in a day and finish the solar day with a lower state of charge than you would have if the solar had been able to float the battery and power the loads all day.

In your situation, you need a float since you will have some 12V loads.

Change to float and set to 13.1-13.2V.

Good advice Steve.........

[twvette]

I see your point about wanting to Float a solar charger. But typically those are separate controllers as is mine. I am using an Outback FlexMax 80 for that while my Magnum MSH3012M Inverter/Charger handles the shore and generator power. My solar charger is setup in a float mode. My rig is stored in a garage so sees no solar in my storage situation.

I get that might need to re-consider my mode or settings when using it with the generator to ensure it delivers a full charge and will see what it does next trip. For now am just trying to figure out what is best for storage.

If helps any, I have had it in the Silent mode now for about 36hours with the ReBulk set to 12.9V and it is at about 13.2V now with just under 100amp hours depleted (had some lights on for a while yesterday) and reporting about 84% State of Charge (math makes sense).

Regardless, I think I agree with you. From the plots the Float stage looks more straight forward to just hold it at a reduced State of Charge rather than Bulk it back up 100% and then "micro cycle" it like you mentioned.
It seems the various voltage settings could also make the two modes do close to the same thing too for storage purposes but the settings would be a bit stranger in Silent mode.

[snoobler]

Originally Posted by twvette

I see your point about wanting to Float a solar charger. But typically those are separate controllers as is mine. I am using an Outback FlexMax 80 for that while my Magnum MSH3012M Inverter/Charger handles the shore and generator power. My solar charger is setup in a float mode. My rig is stored in a garage so sees no solar in my storage situation.

I get that might need to re-consider my mode or settings when using it with the generator to ensure it delivers a full charge and will see what it does next trip. For now am just trying to figure out what is best for storage.

If helps any, I have had it in the Silent mode now for about 36hours with the ReBulk set to 12.9V and it is at about 13.2V now with just under 100amp hours depleted (had some lights on for a while yesterday) and reporting about 84% State of Charge (math makes sense).

Regardless, I think I agree with you. From the plots the Float stage looks more straight forward to just hold it at a reduced State of Charge rather than Bulk it back up 100% and then "micro cycle" it like you mentioned.
It seems the various voltage settings could also make the two modes do close to the same thing too for storage purposes but the settings would be a bit stranger in Silent mode.
Attachment 37804

The silent mode will actually cycle them notably. You'll be pulling down to 12.9 under low current and doing a full recharge. Since you're "storing" the battery, it's more desirable to try and replicate the storage recommendations, i.e., 30-70% SoC.

This is a good exercise in determining your particular battery's state of charge to voltage relationship in this scenario. I would continue the discharge to ~50%, note the voltage and set float at or just under that voltage.

Alternatively, just continue discharging until you hit ~70% and disconnect the battery...

[twvette]

I got some feedback from the battery manufacturer that was interesting. Here is the summary:

- Recommended to bump up my float from 13.6 to 13.8V. This was to ensure have enough power for the RV. I still understand that doing a bit less is less stressful on the batteries if can get away with it. So far 13.6V has worked well for three trips.
- Change Absorb to 14.6V. They say that as the battery approaches full charge the voltage rises quickly, but it is not fully charged so does need a period of a high voltage absorb to truly top it off. I had plenty of juice my last trips going to just 14.4V which should also be a little less stressful on the battery but will keep this in mind.
- Best to keep it stored floating with power to it rather than cycling it. Confirmed better to not deep cycle it. No real need to deep cycle the battery if going to keep a charger on it even though their manual suggests this.
- It was still a bit unclear what the lowest float voltage they recommend is. They still say the "Discharge Disconnect Voltage" should be 10.8V and the "Low Voltage Warning Voltage" should be 11.4-11.6V. These don't seem to be true settings in the chargers I have. Think I will take this as never let the batteries get below the 11.4V (probably the 10% charge point) and damage occurs if get below 10.8V.

Decided to set my charger when in storage to a Float of 12.5V with the final sage being Float as discussed previously. It has been on this a couple weeks and it actually drifted down to about 12.7V after a couple days from the load I have on it and said was at 55% and then took the bulk of this time to finally come down more to the 12.5V and claims about 45% charge.

In piecing all info together I think will do the following if this might help someone else:

Magnum Charger - in storage (no solar)
Absorb: 14.4V (not really applicable as will just be floating in storage)
Absorb time: 1 hr (not really applicable as will just be floating in storage but same setting will use not in storage)
Float: 12.5V (keeps batteries at about 45% state of charge)
EQ: 14.4V (as low as it will go as minimum is tied to the Absorb setting .. EQ is generally never done on lithium so this is just protection incase accidentally done)
Final Charge Stage: Float

Magnum Charger - when in use for generator charging
Absorb: 14.4V (Safe setting...will change to 14.6V if need more capacity)
Absorb time: 1 hr (too long or too short?)
Float: 13.6V (Safe setting...will change to 13.8V if need more capacity)
EQ: 14.4V (as low as it will go as minimum is tied to the Absorb setting .. EQ is generally never done on lithium so this is just protection incase accidentally done)
Final Charge Stage: Float

Outback Solar Charger - when in use for solar charging but I wont change for storage as in a garage with no solar
Absorb: 14.4V (Safe setting...will change to 14.6V if need more capacity)
Float: 13.6V (Safe setting...will change to 13.8V if need more capacity)
Absorb time: 1 hr (too long or too short?)


Let me know if disagree with something above ...

[snoobler]

Originally Posted by twvette

I got some feedback from the battery manufacturer that was interesting. Here is the summary:

- Recommended to bump up my float from 13.6 to 13.8V. This was to ensure have enough power for the RV. I still understand that doing a bit less is less stressful on the batteries if can get away with it. So far 13.6V has worked well for three trips.

Most feedback is generic with no expertise for your particular application.

I would stick with 13.6V

- Change Absorb to 14.6V. They say that as the battery approaches full charge the voltage rises quickly, but it is not fully charged so does need a period of a high voltage absorb to truly top it off. I had plenty of juice my last trips going to just 14.4V which should also be a little less stressful on the battery but will keep this in mind.

This depends. If you're charging at 0.5C, yeah, you might need 30-60 minutes depending on the voltage drop between the inverter and the battery.

Lower current charging (~0.2-0.3C) rarely needs more than 15-30 minutes absorption.

I would stick with 14.4V for generator. 13.8V for shore/solar if you can charge for several hours.

- Best to keep it stored floating with power to it rather than cycling it. Confirmed better to not deep cycle it. No real need to deep cycle the battery if going to keep a charger on it even though their manual suggests this.

Yep.

- It was still a bit unclear what the lowest float voltage they recommend is. They still say the "Discharge Disconnect Voltage" should be 10.8V and the "Low Voltage Warning Voltage" should be 11.4-11.6V. These don't seem to be true settings in the chargers I have. Think I will take this as never let the batteries get below the 11.4V (probably the 10% charge point) and damage occurs if get below 10.8V.

They probably didn't understand the question.

Decided to set my charger when in storage to a Float of 12.5V with the final sage being Float as discussed previously. It has been on this a couple weeks and it actually drifted down to about 12.7V after a couple days from the load I have on it and said was at 55% and then took the bulk of this time to finally come down more to the 12.5V and claims about 45% charge.

IMHO, 12.5V is too low. that's less than 20% SoC. Recommend 12.8-13.0. You're likely seeing some error on the SoC computation. Anything in the 30-70% range is fine.

In piecing all info together I think will do the following if this might help someone else:

Magnum Charger - in storage (no solar)
Absorb: 14.4V (not really applicable as will just be floating in storage)
Absorb time: 1 hr (not really applicable as will just be floating in storage but same setting will use not in storage)
Float: 12.5V (keeps batteries at about 45% state of charge)
EQ: 14.4V (as low as it will go as minimum is tied to the Absorb setting .. EQ is generally never done on lithium so this is just protection incase accidentally done)
Final Charge Stage: Float

Magnum Charger - when in use for generator charging
Absorb: 14.4V (Safe setting...will change to 14.6V if need more capacity)
Absorb time: 1 hr (too long or too short?)
Float: 13.6V (Safe setting...will change to 13.8V if need more capacity)
EQ: 14.4V (as low as it will go as minimum is tied to the Absorb setting .. EQ is generally never done on lithium so this is just protection incase accidentally done)
Final Charge Stage: Float

Outback Solar Charger - when in use for solar charging but I wont change for storage as in a garage with no solar
Absorb: 14.4V (Safe setting...will change to 14.6V if need more capacity)
Float: 13.6V (Safe setting...will change to 13.8V if need more capacity)
Absorb time: 1 hr (too long or too short?)


Let me know if disagree with something above ...

Concerns noted above.

14.6V will never buy you much of anything, especially if you're absorption for 1 hour @ 14.4V.

13.6V for several hours will get you 95%+ charged.
13.8V for several hours will get you 98%+ charged.