Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Discharge after capacity testing?
#21
(11-09-2019, 03:51 PM)daromer Wrote: So if we look at the numbers. Lets say you have used cells and you let them sit at 15c that I do... Lets say I leave them for 3 months. If you look at that white paper you will see that I loose a whopping 1% or so.

I was reading the paper as a 2-5% loss for the 3 months or so.  ...and I always wondered if powerwall users kept a 100% charge or limited things to a bit lower for better cell life.  I'm using cells in ebikes and cars and usually charge to 80-90% except for top-balancing every now and then or heavy usage when I use them as soon as I charge to 100%.
Reply
#22
You basically have same degradation if charged to 80% as to 100%.

Degradation at 25c is said to 3-6% over a FULL year. At 15C and 3 month its fraction of that. At 5C its even lower Smile I only checked the graphs in the document and a bit tricky to see 3 months but 1 year was easier.
I didnt read this specific document in full but I have read alot of others and it varried ALOT between cells, brands and environments on how much they did degrade. But most tend to say up to 6% over the first year. Second year its less and so forth.

To sidetrack. One of my batches i got (4500cells). They where 6 years old and was stored at 60% charge when gotten. At this level you degrade around 3-4% per year in capacity. When testing those cells i meassured the total loss to around 5%.

What we all can agree on is that the degradation due to heavy use is alot heavier than by doing above test. Many people do forget to add all factors in. Even I miss that some time when calculating.
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
Reply
#23
BEST. THREAD. EVER!
Reply
#24
(11-09-2019, 06:58 PM)daromer Wrote: You basically have same degradation if charged to 80% as to 100%.

Not to be argumentative, but even Elon Musk has stated to the contrary.  He has stated his team prefer the 20%-80% range to avoid loss.
Reply
#25
(11-09-2019, 06:58 PM)daromer Wrote: To sidetrack. One of my batches i got (4500cells). They where 6 years old and was stored at 60% charge when gotten. At this level you degrade around 3-4% per year in capacity. When testing those cells i meassured the total loss to around 5%.
  
3% per year over 6 years would be 18% loss per charts?  but you measured 5% loss.   What's the conclusion from the above comment - e.g. charts are wrong and its more like 1% per year?

As an example, I'm testing a large batch of Grey LGABB41865 that I just got from Alarmhookup. They have date codes in 2012 range (7 yrs ago) but are testing 2600+ mah - which is 100% of original ( https://secondlifestorage.com/showthread...&pid=28753 ). Assuming my OPUSs are 10% off/overstating - I'm still in 90%+ of original capacity.. which is more in line with 1% degradation per year. Of course I have no way of knowing the storage history except these obsolete 'modem' battery packs - presumably not used much.
Reply
#26
Digout. Yes you should not charge to 100% when cycling to maintain a gap to have extra energy left. Thats how we run the powerwall later on. I have not read all their reports nor have i Compared their cells with the cells I have had nor the cells in that report. But... It differs but in terms of total loss. Leaving cells fully charge for a test is not a dealbreaker if it can weed out self-dischargers or other faulty cells. As I said: Take in all numbers. Smile

And you can argument for sure! Get some more concluded data from tests and we arrange them in here. Thats why I Argue regarding it since in the beginning people claimed up to 20% loss per year fully charged.....

OffGridInTheCity: The degradation is getting lower per year as well. its not linear....

My Conclusion is that many factors weigh in. The report is with 25C.. Still all numbers. My cells was stored below 10C that I compared to above.
So the numbers in the reports as you also saw can vary ALOT.
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
Reply
#27
It's really only 2 steps:
(1) Charge the cell and note date and voltage at full charge.
(2) One month later, note voltage and run "Discharge" in Opus, not "Charge Test" to determine capacity.

As I mentioned before, about 3% of my genuine cells tested with good capacity initially, DID NOT SELF DISCHARGE, and then 8-12 months later tested with only 40% of original capacity. My guess is this is what happened with Mike/LithiumSolar's low capacity pack.

I know this is extreme, but I would argue that each week that a cell is left at full charge is equivalent to a full charge/discharge cycle on that cell. There is noticable degradation from full charge storage.

I also don't treat datasheets or white papers as scientific law. In my own tests, cell cycle life has blown away what a data sheet stated. The cells being tested by white papers are usually very different from the ones you get from laptops. Personal experience trumps paper in my opinion.

Also, I really want to stress the difference between "% of original capacity" and State of Health. You could have a 5% loss from the original capacity of a cell, but that could be a 20% reduction in the number of cycles a cell has remaining in its life as a cell may not degrade linearly or it may start to self-discharge once reaching a certain % of original capacity.

DiggsUt - It's not that we want to store them for months at full charge, it's that we underestimate the time it takes to build a powerwall and don't want to start building packs until we have a certain amount of cells, and that amount takes longer to acquire/break down/test/etc than originally thought.
Formerly known as Dallski
Reply


Forum Jump:


Users browsing this thread: 1 Guest(s)