Self-discharge rate and SoC @ end of life for solar batteries

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Bob Smith

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Hello

I have learnt in the past few months that you can't really expect to charge your batteries at 100% during summer time and then starting using it in winter when the panels will hardly be able to fully charge the battery on a daily basis.

This is because of self-discharge. That is a idled battery will lose its charge no matter what over time.
I am trying to understand what this over time is

I have find in a specs for a battery (here this is Sunsynk)

1659269221886.png

I understand that the rate of <5% has been measured for a battery charged at 50% (SoC = State of Charge) and at 25°C
My first question is about the time/duration. How long will it reached 5% discharged? Is it per week, month, year (that would be nice though).
I also cannot find any diagrams from the batteries specs that would show how the battery discharge based on the temperature or SoC.

Also is it actually an important factor when chosing a battery or is it as important as the colour of the cover?

Regarding the SoC @ end of line
1659269930810.png

Similar question. I cannot find when that end is (is that 10, 15, 40 years)

I noticed that I could not find that self discharging rate nor the SoC @ end of product line on Growatt and Pylontech datasheets. Is that because Sunsynk is more transparent or it is actually useless information?


Thank you in anticipation for making more knowledgable :)
 

binky

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Hello

I have learnt in the past few months that you can't really expect to charge your batteries at 100% during summer time and then starting using it in winter when the panels will hardly be able to fully charge the battery on a daily basis.

This is because of self-discharge. That is a idled battery will lose its charge no matter what over time.
I am trying to understand what this over time is

I have find in a specs for a battery (here this is Sunsynk)

View attachment 13832

I understand that the rate of <5% has been measured for a battery charged at 50% (SoC = State of Charge) and at 25°C
My first question is about the time/duration. How long will it reached 5% discharged? Is it per week, month, year (that would be nice though).
I also cannot find any diagrams from the batteries specs that would show how the battery discharge based on the temperature or SoC.

Also is it actually an important factor when chosing a battery or is it as important as the colour of the cover?

Regarding the SoC @ end of line
View attachment 13833

Similar question. I cannot find when that end is (is that 10, 15, 40 years)

I noticed that I could not find that self discharging rate nor the SoC @ end of product line on Growatt and Pylontech datasheets. Is that because Sunsynk is more transparent or it is actually useless information?


Thank you in anticipation for making more knowledgable :)
Never considered such things to be honest. Life span is normally quoted as charge cycles, 6000 being normal for lithium. So one full charge cycle per day is something like 15years, but that's full cycles. In reality you will see a lot of partial cycles, but I've never seen any guidance on how that affects battery life.
 
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2% Per Month is the self discharge rate of LifeP04 cells, I think you have little to worry about there.

Its not a good idea to charge to 100% or discharge to zero. To enhance the life of the batteries look at 10% and 90% for min and max values.

Battery life should be well over 16 years.
 

Bob Smith

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2% Per Month is the self discharge rate of LifeP04 cells, I think you have little to worry about there.

Its not a good idea to charge to 100% or discharge to zero. To enhance the life of the batteries look at 10% and 90% for min and max values.

Battery life should be well over 16 years.
When you say battery life should be well over 16 years. Do you mean with the same performance or with some degree of degradation (like PV panels) and if so where I can see (under what name/acronym) that specs about the degradation?
 
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When you say battery life should be well over 16 years. Do you mean with the same performance or with some degree of degradation (like PV panels) and if so where I can see (under what name/acronym) that specs about the degradation?
Bob,

The general understanding is 80% of design capacity is end of life, clearly it isnt but thats the accepted figure. LifeP04 cells are again generally rated for 6000 cycles (charge / discharge) they are deemed to be 10% to 90%. If you halve the amount of charge you double the number of cycles. 6000 cycles = 16 years. This all correlates with the EV motor industry, my BMWi3 has now done 75,000 miles and is 5 years old, the battery capacity has deteriorated by 2.2%.

I think you worry little about battery life.
 

Bob Smith

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you mean I worry too much ;)
Just trying to find parameters to compare like for like the different brands.

Sunsynk mention SoC at end of life 40%. I'll ask them what that actually means.
 

Bob Smith

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I just talked to the technical support of Sunsynk and they told me the following

  1. SoC @ End of product line (should be life and not line) 40%. It means that the battery will only hold 40% of the initial capacity after 10 years.
  2. The self-discharging figure is for a period of 1 month.

I am not too worried about the self-discharge because I will need to use the battery every night and in some days. However, I see from the specs I read from Growatt or Pylontech so far that only sunsynk is providing SoC info at 10 years. This is similar to the degration of PV panels. However, I am concerned that this will be 40% only after 10 years.

Does anyone have any info about other manufacturers, are they all in the same boat +/- some % ?

This means that it changes the calculation of the ROI a fair bit.
 

binky

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I just talked to the technical support of Sunsynk and they told me the following

  1. SoC @ End of product line (should be life and not line) 40%. It means that the battery will only hold 40% of the initial capacity after 10 years
That sounds rubbish to me! Might be worth a call to Pylontech and see what response you get from them.
 

Bob Smith

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I do not know about the grade of the cells. I was also surprised 40% is a LOT.

I sent an email to Pylontech and Growatt because the only phone number I could find was aboard. Maybe I will get a reply maybe not...
 

binky

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It does indeed sound pretty poor, maybe their BMS is configured to 0% for empty and 100% for full and/or theyre using B Grade cells.
limiting discharge / charge is how they extend battery life they use to get to 6000 charge cycles, so perhaps they are allowing full discharge? I would still love to know how partial charging affects total cycles achieved - can't find any info on that at all. I'm also suspicious that other compenents will fail after say 15 years or so anyway. Part of the reason I like Pylontech is that they seem well made, externally, but then I still can't see the quality of internal components without getting the screwdrivers out and invalidating any warranties.
 

Bob Smith

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limiting discharge / charge is how they extend battery life they use to get to 6000 charge cycles, so perhaps they are allowing full discharge? I would still love to know how partial charging affects total cycles achieved - can't find any info on that at all. I'm also suspicious that other compenents will fail after say 15 years or so anyway. Part of the reason I like Pylontech is that they seem well made, externally, but then I still can't see the quality of internal components without getting the screwdrivers out and invalidating any warranties.
I was told by apparently the son of the CEO of Sunsynk that you can discharge the battery to 100%. However here is the trick. They show a capacity of let's say 5kWh and claim this is also the useable capacity thus 100% discharge. However, the battery keeps a reserve of 10% (I think it is) that the user should never take from. So in other words if you play with the BMS you could discharge more than 100% and take from the reserve. It is a kind or hidden capacity so they can claim a 100% discharge.

It is just a way to market the product. Technically speaking it is like any other batteries. drain them to death and they die.


This does not resolve my problem of the SoC @ end of life. I hope Growett and Pylontech will respond to my email.

What I would be interested in seeing are curves of degradation as you can see from PV panels
 

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