Lifetime cells / # cycles

roeln

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Hi all

new to this but very well enthousist about the project! Great work, super video's, huge knowledge, ... I like it!
But I also have a lot off questions when thinking about building such a powerwall myself.
One of them is regarding the used cells. In an old facebook post of Peter on HBPowerWall I've red he was looking for irc18650-28a batteries. I got curious and googled around for the datasheet of that cell. Seems that this cell has a 70% DOD lifespan of 300 cycles. Andthat is if these cell are new.

Now, I've got a PV-System and a lot of data regarding power consumption and generation. I can use all that data to import into SMASunnyDesignWeb. On that site I can simulate battery-systems. I'm simulating with these specs:

Lithium
9,8kWh nominal capacity
88% usable capacity
95% roundtrip

It tells me this battery will make 200 cycle each year. When I compare this to the lifespan of a new irc18650-28a cell with the calculated prediction I get depressed.
With new irc18650-28a cells, the DIYpowerwall will last 1.5 years. So how long will it last with used cells?

Am I missing something? It must be, I hope!
Any comments are welcome!

Kind regards, Roel
 
You need to be able to build considerably larger capacity than your energy needs to reduce your cycle count and cell life significantly
 
No your reasoning is completely sound. There's a sweetspot between how many cells and for how much and how much you need for how long, that determines the viability of the DIY salvage approach. That's putting aside the cost of electricity generation altogether. You would have to be in a unique situation to acquire the cells you need to accomplish your goals relatively quickly too.
Daromer has a system closest to what you wanthttp://secondlifestorage.com/t-Esperyd-Solar-and-powerwall-system
 
I don't remember the figures offhand but have you looked at the Tesla modules from salvaged battery packs?

Personally I'm only assuming atmost 5 years of use out of anything I build today mainly because I assume the cost for the same capacity will be several factors less by then
 
I definitely don't want to scare you off, I would love to see a 300kwh system :) I just think it would be very difficult to meet the goal of cheaper than buying off the shelf at that size but not impossible
 
One of the key with powerwall is to reduce charge and discharge bandwidths to increase cells lifespan.
Manufacturer's lifespan is given for 100%-0% cycles, if you cycle your cells between 90 and 10% for exemple you increase the lifespan by at least 2 or 4 time(some will say 10x).
Most of us use our cells around 4.0v(or 4.1v) to 3.3v as you loose no too much stored energy compared to cycles gain.
 
First:
DOD 70% after 300 cycles for irc18650-28a is for 100% usable capacity, it will increase a lot with 88% cycles(probably not 6000 too).
6000 cycles for 70% DOD is partly due to the 88% of usable capacity, at 100% they will cycle a lot less.

Secondly:
DOD 70% doesn't say the cell is dead, just you lost 30% of capacity so manufacturer don't guaranty more cycles as it's industry standard testing.

So you don't need 20x more cells, but it's hard to estimate how many you need.
 
Im not sure where you got your numbers from for that LG pack?

If i check the warranty it says that they guarantee 60% capacity left after 10 years on normal use. Thats 27,4MWh
Thats compare to run 2800 cycles
If we do the 88% instead it would be 3100 cycles. But this is not accounted for the loss so if we go with that its 60% left after those 27MWh used you may have 5000 Cycles.

Note that the number of cycles is quite irrellevant unless you talk about the total energy that you got out of the pack.
Useable capacity is merely depending on your wallet :) And the space you have available. For instance lead acid really shouldnt be above 20% DOD if you want them to work a couple of years. Yes some state 50% and it do work on some...

LiIon mix from laptop generally do 500-1000 Cycles with 80+DOD but on the other hand if you go 60% DOD you might get 2500 cycles. Compare to LiFepo4 that can do 5000 cycles 80% DOD with >60% remaining capacity.

So for it to be worth its all about how much accumultative energy you can get out of it compare to that cost.

What i can see LG use NMC and they are often said to have 1000-2000 Cycles with 80% capacity retained so they can most likely do 3-5000 cycles with 60% left.

Common in laptops are LCO that is 500-1000 cycles today.

I hope i got some point in it all. My point is that you need to bring in the total used energy factor in the scenario. And balance that with the DOD/Cycles/cost
 
Ok, something that's being over looked here. The life span of a cell dramatically increases as it's discharge current is reduced and if charge/discharge is below/above voltages.

In meaning,
* charge to 4.1V instead of 4.2V
* discharge to 3.6V instead of 3.4V
* discharge at 5 mA instead of 1 Amp

These will dramatically increase the life expectancy of the cells.

If a cell is rated at 1C with a life cycle of 600, running that cell at .5C will increase the cycles dramatically. Maybe not twice, but could be as much as a third. So you get 1200 cycles. Going at .25C, you could possibly double or more the cycles.
The same goes for the voltage ranges. A cell is usually rated at 4.2V full charge, 3.2V full discharge. Again, if you charge to 4.1V and discharge to 3.4V, that increases the cycles; and likewise to the current, going even shallower of 3.6V will dramatically increase life cycle.
 
Ur not wrong. Its just that u cant estimate on 2nd hand cells since u dont know their life cycle.

I doubt you need 20x but remember its different type of cells and chemistriemies.
 
roel.nagels@telenet.be said:
;-) as said above. I'm curious where I'm wrong ;-)
So more comments are still welcome!

That said I think you've found the spec of the first RESU version batteries.
I'm talking about this version (it states 80% DOD within 10 years)

LG Chem RESU info http://www.mg-solar-shop.de/media/products/111954_data sheet_lg_resu_3_3-6_5-10.pdf
(Page 1 , Powerful performance.... 80% of capacity retention after 10 years)

http://www.batteryenergystoragesystems.com.au/index.php/battery/lg-chem-resu-range
http://www.myenergysolar.com.au/lg-chem-resu/
...

Never the less I'm going to search for a nice way to do create a DIY powerwall. Just because that's a lot more fun :)

Obviously, you guys are all missing something - and that's capacity testing (which we spend about 70% of our project time doing).

I just pulled out a battery from a tester and the capacity is 1677mAh, the manufacturer's rating is 2200mAH. So it means after about 600 cycles (2 years) in a laptop, the capacity is now 76% of original.
If I used 200 of this cell (150 pcs of brand new)in my powerwall then my current capacity is about 330AH.
If I cycle for another 2 years my powerwall's capacity will probably reduce to about 220AH (new cells or not).
The only difference is that you will need more old cells than new cells.
The new cells will be about 70% of the manufacturer's capacity while the old cells will be 49% (70% of 70%) of manufacturer capacity.
And like other have said, if you only pull 0.2c then you may have more cycles.
 
And that's the primary reason for using shunts and monitoring the amps in both directions. By doing so we can see how much capacity is being charged to, pulled from, and be able to graph out the life expectancy of the cells remaining.
Personally, I would shunt each pack to really narrow down the low-lifers; if not each set of series strings. But that's me :)
 
Lawcomleo
Note that the capacity is not linear compare to number of cycles. Especially not on laptop packs since you do not generally know how they were cycled.
Also that most people use cheap testers and that can vary alot compare to factory capacity. Easily 10++ % And that could potentially be 100s of cycles :)
Cycles also depends alot on as mentioned on how much they are cycled and alls.

With that said you still get an indication as you said and thats what is important.

I have tried to compare cycles to mAh left and all that but never got any numbers that were sufficient. I had packs/cells that have degraded heckalot faster than others even though they should have been at same "number of cycles" based on the capacity compare to original.

And i cant state i spend 70% of my time on testing compare to building. Perhaps 2% :) I put in and take out the cell from the tester and then thats it :)


Its not easy to determine 2nd hand cells and thats what make it hard to say and compare to new packs. You can only guess. Not even estimate :)
 
I try to limit the cells I use to be at least 80-85% of original capacity, I wouldn't use a cell that tests 2000-2100mah if the original capacity was 2900-3000 in my PowerWall. Perfectly fine for use otherwise. It's just my way of narrowing down the cells to similar wear levels in the hope they will age similarly
 
As long as you have plenty of cells to work with, that's a great way to do it. Or, you just build each pack with all similar mAh within 100-200 mAh variance.
But I do agree with you Scepterr with not wanting to put into powerwall production such shortened life cells. Those would go into smaller packs used for other purposes or for flashlights or such.
 
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