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General battery pack/PV planning
#1
Hello,

I am currently building a house (in Austria, Europe). It will have PV (I want to produce more energy than I use).

The yearly usage will be approximately 6000 kWh. Base load (Servers, infrastructure, ventilation...) will be around 200W. I need 3 Phases.

1) What would be an appropriate sized Battery?
2) What is currently an easy/cheap battery type and source? Used 18650 Cells? Used EV-Batteries?
3) What is an approximate total cost for lets say 10kWh diy-battery usable in Europe?
4) What is the recommended Infrastructure? PV-hybrid inverter that can handle batteries? Normal Inverter and parallel battery structure?

thanks
Daniel
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#2
1)What would be an appropriate sized Battery?
  • Double your daily consumption would be ideal to discharge to 50% SoC.  Which allows for some headroom on low PV production days and often times helps long term cost of ownership by not going deep on cycles during regular use.  I personally oversize my batteries as much as possible. 
2) What is currently an easy/cheap battery type and source? Used 18650 Cells? Used EV-Batteries?
  • Cost of batteries based and remaining service life, quality, consistency will all play a roll.  I prefer used EV cells by a large margin.  Less labor and cost to construct packs, easier to find a large batch which is great for consistency, and depending on the vehicle mileage may have a much longer service live than used laptop packs which are probably at the end of their service life.  Obtaining used 18650 of good quality (medical packs) in LARGE quantity is harder than just buying EV cells. 
3) What is an approximate total cost for lets say 10kWh diy-battery usable in Europe?
  • In USD, expect to pay about $100 - $225 per kWh for raw materials in EV cell cost.
4) What is the recommended Infrastructure? PV-hybrid inverter that can handle batteries? Normal Inverter and parallel battery structure?
  • 100% depending on your needs and design.  Something like a Victron Quattro would make for a VERY versatile setup.  Allows the system to be configured in various ways including Grid or Battery priority which allows the system to act as a UPS, or use battery and use grid or gen as a supplement for higher loads too.  On a side note, I would not charge though inverter, I would have a separate MPPT charge controller for PV.
dewki and completelycharged like this post
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#3
I would need to get to about 100€/kWh usable (for whole powerwall, not only battery) with a life of 1000 cycles to make it competable to the grid.
As this is not the only reason I will go (a bit) over that, but it must definately be <<150€

Are there any alternatives to recycled 18650 cells in that price range?
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#4
Finding batteries worth using is the core challenge everyone here faces.  There is no consistent inexpensive source for good used cells as supply and inventory is constantly changing.  If you really want to use 18650 cells, research the cost per kWh of making cells into a battery and let that play into your decision.  Using cell holders and solder vs spot welding vs PCB builds all have different costs and labor challenges involved.

For the sake of conversation… I live in an area where grid power is reliable and affordable.  If I look at my project and try to complete with the grid, I already lost.  The ROI is poor from that standpoint, but I didn’t start this project to complete with the grid.  I did this project because I enjoy this stuff as a hobby and find it fun, like to learn, and thought it would be good for emergency use.  Yes it makes power, saves me money, but my ROI may not be realized for years to come assuming the system even last that long.  On top of that, I keep changing things around, and or buying new interesting things like a high end inverter vs a basic unit or a new MPPT controller.

On to your comments… 1000 full cycles could be considered a lot for a lithium-ion chemistry.  Brand new Chinese cells (Revolt) which have been well tested and documented by members here have shown significant degradation as soon as 335 cycles and continues going down as cycle count increases.  Contrast that to brand name cells like Samsung, which still degrade, but hold up much better at higher cycle counts, but still show some fatigue at 500 full charge/discharge cycles with perhaps a 15% to 20% reduction in capacity.

Point is, don’t cycle too deep and over-provision capacity to give your cells a fighting chance.  So if you use 10 kWh per day, I would add at least 20% more to combat against capacity loss over the course of service life, and then double that to reduce the depth of discharge, and the need to fully charge which can also negatively impact cell life.  So if you use 10 kWh per day, I personally would want a battery that has at least 20 to 24 kWh and I live in an area where I actually get a lot of sun.  You might not need a bigger battery, but you might need even more panel to charge that capacity if you don’t get a lot of sun hours otherwise you end up with a deep depth of discharge because you cant top off for the next days loads.

Alternately, you could consider using LiFePO4 chemistry lithium batteries…  those typically have a significantly larger cycle life (3000-4000 cycles), the cells also cost more money, but they can be easily sourced new in large prismatic cells which save a lot of time and money when building packs with 18650 cells.

Perhaps start with a small system, and build your own proof of concept before “going big”.  You might learn some things along the way that might change how you approach your own project.
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#5
Cell cost for me was £260/kWh (all in, imported, delivered to door) however they are LTO chemistry and have 1000's of cycles life remaining (and can do 90% DoD /charge each day) so the cost per kWh over the next 15/20 years works out low enough to compete with the grid for solar storage and off peak charging in winter. It's not always the cheapest cells that make sense :
https://secondlifestorage.com/t-How-much...-batteries

Also for 3 phase you may need to consider what your doing a bit more carefully :
https://secondlifestorage.com/t-Controll...0#pid53140

"Servers, infrastructure, ventilation" with 200W ? I can't leave mine powered up as they use nearly 1kW.

Summary I would say, if you want a large capacity, either go for large format cells or spend a LOT of time testing, soldering, building with 18650's.

3-phase - do your homework carefully.
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#6
I have a 7kwh PV array - producing 9,5000kwh per year. So far this year (thru Aug 11) the house has consumed 2,654hrs of inverter power at an average of 2,197watts per hr. In summer, the 7kwh PV array produces 40-45kwh/day of which 30kwh is used 'directly' (during PV array hrs) and 15kwh excess is strored in battery and consume thru night.

My goals are
1) consume 100% of PV array input - e.g. I never go into 'float'
2) try to get my money back (grid power is only 0.12c/kwh in my situation)
3) run my home 'just fine' if things go awry like Puerto Rico and power is lost.

For #1 - I need a battery bank big enough to charge/discharge 15kwh/day in max (summer) days.
For #2 - I need the system (including the battery) to last 20yrs - e.g. 7,000 daily charge/discharge cycles.

It has cost me $307/kwh made up of ~6,500+ cells, holders, solder, wire, Batrium/longmons, failures, shelves, misc .. to build a 53kwh battery of 18650s.
The 15kwh/day is a DOD of 29% in the range of 4.0v/cell -> 3.4v/cell.

As the battery degrades.. even when it's down to 50% of current capacity (26.5kwh) I can still get my 15kwh / day at 56% DOD. 20yrs is too long to make any meaningful predictions but charts on Battery University give me 'hope' that it might be possible.

So PV array input, battery size/expected-life, consumption, cost all work together BUT can also be like pushing both ends of a string - make a tangled mess. I had no idea when I started this almost 2yrs ago but have come to focus on GOALS. So I think if you work on your goals it will give you a framework to weight choices and costs etc.

For example - one of my goals is 20yr ROI. So I can ask myself questions like:
1) Can I get there using 18650s?
2) Is there a DOD sweet spot that will get me there vs $$ to increase my battery size to lower the daily DOD? (in other words, does 50% reduction in DOD get me 400% longer life??)
3) Should I switch to LifePO4 or LTOs ... what do they cost?
4) Should I pay $250.00 for 100 used cells or $104 per 100 and risk a 25% failure rate/break the packs apart myself.

The answers are not immediately clear ... but at least you can fix one end of the string Smile
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#7
1000 Cycles was just used for my calculation.

Current price for electricity is 0,153€/kWh when I buy it, and ~0,05-0,06€/kwh when I sell it.

So every kWh that I sell to the provider and have to buy again (for instance at night) costs me ~0,1€
If I store it I calculate with 80 % efficiency, so I save 0,08€/kWh.
A battery that lasts 500mcycles can cost 40€/kWh usable
For 1000 cycles 80€/kWh usable
For 3000 cycles: 240€.
I am hesitant to calculate with more than 3000 cycles.

I know that this ignores the potential rise of energy prices.

Are there better suited batteries for me other than LiIon/18650?
LTO-Cells seem inhteresting, especially if new, what are the sources to buy in bulk?
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#8
LiFePO4 as mentioned above would be another chemistry worth looking at.

Most EV cells are the same/similar chemistry as your typical 18650, just in a larger form factor.  At least from a voltage range standpoint, the same.

LTO cells are still in a segment that is still developing.  I expect many changes here over the years.  Buying now I would feel like an early adopter, but I also have limited experience with this chemistry.

I still feel some of the absolute best deals around are EV modules… I got these from a salvage EV that only had less than 10k miles on it.  All the cells are very healthy and consistent in terms of performance.  Sure beats scraping the bottom of the barrel for 18650’s, and the savings compared to 18650 time/cost to build into packs was just night and day.  I actually ended up selling 400-500 lbs of laptop packs after I got a hold of these and never looked back.

Perhaps reach out to your local automotive parts yard / salvage companies and get a hold of a EV pack and take it apart yourself.

33kWh
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#9
"I know that this ignores the potential rise of energy prices." - this is more a question of comparing 1 EUR in the bank earning a return higher than the inflation (and volatility) of energy prices.

If "For 3000 cycles: 240€" is an issue you need to figure out what your going to do for the 3-phase supply and inverters before looking at cells

For your inverter, charger and any other components in the system work out a lifetime cost per kWh and factor this into your storage throughput cost as this will lower your 0.08 down a bit and then re-evaluate the cell position. Part of the value is in powered on hours and part of it is based on kWh throughput.

Solar all-in costs should be around 0.04/kWh or lower (making the margin over 0.1/kWh)

If you get an export value, over size the solar rather than buy more battery capacity... to balance more of the winter and cycle the cells


EV packs ar as mentioned, if cheap enough (base on remaining cycle life), are the best route for a build. This is probably the route I would suggest. Select carefully and research well...


I'm biased, however, for me LTO deciding factors were :
1. No issue with low temperature (no heating / cooling of cells) so they can sit outside or wherever (point 9)
2. Long cycle life and low maintenance (once built no messing with the cells for a long while, hopefully)
3. Worst case discharge to 0V does not do any real damage
4. Self discharge is near zero - tested a cell left in the shed a year...
5. Portable packs (ok not really chemistry dependent, unless you take point 1 into account)
6. I have fixed my energy cost (with solar) for the next 10+ years, hopefully 20, electric price is only going one way.... tax.
7. Just as good as a pension, appart from you have the asset and can look at it, even put your feet up on it while you have a cup of tea (boiled in a kettle powered by the pack !)...
8. Mature technology, although lacking actual cycle life testing to 30,000 cycles (I think they only had about 15,000 cycles in the test data). It is still developing though
9. No bursting into flames if the cells are damaged, very difficult to set on fire

I like options....
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If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#10
I agree with all said here. For me - the biggest / key issue that I've tripped on is *trustworthy* battery purchase. Its complicated because you don't (typically) have the $10,000-$15,000 (or whatever - big number) to buy 40kwh (or whatever) worth of batteries all at once. I bought mine in 20 separate purchases over last 2 years - a few hundred here, a $1000 there, etc. So it became important that:
1) I had trustworthy source. I tried several ebay (china) and got burned badly. I'm afraid of Alibaba.
2) Stead source. I have come to believe its better to have same cells for my batteries as much as I can manage... so I need to re-purchase additional cells over time.
3) I rejected the 'troll laptop stores for dead laptop batteries' as its too much of a crap shoot - I feel better buying used batteries that have had some known history.

I wound up with Alarm Hookup and Power2Spare as the only 2 sources that I feel comfortable with thru much experience. They both give me 1) reliable delivery as adverstised AND 2) I can repeat purchases so that when I build a battery (1,680cells) I can purchase like 400 at a time and be fairly sure I can get 1600 all the same over 9 months.

These 2 sources are pretty much 18650 format.

The price of LTO as far as a few quick queries is *way out there* and LifePO4 CALB keep coming up double/triple compared to 18650. I've longed for Nissan Leaf(s) as they are 3.7 volt chemistry, but again - its a lot of money to find out I was delivered 45ah batteries instead of 66ah. Unfortunately I do not have any contacts for battery sources... that would be invaluable! so I'm limited to retail purchase.

One advantage of 18650 is I can repair / adjust a 'battery' by adding/replacing a few cells. If I did 200ah prismatics, I'd have to replace the entire 200ah cell. The downside is 18650 is A LOT OF WORK! I now have 6,700 cells and I'm pretty much committed at this point.

Bottom line - figuring out some good sources of batteries is (in my opinion) a key issue and depending on their stock and price points - it can well direct one into certain paths.
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