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locked Batteries starting to get weak - MMM - What to do -


Scott SV Tengah
 

Oliver, that has not been my experience. Perhaps Victron makes bad internal cell-cell connections but I surmise it might have to do with my keeping the Amel configuration of 12v batteries paired up in serial to create 24v and paralleled to create the total AH. This is useful in case your starter battery dies and you need to borrow a 12v battery to start engine/genset. I opted to retain that redundancy.

To my knowledge, BMS systems do not typically balance between serial pairs of batteries. As such, I've had a few cases of meaningful imbalance in the last 2 years of operation. By meaningful, I mean high enough to cause a cell to go over the cutoff threshold and cause the BMS to tell the chargers to stop charging.

Note that these voltage imbalances didn't show up unless I charge to 90% SOC or greater, because of the shape of the lithium voltage curve.

I agree that active balancing doesn't seem to provide any benefit and is of questionable utility.  

--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com


Oliver Henrichsen, SV Vela Nautica
 

Hello,

Good BMS systems measure 0.001 Volt with +/- 2 exact. 

Balancing is over evaluated in the thread.
A good batterie setup stays balanced for years after initial balancing if needed at all. You may check on it from time to time but a good system will hand a banalce Alarm anyway.
Active balancing has no real advantage agains passive balancing. It only reuses a tiny bit of energy instead of dissipating it.

Oliver from Vela Nautica 
A54#39 
Martinique 

On Wed, Sep 23, 2020, 18:18 Arno Luijten <arno.luijten@...> wrote:
Hi Scott,

Don't get me wrong. I'm not against Lithium. I'm just trying to bring some nuance to the many über-happy stories about Lithium. These things are bloody expensive and people should know what they are getting into.
Actually I'm really happy that you bought Lithium because that means you can tell me about how this works in the long term 😎.

In the electric car world you see similar discussions about charging speed and charging level. People are now starting to find out that these batteries are degrading much faster when you are a super-charging junkie and keep pushing it to 100% SOC
So if you buy a used Tesla from a predominately home charging person that doesn't use his car as racing vehicle you are much better off then when you buy a mostly supercharged race-car. There is a reason Tesla keeps all these metrics on their cars.
Latest EV cars are charging with more then 2C (250 kW) using water cooled charging cables. I really wonder how that will work out on the long term.

Anyway I don't want to talk anyone out of Lithium and if my use case was different I probably would have bough an all singing and dancing Lithium pack by now. But I would expect a 8-10 year lifespan of it.

Regards,

Arno Luijten
SV Luna,
A54-121


Arno Luijten
 

Hi Scott,

Don't get me wrong. I'm not against Lithium. I'm just trying to bring some nuance to the many über-happy stories about Lithium. These things are bloody expensive and people should know what they are getting into.
Actually I'm really happy that you bought Lithium because that means you can tell me about how this works in the long term 😎.

In the electric car world you see similar discussions about charging speed and charging level. People are now starting to find out that these batteries are degrading much faster when you are a super-charging junkie and keep pushing it to 100% SOC
So if you buy a used Tesla from a predominately home charging person that doesn't use his car as racing vehicle you are much better off then when you buy a mostly supercharged race-car. There is a reason Tesla keeps all these metrics on their cars.
Latest EV cars are charging with more then 2C (250 kW) using water cooled charging cables. I really wonder how that will work out on the long term.

Anyway I don't want to talk anyone out of Lithium and if my use case was different I probably would have bough an all singing and dancing Lithium pack by now. But I would expect a 8-10 year lifespan of it.

Regards,

Arno Luijten
SV Luna,
A54-121


Scott SV Tengah
 

Arno,

My Victron BMS reports cell level voltage down to 0.01v. I can view it via bluetooth and have seen it vary by 0.01v.  I can attest that when I had an imbalance, up to 90% SOC, the cells had the exact same voltage, down to 0.01v. Above that, the variance reared its ugly head. This is not theory - this is my experience living with this system fulltime for 2 years. So I would be very skeptical of any active balancer that has the same resolution as you may be unknowingly exacerbating an imbalance by relying on marketing promises.

If I haven't stated before, I do agree that if you want a system that requires no planning and very little monitoring aside from making sure you don't go below 50% SOC, then lead batteries are fine. If I were in your position and spent most of my time plugged into marinas, I would probably NOT get lithium even if someone paid me to take it. 

One guy may have had his gel batteries last 9 years, but longevity is not the main reason we got lithium. As we speak, I am running the dishwasher and washing machine and the admiral just turned on the microwave - all on battery/inverter. The continuous draw is around 160amps and the voltage is quite stable. Because of the huge boost in charge efficiency compared to lead, I won't be running the genset to recharge the batteries, either. Our solar is more than enough.

But as economists like to say - there is no free lunch. Lithium requires planning and thinking prior to implementation and if you plan it right, it is set it and forget it. I am not quite there but moving in that direction slowly. The same thing applies if you compare roller electric in-mast furling vs. manual stacking mainsails. The former requires some thinking but, at least to me, is well worth the effort. For my use case, lithium is the same. 

I hope my contribution to this forum is useful in that those who are off-grid and would rather not run the genset all the time are given some useful info that they can use to plan their systems. It is beyond my risk tolerance to try to assemble battery cells and rely on them in the middle of the ocean, so I went with a mostly-one-brand solution that allows me to monitor every single of the 24 cells via bluetooth and if a cell fails, quickly remove that battery pair and continue on with my trip. Luckily I haven't had to do that yet.

On Wed, Sep 23, 2020 at 9:34 AM Arno Luijten <arno.luijten@...> wrote:
Hi Scott,

Typically good BMS-es can measure at 0.01 resolution. So even at lower SOC you will be able to see differences. Charging up to 100% can be postponed longer with continuous balancing. Active balancing means you are using the charge of the fuller cell to charge the less full cell. That improves the speed of balancing and prevents useless heat generation.
Obviously it's not as world of difference all together but if you design a BMS for lithium you may as well make it top notch. The system is expensive enough to push it to the limits of longevity.
Your battery-monitor has a easier life with lithium as the charge/discharge losses are far less so it will keep an accurate reading longer, especially if you can set the Peukert value yourself.

If you are less concerned about those things than I think the Battle Born 24V batteries are a good choice, just do not expect 10-15 years of life. If you do achieve 10-15 years you can always celebrate at that time, but at least you will not be disappointed.
As said before and as said by others as well; there is still so much development in this area that I prefer to wait a few years to see what the next best will be. I just read something about a person managing 9 years on some Gel batteries so it's not that all other chemistries are useless.

Regards,

Arno Luijten
SV Luna,
A54-121


--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com


Arno Luijten
 

Hi Scott,

Typically good BMS-es can measure at 0.01 resolution. So even at lower SOC you will be able to see differences. Charging up to 100% can be postponed longer with continuous balancing. Active balancing means you are using the charge of the fuller cell to charge the less full cell. That improves the speed of balancing and prevents useless heat generation.
Obviously it's not as world of difference all together but if you design a BMS for lithium you may as well make it top notch. The system is expensive enough to push it to the limits of longevity.
Your battery-monitor has a easier life with lithium as the charge/discharge losses are far less so it will keep an accurate reading longer, especially if you can set the Peukert value yourself.

If you are less concerned about those things than I think the Battle Born 24V batteries are a good choice, just do not expect 10-15 years of life. If you do achieve 10-15 years you can always celebrate at that time, but at least you will not be disappointed.
As said before and as said by others as well; there is still so much development in this area that I prefer to wait a few years to see what the next best will be. I just read something about a person managing 9 years on some Gel batteries so it's not that all other chemistries are useless.

Regards,

Arno Luijten
SV Luna,
A54-121


Scott SV Tengah
 

Does anyone understand "continuous cell balancing", because I don't?

When I have had unbalanced batteries, the batteries are look perfectly balanced until you get to 90% + SOC. This is because of the flat voltage curve of lifepo4. Above that, the imbalances show up. How does an active balancer reliably detect an imbalance continuously when things look perfect below 90%? Through coulomb counting to determine amps out to see if the imbalance shows up during discharge? That wouldn't be reliable as cells age at different rates. 

Note that imbalanced batteries generally just reduce your total capacity until the imbalance is corrected. This is because if one of the 4 cells in the battery see an overvoltage condition, the BMS should stop charging altogether, even though the other cells could still accept more charge. Charging is restarted once the overvoltage cell drops below the high voltage threshold, either through a resistor to draw charge from it or a method to transfer that excess voltage to the other cells. Then the charging resumes. The charging will start and stop multiple times as during the cell balancing process. I have seem this before on my Victron setup and have been assured by Victron techs and my own research that it is totally normal.

The desire for active cell balancing seems a bit misplaced. Even if somehow they detect cell imbalances when there is no voltage difference, you still need to fully charge once in a while to reset the battery monitor as I believe all battery monitors use (amps in - amps out) to determine SOC and as batteries age, that needs to be recalibrated if you want any sort of accuracy. So solving the former problem still leaves the latter.


--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com


Denis Foster
 

Hello,

Just had Flash battery Kaiken that does continuous cell balancing... but they don’t do batteries for yachts and private owners just for industrial companies.

so that s out for us.

regards

Denis


Arno Luijten
 

Hi Paul,

This Best Lithium BMS still uses passive balancing and it does not say if it only does top-balancing. So from that perspective it's not a great BMS.

Arno


Denis Foster
 

Thank s Paul,

i have already seen this Swedish company. Very interesting in the DIY Winston Thundersky  cell assembly. They will balance the cells before delivery that are from the same batch. Looks like a more cost efficient alternative to Lithionics.

The website is more in Swedish than English.

They have to 24v 400Ah cell packs One with 8 cells the other with 16 cells not as tall.

one interesting features seems to be in case one prismatic cells fail apparently they can be exchanged at moderate cost and the whole setup can be rebalanced. Maybe more useful than a theoretical 10 year guarantee.

progressing towards the truth of lithium and following Oliver of Nautica Vela who has engineering skills I lack.

regards

Denis

 


Elja Röllinghoff Balu SM 222
 

Hi Denis
i agree with you , the last 8 weeks have been dealing with Licium batteries , but it was not happy that too many complex questions are still open to me ,
then I have co2 and carbon
batteries are busy, not matured on sailing boats at the moment

Now the decision is probably on AGM Victron super cycle with it I get to the SM battery compartment about 580 AH /24 V

ich hoffe damit die nächsten 5-10 Jahre zu überstehen und dann schaue ich mal neu

Greetings Elja
SM Balu
222

Von meinem iPhone gesendet


Paul Osterberg
 

I looking into this BMS which I will probably go for, expensive but I got confidence in my contact, he provided a lot of valuable information 
http://www.bestlithiumbattery.com/
Paul on sykerpa SM 259 


Denis Foster
 

Hi Arno,

The Kaiken Flash seems to be permanent equalizing.

That said sometimes I have the same feeling as you about Lithium. The maturity for the cruising yacht market doesn t seem that established leading to complex not fail safe systems probably impossible to fix in remote places.

For the moment we have a system that works with Gel batteries (9 years old) religiously maintained and strangely still working. I haven t excluded the possibility of replacing them by the same brand and model. I was also attracted by the Firefly carbon foam AGM. And spending the money saved on a high quality Solar installation that would be ready when Lithium will be at maturity for our specific needs. I know the industrial research on energy storage by Lithium batteries is very active now and the results will pass one day to our little niche market.

Robust, reliable and easy to fix systems have there attraction....

Regards

Denis


Arno Luijten
 

Hi Dennis,

This is one of the problems I have with many BMS systems, if not all. They use passive top-balancing, meaning they can only balance the cells when full and do this by dissipating the energy of the fullest cells.
It seems that the one you found is more clever. It does not say if it uses active balancing as far as I can see.
Some time ago I found this system that does it all: https://enerstone.fr/en/
However this also needs some additional logic to protect the batteries for under/over voltage. I sort of gave up on the Lithium stuff because if you want to do this right it is immensely complex and you will have to come up with a bespoke solution that I find undesirable on a sailing yacht.
Fact is that the market for lithium systems on yachts is quite small compared to other markets so not many companies develop specialized solutions for it.

Regards,

Arno Luijten
SV Luna,
A54-121


Denis Foster
 

Hello,

During research I found this particular cell balancing technique of FLASH Battery in Italy. Seems to have a lot of industrial applications/

What do you think of this .

Regards

Denis
 
 
 
 

FLASH BALANCING SYSTEM

 
 
 
 
 
 

The exclusive Flash Battery electronic balancing system is called the Flash Balancing System.

Traditional balancing systems apply resistance to the cells with the greatest charge in an attempt to “lower” them to the level of the lowest cell.

Although it’s a simple and economical method, it’s also extremely slow: 4 to 8 hours on average are required for balancing alone (in addition to the earlier charging step).

OUR FLASH BALANCING SYSTEM OFFERS MUCH HIGHER BALANCING POWER THAT IS APPLIED NOT ONLY AT THE END OF THE CYCLE, BUT ALSO ACTIVELY DURING DISCHARGE

This results in 20 times faster balancing without increasing overall recharge time to a significant extent.

Traditional balancing systems require a much longer time, so when they’re interrupted because the battery needs to be used, the result is a gradual reduction in available energy over time: vehicles and machines thus lose range at every cycle.

THE ULTRA-FAST BALANCING TIME OFFERED BY THE FLASH BALANCING SYSTEM DURING BOTH CHARGING AND DISCHARGING ELIMINATES THIS PROBLEM AND MAXIMISES RANGE.

If unexpected repairs are need, the Flash Balancing System makes it possible to add or replace cells throughout the life of the battery, which overcomes the limits of traditional systems in keeping the higher and lower-performing cells balanced.


Scott SV Tengah
 

Joerg,

As I mentioned before, I believe, it is recommended (and I do) charge to full once a month. That resets the battery monitor to 100% and also balances the batteries since for every battery I know, balancing only occurs when nearly full. That has to do with the very flat voltage vs. SOC curve. There's bottom balancing, too, but that's beyond my comfort level.

There is a significant difference between charging full once a month and keeping it full. The former is recommended, the latter is generally accepted to be detrimental with no known benefits. 

--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com


Joerg Esdorn
 

An interesting tidbit from the manual for my MV batteries. One of the „events“ causing an alarm on the Masterbus system is that the batteries HAVE NOT been fully charged for a period - default 31 days:  „ Last time fully charged serves as an event source. To prevent damage the batteries must be charged to the full 100% regularly. It is a common misconception that Lithium- Ion batteries should not be fully charged.“   


Joerg Esdorn
Amel 55 #53 Kincsem
Vigo, Spain


Scott SV Tengah
 

I used to own and race BMWs and was a big fan of them.

Previously, they recommended oil changes every 3,000 miles. When they started including free oil changes, suddenly their recommendation was that you could go 20,000 miles or more between oil changes. Nothing changed in the engine nor the oil they used.

From a business perspective, it makes complete sense. Since BMW started including the oil changes, they rationally, as a business, determined that they could save money by only giving you 2 oil changes before your warranty ran out. Any damage due to extended oil change intervals would probably occur after that. When the customer was paying for oil changes and they wanted to protect their reputation, you were supposed to change the oil 6-7 times as often.

If I was a profit maximizing business, I would want my batteries to last the warranty period + 1 day. Or for you to get enough loss of capacity that you buy new ones. They go out of business if their customers only buy once every 10-15 years.

For this reason, I rely on statistically valid research from people who are not conflicted to make my decisions. When I leave the boat, I setup a simple programmable relay that drives a load will keep the SOC between 40-70%. Sure, there's one guy who seemed to do ok (in 1 out of 2 tests) when he goes against the non-conflicted researchers' advice...but he also sells lithium related products. It's not much effort and there's very little known downside to doing what the research says.

Or you can trust the guys trying to sell you stuff and who have a strong incentive for you to buy from them again once the warranty period ends.



--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com


Porter McRoberts
 

Mark. I think your insights are spot on. Both #1 and # 2. 

However, It seems to me that with access to solar or other power a charger should be able to be programmed to perfectly care for a battery, so that degradation likelihood is at its lowest potential. 

I’m sure the lack of data is from your 1 and 2. 

Interesting the difference in suggestions for parking. 


Porter McRoberts 
S/V IBIS A54-152
WhatsApp:+1 754 265 2206
Www.fouribis.net

On Sep 19, 2020, at 6:44 AM, Mark McGovern <mfmcgovern@...> wrote:



Porter,

That's part of the issue.  If you read each manufacturers instructions for how to store their LiFePO4 batteries you will get very different answers.  There is some level of commonality between some of them, but the specific recommendations are quite different from manufacturer to manufacturer.  See below for the details. 
Given that these batteries are all made from cells of the same chemistry (LiFePO4) and, in some cases, possibly the same exact cell manufacturer, that just doesn't make sense to me.  There should be one "best way" to store these batteries.  My conclusions from this lack of consensus seven or eight years into selling these batteries commercially are:

1.  The battery manufacturers don't really know for sure what the best storage method is

and

2.  The storage method does not affect the longevity/capacity of the batteries significantly enough to warrant investing the time and resources to figure it out

I am not stating that storage method does not affect the longevity/capacity of LiFePO4 batteries.  I actually believe that it does based on the reading and research that I have done.  But what I am saying is that if storage method was significantly affecting the manufacturers warranty costs and/or reputation, I believe that they would have figured that out by now and they would all have very similar recommendations for how to best store these batteries.  

Contrast this complete lack of consensus on storage method to the manufacturers recommended/built-in Low Voltage Cutoffs and High Voltage Cutoffs.  It is accepted fact that over-charging (voltage too high) and over-discharging (voltage too low) is what kills LiFePO4 batteries prematurely.  Every LiFePO4 battery manufacturer that I have researched has set a High Voltage Cutoff at 3.75-3.90 volts per cell and Low Voltage Cutoff at 2.5-2.8 volts per cell. While they are not exactly the same voltages, they are all based on a defined voltage level and they all are within the consensus "acceptable" voltage of 2.5 to 4.0 volts per cell for LiFePO4.

As I stated earlier in this thread, I've only seen one person actually experiment on what happens to LiFePO4 batteries when stored at a high SOC% (link referenced earlier in this thread).  The two experiments he conducted showed a permanent capacity loss of 4-12%.  This was after leaving LiFePO4 cells that were charged to 100% SOC for about 12-13 months with no charging or discharging during that time.  To me, 4-12% is not a huge loss of capacity given how long the batteries were completely neglected.  After all, most of us would never leave our boats unattended for 12-13 months at a time (at least on purpose).  And most notably, that capacity loss is well below any threshold that I have seen where a manufacturer would have to replace the battery under warranty.


If anyone is interested in the details on the various LiFePO4 manufacturers storage recommendations, below are some links and "copied and pasted" excerpts from some of the better known LiFePO4 battery manufacturers for your reference.  I've bolded and italicized some of the more pertinent information:  

Battle Born:  https://battlebornbatteries.com/faq/

The storage temperature range is -10°F to 140°F (-23°C to 60°C). We recommend bringing the Battle Born Batteries to a 100% charge and then disconnecting them completely for storage. After six months in storage your batteries will remain 75 – 80% charged. 

Victron:  https://www.victronenergy.com/upload/documents/Manual-Lithium-iron-phosphate-batteries-Smart-EN-NL-FR-DE-ES-IT.pdf

Recommended storage/float voltage: 13,5V resp. 27V per battery. Batteries must be regularly (at least once every month) charged to 14V (max. 14,4V) in order to fully balance the cells. Two or four batteries in series should be charged regularly to 28V resp. 56V.

MasterVolt:  https://images.mastervolt.nl/files/10000015396_14_manualMLIUltra27505500_EN.pdf

The battery should be stored in a dry and well-ventilated environment. The rate of self-discharge is less than 5% per month. High or low ambient temperature affects the self-discharge rate of the batteries and natural aging. If the battery will not be used for a period exceeding 3 months, we advise the following:

If external AC power is available switch off all loads and switch on the charger. Apply a float voltage as specified in the following table.
Model - Float voltage setting
12V - 13.5V 
24V - 27.0V

If no external AC power is available: - Charge the battery to > 80% of its capacity before storage. - Set the safety relay knob to “LOCK OFF”, see page 12. - Make sure MasterBus powering is not set to "Always on" (see Configuration tab in MasterAdjust). In this setup the batteries can be kept at least 6 months without maintenance. However, it is highly recommended to charge the battery to > 80% of its capacity every 100 days.

RELiON:  https://ceb8596f236225acd007-8e95328c173a04ed694af83ee4e24c15.ssl.cf5.rackcdn.com/docs/product/RelionInstallationManual-8.5x5.5-081720.pdf

5. BATTERY STORAGE

5.1. Storage Temperature LiFePO4 can be stored between 23 to 95°F (-5 to 35°C). For storage longer than 3 months, the recommended temperature range is from 32 to 77°F (25 to 40°C).

5.2. Storage Conditions It is recommended to store LiFePO4 batteries at 50% state of charge (SOC). If batteries are stored for long periods of time, cycle the batteries at least every 6 months.

Lithionics:  https://lithionicsbattery.com/wp-content/uploads/2019/02/Lithionics-Battery-Storage-Procedure.pdf

Storing your battery at the correct specifications is important as it keeps the battery in the healthiest state possible for the fastest deployment when needed. Consult the table below for proper storage conditions.

Typical storage scenario < 3 months:
1. Fully charge the battery.
2. Turn the battery OFF by the On/Off/Storage switch.
3. Keep the battery in an environment according to the specifications shown above.

Typical storage scenario > 3 months:
1. Reduce the battery SOC to 3.3V/cell which is 50% ±10% SOC. Note: See chart below for cell voltage calculation.
2. Turn the battery OFF via the On/Off/Storage switch.
3. Keep the battery in an environment according to the specifications shown above.
4. Every 6 months charge the battery to 100% SOC, then discharge the battery to LVC, then charge it back to 50% ±10% SOC. 


Mark McGovern
SM #440 Cara
Deale, MD USA


Mark McGovern
 
Edited

Porter,

That's part of the issue.  If you read each manufacturers instructions for how to store their LiFePO4 batteries you will get very different answers.  There is some level of commonality between some of them, but the specific recommendations are quite different from manufacturer to manufacturer.  See below for the details. 
Given that these batteries are all made from cells of the same chemistry (LiFePO4) and, in some cases, possibly the same exact cell manufacturer, that just doesn't make sense to me.  There should be one "best way" to store these batteries.  My conclusions from this lack of consensus seven or eight years into selling these batteries commercially are:

1.  The battery manufacturers don't really know for sure what the best storage method is

and

2.  The storage method does not affect the longevity/capacity of the batteries significantly enough to warrant investing the time and resources to figure it out

I am not stating that storage method does not affect the longevity/capacity of LiFePO4 batteries.  I actually believe that it does based on the reading and research that I have done.  But what I am saying is that if storage method was significantly affecting the manufacturers warranty costs and/or reputation, I believe that they would have figured that out by now and they would all have very similar recommendations for how to best store these batteries.  

Contrast this complete lack of consensus on storage method to the manufacturers recommended/built-in Low Voltage Cutoffs and High Voltage Cutoffs.  It is accepted fact that over-charging (voltage too high) and over-discharging (voltage too low) is what kills LiFePO4 batteries prematurely.  Every LiFePO4 battery manufacturer that I have researched has set a High Voltage Cutoff at 3.75-3.90 volts per cell and Low Voltage Cutoff at 2.5-2.8 volts per cell. While they are not exactly the same voltages, they are all based on a defined voltage level and they all are within the consensus "acceptable" voltage of 2.5 to 4.0 volts per cell for LiFePO4.

As I stated earlier in this thread, I've only seen one person actually experiment on what happens to LiFePO4 batteries when stored at a high SOC% (link referenced earlier in this thread).  The two experiments he conducted showed a permanent capacity loss of 4-12%.  This was after leaving LiFePO4 cells that were charged to 100% SOC and then left alone for about 12-13 months with no charging or discharging during that time.  To me, 4-12% is not a huge loss of capacity given how long the batteries were completely neglected.  After all, most of us would never leave our boats unattended for 12-13 months at a time (at least on purpose).  And most notably, that capacity loss is well below any threshold that I have seen where a manufacturer would have to replace the battery under warranty.


If anyone is interested in the details on the various LiFePO4 manufacturers storage recommendations, below are some links and "copied and pasted" excerpts from some of the better known LiFePO4 battery manufacturers for your reference.  I've bolded and italicized some of the more pertinent information:  

Battle Born:  https://battlebornbatteries.com/faq/

The storage temperature range is -10°F to 140°F (-23°C to 60°C). We recommend bringing the Battle Born Batteries to a 100% charge and then disconnecting them completely for storage. After six months in storage your batteries will remain 75 – 80% charged. 

Victron:  https://www.victronenergy.com/upload/documents/Manual-Lithium-iron-phosphate-batteries-Smart-EN-NL-FR-DE-ES-IT.pdf

Recommended storage/float voltage: 13,5V resp. 27V per battery. Batteries must be regularly (at least once every month) charged to 14V (max. 14,4V) in order to fully balance the cells. Two or four batteries in series should be charged regularly to 28V resp. 56V.

MasterVolt:  https://images.mastervolt.nl/files/10000015396_14_manualMLIUltra27505500_EN.pdf

The battery should be stored in a dry and well-ventilated environment. The rate of self-discharge is less than 5% per month. High or low ambient temperature affects the self-discharge rate of the batteries and natural aging. If the battery will not be used for a period exceeding 3 months, we advise the following:

If external AC power is available switch off all loads and switch on the charger. Apply a float voltage as specified in the following table.
Model - Float voltage setting
12V - 13.5V 
24V - 27.0V

If no external AC power is available: - Charge the battery to > 80% of its capacity before storage. - Set the safety relay knob to “LOCK OFF”, see page 12. - Make sure MasterBus powering is not set to "Always on" (see Configuration tab in MasterAdjust). In this setup the batteries can be kept at least 6 months without maintenance. However, it is highly recommended to charge the battery to > 80% of its capacity every 100 days.

RELiON:  https://ceb8596f236225acd007-8e95328c173a04ed694af83ee4e24c15.ssl.cf5.rackcdn.com/docs/product/RelionInstallationManual-8.5x5.5-081720.pdf

5. BATTERY STORAGE

5.1. Storage Temperature LiFePO4 can be stored between 23 to 95°F (-5 to 35°C). For storage longer than 3 months, the recommended temperature range is from 32 to 77°F (25 to 40°C).

5.2. Storage Conditions It is recommended to store LiFePO4 batteries at 50% state of charge (SOC). If batteries are stored for long periods of time, cycle the batteries at least every 6 months.

Lithionics:  https://lithionicsbattery.com/wp-content/uploads/2019/02/Lithionics-Battery-Storage-Procedure.pdf

Storing your battery at the correct specifications is important as it keeps the battery in the healthiest state possible for the fastest deployment when needed. Consult the table below for proper storage conditions.

Typical storage scenario < 3 months:
1. Fully charge the battery.
2. Turn the battery OFF by the On/Off/Storage switch.
3. Keep the battery in an environment according to the specifications shown above.

Typical storage scenario > 3 months:
1. Reduce the battery SOC to 3.3V/cell which is 50% ±10% SOC. Note: See chart below for cell voltage calculation.
2. Turn the battery OFF via the On/Off/Storage switch.
3. Keep the battery in an environment according to the specifications shown above.
4. Every 6 months charge the battery to 100% SOC, then discharge the battery to LVC, then charge it back to 50% ±10% SOC. 


Mark McGovern
SM #440 Cara
Deale, MD USA


Steve Bell
 

There is an interesting detailed article on www.marinehowto.com website on lithium batteries. check it out