LFP SOC for Storage and Use

Hello all,

50% State-of-Charge is the recommendation for storing LFP cells from all I've read. I have a few questions to help me understand that recommendation better:

1. In looking at the charge profile on the spec sheet, it appears the 50% SOC mark is about 3.28V per cell, or 13.12V for the battery pack. Is this voltage about right for the 50% SOC benchmark (assuming roughly 15mv delta on cells)?

2. What is the timeframe for the battery to be considered 'in storage'? Is it a week, a month, 3 months, or whatever? The reason I ask is I use my LFP pack on an occasional basis, not like someone using for on-grid or off-grid purposes cycling the batteries regularly. I use for an occasional RV trip, an occasional Ham radio event, and the 'who-knows-when' power outage at my home.

3. Is there a tolerance to that 50% SOC for storage? Is it 40-60% SOC, or 30-70% SOC, or whatever?

4. And lastly from what I read, use only 80% of stored power. I read that as don't fully charge to 3.65 V, and do not fully discharge to 2.5V. Again looking at the specs and the Charge Curve, I read that to keep the SOC about 90% of fully charged at 3.65V, and 10% above fully discharged at 2.5V...or about 3.3V to 3.1V range. What are your thoughts?

Any insights appreciated.

Thank you,
Mike

TheGriz said:

1. In looking at the charge profile on the spec sheet, it appears the 50% SOC mark is about 3.28V per cell, or 13.12V for the battery pack. Is this voltage about right for the 50% SOC benchmark (assuming roughly 15mv delta on cells)?

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Sounds about right, though measuring terminal voltage as an analogy of SoC for a lithium-ion battery is somewhat hit and miss. That said, I've seen 'storage SoC' recommendations from 30% - 80% so I don't think it matters too much.

TheGriz said:

2. What is the timeframe for the battery to be considered 'in storage'? Is it a week, a month, 3 months, or whatever? The reason I ask is I use my LFP pack on an occasional basis, not like someone using for on-grid or off-grid purposes cycling the batteries regularly. I use for an occasional RV trip, an occasional Ham radio event, and the 'who-knows-when' power outage at my home.

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AFAIK, storage means forever. I mean, the battery will calendar age as all things do (entropy) but it's not like you can remove the electrolyte like you can in a lead-acid for example.

TheGriz said:

3. Is there a tolerance to that 50% SOC for storage? Is it 40-60% SOC, or 30-70% SOC, or whatever?

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I don't think its critical at all, I've heard anywhere between 30% - 80% as being recommended storage SoC. Of course, you should follow your battery manufacture's guidelines.

TheGriz said:

4. And lastly from what I read, use only 80% of stored power. I read that as don't fully charge to 3.65 V, and do not fully discharge to 2.5V. Again looking at the specs and the Charge Curve, I read that to keep the SOC about 90% of fully charged at 3.65V, and 10% above fully discharged at 2.5V...or about 3.3V to 3.1V range. What are your thoughts?

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It's the DoD that will be the primary factor in aging any battery, the higher you charge, the more it ages, the lower you discharge, the more it ages. Some people swear by never discharging below 10%, never charging above 90%, but for lithium ion technology the number of additional cycles you glean from adopting this approach is, imho, meaningless in the grand scheme of things.

A typical, modern LiFePO4 battery can already go through 3,500 - 5,000 cycles, that's already 9 - 14 years!! So you limit your charge/discharge and gain an extra year ... who cares!! The next battery technology will be online in a few years and we'll all be swapping to that - our LiFePO4's will be ten-a-penny on eBay.

Personally, I don't think it's worth the effort. Charge to 3.65V, discharge to 3.0V and get on with your life.

I have tested 80 cells measuring the capacity. The midpoint ranges from 3.171 to 3.31 volts

Craig said:

I have tested 80 cells measuring the capacity. The midpoint ranges from 3.171 to 3.31 volts

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Do you have a writeup on this? I'm curious to know how you capacity tested and your thoughts behind the benefit of doing it.

I am working in something. I capacity tested them because I told those who ordered from me I would test them first. I capacity tested each individually using some resistors and a computer connected shunt recording voltage and current draw every 15 seconds. Added up all the results for capacity. Have some good data. Still have some tweaks on the IR testing to do though on next batch

The midpoint ranges you posted above have quite a variance. Is that all within the same batch of batteries?

YEP we are told to store all of our LiFEPO4's at 45% ... and of course we have no idea what 45% is ... so we just get it above 3.27 and call it a day ... seriously ... with a discharge rate of virtually nothing we can come back in 5 years and they will still be good ...

HRTKD said:

The midpoint ranges you posted above have quite a variance. Is that all within the same batch of batteries?

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Yes it is. Ideally I could take every set of cells and match them perfectly with the data. But I need to make some sort of program to analyze the data and automatically match cells. In the end this could make some well balanced packs. Although figure a 4s pack is way easier to match than a 16s pack

Thanks all...some excellent feedback!!! Look forward to anyone else tossing out their thoughts and experiences in this line of thoughts!!!

TheGriz said:

Thanks all...some excellent feedback!!! Look forward to anyone else tossing out their thoughts and experiences in this line of thoughts!!!

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My thought is that it would be nice if the BMS supported multiple saved charge profiles. Like "home" and "away". When my RV is in the storage lot I don't need the charge to go to 100%. If I have to fiddle with the BMS every time, that's going to get old real fast.

HRTKD said:

My thought is that it would be nice if the BMS supported multiple saved charge profiles. Like "home" and "away".

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Why would the BMS ever need to know or care that you are home or away (have different charge profiles)? BMS's should not be used to manage charging. Do you mean your SCC?

With Victron SCCs you can store two custom profiles that you can easily switch between ... remotely, in the pub, whilst on holiday, for example.

Thanks Craig: for your time and work. It's going to help us all to understand ,why and how these batteries preform .

tictag said:

Why would the BMS ever need to know or care that you are home or away (have different charge profiles)? BMS's should not be used to manage charging. Do you mean your SCC?

With Victron SCCs you can store two custom profiles that you can easily switch between ... remotely, in the pub, whilst on holiday, for example.

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I'll take whatever works. BMS or SCC. It's good to know the Victron can handle that. That's what I'm planning to use.

I think the important thing is to not store the battery fully charged and even more important don't use a float setting like it is a lead acid battery that is constantly topping off the battery.

My use case is like yours. In addition, I plan on removing the battery from my van when not using it since I live in Phoenix AZ and the summer heat here is deadly.

Spec on most LFP cells are 3% per month of capacity self discharge. Typically in 1-2% range. Older or more 'abused' cells will be higher.

Should initially charge between 50% (3.300 v) and 80% (3.335 v) SOC before storage. Cells can go down to 30% SOC due to self leakage which should set the limit time of storage.

Closer to 80% SOC, the longer you can store.
80% to 30% SOC at 3%/mo. self discharge is 16 months. Considering accuracy of determining 80% SOC, I would not go more than 12 months.

Self leakage goes up above 80% SOC caused by other cell stressing effects (without external loading) which is why should not be stored starting above 80% SOC.

As you can see the voltage range is not too wide so you need an accurate DVM.

Cheap DVM's are not accurate enough. The most accurate low cost DVM I have found is UNI-T UT61E (about $60). It is about 0.1% +/- 2 counts at lowest significant digit accuracy (about +/- 4 mV at 3.3v) Even that accuracy cannot tell difference between about 70% and 90% state of charge.

Rested open circuit (no load) voltage. Between -10C and +40C temp range there is almost no change in no load rested voltage.
100% > 3.450 v
90% = 3.338 v
80% = 3.336 v
50% = 3.300 v

Attached is a no load rested LiFePO4 rested open circuit cell voltage curve I have put together from several sources from controlled measurements and modeling predictions.

It is very difficult to achieve absolute accuracy as it is very difficult to be sure of SOC condition with a high degree of accuracy. This is why I combined averages from what appeared to be sources with reliable measurement procedures as well as sources with modeling predictions.

If there is any other additives in cell, like Winston Yitrium additive, the curve will be slightly different.

Again, you must have a high accuracy DVM to make measurements for SOC estimation.

Loaded voltage measurements are a whole new discussion and is very complicated with several factors involved. On first pass estimation, for short burst, you can assume about 0.6 to 1.0 milliohm equivalent series resisstance per cell per 100 AH capacity. I only mention this because the attached curve is only for open circuit rested cell, as you might get on a just received cell. Do not expect to get these voltages on a loaded battery pack.

RCinFLA said:

Attached is a no load rested LiFePO4 rested open circuit cell voltage curve I have put together from several sources from controlled measurements and modeling predictions.

It is very difficult to achieve absolute accuracy as it is very difficult to be sure of SOC condition with a high degree of accuracy. This is why I combined averages from what appeared to be sources with reliable measurement procedures as well as sources with modeling predictions.

If there is any other additives in cell, like Winston Yitrium additive, the curve will be slightly different.

Again, you must have a high accuracy DVM to make measurements for SOC estimation.

Loaded voltage measurements are a whole new discussion and is very complicated with several factors involved. On first pass estimation, for short burst, you can assume about 0.6 to 1.0 milliohm equivalent series resisstance per cell per 100 AH capacity. I only mention this because the attached curve is only for open circuit rested cell, as you might get on a just received cell. Do not expect to get these voltages on a loaded battery pack.

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Exceptional effort and information! Thanks for taking the time to compile and share!

RCinFLA said:

Attached is a no load rested LiFePO4 rested open circuit cell voltage curve I have put together from several sources from controlled measurements and modeling predictions.

It is very difficult to achieve absolute accuracy as it is very difficult to be sure of SOC condition with a high degree of accuracy. This is why I combined averages from what appeared to be sources with reliable measurement procedures as well as sources with modeling predictions.

If there is any other additives in cell, like Winston Yitrium additive, the curve will be slightly different.

Again, you must have a high accuracy DVM to make measurements for SOC estimation.

Loaded voltage measurements are a whole new discussion and is very complicated with several factors involved. On first pass estimation, for short burst, you can assume about 0.6 to 1.0 milliohm equivalent series resisstance per cell per 100 AH capacity. I only mention this because the attached curve is only for open circuit rested cell, as you might get on a just received cell. Do not expect to get these voltages on a loaded battery pack.

Click to expand...

This pretty much concurs with my data as well.