Long term storage of Lithium Phosphate batteries

[harpo]

I have been reading up on these batteries because I want to replace my AGM's at the cabin (remote cold winter location). Wil recently reviewed a BMS with a low temperature cut off that actually works which is one concern I had. (I want to build my own battery). Another concern I had was long term storage. This was not much of a concern because I thought Wil indicated these batteries don't degrade as fast as a lead acid variety. Then I read on one solar site that these batteries should not be stored at full charge but something much less and, in the same light, they should not be subject to a float charge which, of course, is designed to keep the battery at full charge. I have only seen this information on this one particular site so I don't know if this is accurate or not. Anyone care to comment or have references to support or refute this information?

 

[Prefersdirt]

I don't have a link, but frequently mentioned here is long term they should be stored at about 50% charge. Another thing to consider for your situation is the cold -- how will you use them? They do not do well when they are below freezing.

 

[Jim Burrow]

I hope more people in the group chime in on your questions. I've heard the same time about not fully charging your Lithium batteries. But I'm not sure I want to believe that story. The reason is that hundreds of thousands of people now have all-electric vehicles including me. I have a 2016 Chevy Spark Lt2 ev. Everyone with electric vehicles recharges their Lithium battery to 100% full charge and most on a daily bases and it does no harm to the battery.

Why should it be just fine to recharge car Lithium batteries to 100% full charge and not the Lithium batteries that are used for Solar?

 

[RCinFLA]

High temp should be avoided. Keep under 30 degs C (under 86 degs F). The cooler the better for longevity,

They can sit for up to 12 months starting at 50% state of charge. Worse case leakage is about 3% capacity of cell per month, but typically 1-2%. Heat raises self discharge rate.

If using PV continuously during absense, set charge voltage to no more 3.35v per cell. Keeping full charge degrades but not nearly as bad as high temp. High temp and full charge is worse for aging.

Don't charge below freezing. Only takes one time to degrade cells. Don't discharge below -20C ( -4 F). Capacity will be reduced at cold temps.

 

[Prefersdirt]

Why should it be just fine to recharge car Lithium batteries to 100% full charge and not the Lithium batteries that are used for Solar?

The difference is long term storage. How many park their EV for a year? When I went overseas, I loaned my gas car out to a buddy so it would not just sit. I would do the same with an EV.

 

[harpo]

Thanks to the guys that responded. Hoping more chime in with time.
To clarify, this is a cabin that will be used up until about December 20 and then not until late April or early May. It will get well below freezing. During the summer the cabin is used sporadically but even when used the electrical demand will be small and of a short duration. Temps inside the cabin during the summer could reach 100F.
This primarily a hunting cabin used pretty much Oct. 1-Dec. 20 and then April 25 - May 30. The rest of the time the cabin sits empty.
There was a suggestion regarding removal of the batteries when not in use. I agree that would be ideal but we have 10 members who come and go at will. If one member takes the batteries with him and a member decides to visit the cabin for an evening or two they will have no electricity for them to use and, of course, they will NOT be happy. So, the batteries pretty much have to stay.
The small (220Ah) solar system is about 10 years old. We are on our second set of Trojan AGM batteries. They have worked "ok" but we can only use half of the 220Ah and the cycle life is short compared to the Lithium chemistry.
So, now knowing all of this what direction would your plan of action be? Say with AGM? Go as planned with Lithium Ion Phosphate----if so, what precautions on how to use them in our situation so that we get good longevity?
Many thanks for your suggestions.

 

[Prefersdirt]

Can you put the batteries deep in the ground (below frost line)? That would keep things reasonable year round.

 

[harpo]

Can you put the batteries deep in the ground (below frost line)? That would keep things reasonable year round.

Kind of novel idea about underground storage. Only one problem or I might try it, water table is way high around the cabin and we are on the "high ground"! To give you an idea; the outhouse hole usually has so much water in it sometimes your bottom gets splashed.

 

[Prefersdirt]

High water table does make things interesting. Someone here will have an idea. I am wondering if you could put a small plastic tank down and cover it. Have to do some math to make sure it is not buoyant. Partially bury it and cover it with 3-4 feet of dirt (make a large mound)? Will have to think on this one.

 

[Deleted member 9967]

Lifepo batteries should never be floated for more than a week at full charge as they can be damaged by this.
For storage. Every recommendation I have seen says between 3.3 and 3.4 volts per cell for long term storage like six months to a year.

As for the effects of cold on discharging.
At minus 20 C it will only have 57% of its capacity. However, if you let the battery warm up to 20 C. Then it will recover and give you full capacity once again.
At minus 10 you will get only 68% of capacity.
And at 0 C, 82 % capacity.
At 40 C you go down to 95 % of capacity. So avoid heat as heat can kill a battery. Where as cold slows it down but the battery can recover from it.

Warm the batteries up before charging them however. Zero is safe to charge at but use fewer amps when doing so. At least 10% or less of the cell volume. For example a 100 Ah cell should not be charged at more than 10 amp when at Zero degrees Celsius.

Hopefully this is helpful.

 

[JoeHam]

For the EV comparison I would note that is a completely different lithium chemistry and I would suggest is not a good analogy.

In my mind the only hard rule is to not charge below freezing.

Best practice is to store LiFePO4 around 50% charge, which is hard to determine accurately without coloumb counting.

To store at full charge will shorten the amazingly long life span of theses batteries somewhat.

True confession: I have a couple LiFePO4 batteries that I have stored for months at a time per year at full charge for 5 years running. The first few years I didn’t know any better and now I feel I have better things to worry about ?

Undoubtedly this will cost me some amount of lifespan but life goes on.

My newer bigger (more expensive) batteries are benefiting from what I learned on the older ones and will be treated better.

 

[harpo]

Lifepo batteries should never be floated for more than a week at full charge as they can be damaged by this.
For storage. Every recommendation I have seen says between 3.3 and 3.4 volts per cell for long term storage like six months to a year.

As for the effects of cold on discharging.
At minus 20 C it will only have 57% of its capacity. However, if you let the battery warm up to 20 C. Then it will recover and give you full capacity once again.
At minus 10 you will get only 68% of capacity.
And at 0 C, 82 % capacity.
At 40 C you go down to 95 % of capacity. So avoid heat as heat can kill a battery. Where as cold slows it down but the battery can recover from it.

Warm the batteries up before charging them however. Zero is safe to charge at but use fewer amps when doing so. At least 10% or less of the cell volume. For example a 100 Ah cell should not be charged at more than 10 amp when at Zero degrees Celsius.

Hopefully this is helpful.

Thank you for this information.

 

[Deleted member 9967]

Thank you for this information.

Anytime. That is what this forum is for.

 

[SomebodyInGNV]

Related to this issue, I'm installing 2 12V 100A LFP batteries in my travel trailer, wired in parallel. They'll be connected to an MPPT SCC.

Is there an easy way to change the charge parameters so the batteries are charged only to 50%-75% for storage. It may be unused for months at a time and it sounds like storing for a long time at a high SOC is not a good idea. This is in Florida so cold temps aren't an issue. High temps might be.

I'll configure my Epever Triron SCC as recommended by the battery manufacturer, SOK. That will result in a 100% SOC. But how do I change that to charge only 50-80 on the trip home? I know I can just turn off the breaker between the SCC and the batteries but I can't always monitor that closely and would rather not have to bother.

An online acquaintance has a setup involving the relay on a Victron SCC, a PD9160AL converter, a "shutoff module" that plugs into the converter, and an inline SPST switch that disables the shutoff module. I don't fully understand the details and can't replicate it exactly because I don't have the same equipment. In a nutshell, the SCC relay is set up to stop charging when the battery is at 90% SOC and start when it reaches 40%. The SPST switch disables the cutoff when camping, so it charges to 100%. When ready to return home, turn the switch back on and it only charges to 90%.

Is there a way to accomplish something like that by revising the charge parameters on the SCC? Is it easy to toggle back and forth between the two states? Does this even matter?

 

[HaldorEE]

Related to this issue, I'm installing 2 12V 100A LFP batteries in my travel trailer, wired in parallel. They'll be connected to an MPPT SCC.

Is there an easy way to change the charge parameters so the batteries are charged only to 50%-75% for storage. It may be unused for months at a time and it sounds like storing for a long time at a high SOC is not a good idea. This is in Florida so cold temps aren't an issue. High temps might be.

I'll configure my Epever Triron SCC as recommended by the battery manufacturer, SOK. That will result in a 100% SOC. But how do I change that to charge only 50-80 on the trip home? I know I can just turn off the breaker between the SCC and the batteries but I can't always monitor that closely and would rather not have to bother.

An online acquaintance has a setup involving the relay on a Victron SCC, a PD9160AL converter, a "shutoff module" that plugs into the converter, and an inline SPST switch that disables the shutoff module. I don't fully understand the details and can't replicate it exactly because I don't have the same equipment. In a nutshell, the SCC relay is set up to stop charging when the battery is at 90% SOC and start when it reaches 40%. The SPST switch disables the cutoff when camping, so it charges to 100%. When ready to return home, turn the switch back on and it only charges to 90%.

Is there a way to accomplish something like that by revising the charge parameters on the SCC? Is it easy to toggle back and forth between the two states? Does this even matter?

I would suggest turning off the charge sources and run the batteries when you get home to 50% of capacity. Do you have a battery capacity meter in your system? Why not, they can be had for not a lot of money. Even the very top of the line Victron BMV-712 is only $200.

 

[SomebodyInGNV]

I do have a battery monitor. I guess reducing the SOC by consuming power is probably the easiest way.

 

[HaldorEE]

I do have a battery monitor. I guess reducing the SOC by consuming power is probably the easiest way.

That is my plan. Turn off the charge sources on my way home and run the lights/fan until I get close to 50% SOC.

Now that you mention this, one cool idea would be to add a circuit that disconnects at 50% SOC (as measured by my battery monitor) and power a resistive heater with it.

 

[SomebodyInGNV]

... Now that you mention this, one cool idea would be to add a circuit that disconnects at 50% SOC (as measured by my battery monitor) and power a resistive heater with it.

I have an automatic transfer switch. Depending on whether or not I need fully charged batteries at my destination, I plan to test using my inverter to run the two-way refrigerator on AC. That way I won't have a propane flame while driving. If I do that on the way home, it will reduce the SOC.

I have no idea how to build in the automatic cutoff.

 

[HaldorEE]

I have an automatic transfer switch. Depending on whether or not I need fully charged batteries at my destination, I plan to test using my inverter to run the two-way refrigerator on AC. That way I won't have a propane flame while driving. If I do that on the way home, it will reduce the SOC.

I have no idea how to build in the automatic cutoff.

This is how I would do it.

Victron Direct

OK, it has been hours since I stirred the pot. I was looking at another thread and once again was wishing that the BMV-712 had an extra relay output. Got me to thinking. The BMV-712 communicates using Victron.Direct and the communication protocol for Victron.Direct is pretty simple...

diysolarforum.com

 

[JAS]

Just noticed that Battleborn recommends charging to 100% and then disconnecting for long term storage...