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Powerwall using 50 AH cells
#21
(06-20-2019, 09:35 PM)tytower Wrote: I have to disagree with the top voltage comment I am afraid . In fact the plate voltage should not exceed approximately 3.6 Volts and this is pretty universally stated by all manufacturers I have come across. Exceeding this voltage damages the cell . BMS's suited to LiFePo4 or LFP are set to that top voltage . One important point to store is that the BMS you use is set to LFP top voltage  .It is easy to order the wrong  one suitable for Lithium ion.

Depth of discharge is the main influence on cycles 80% discharge might give 4000 cycles and 30% will give 12000.(Manufacturers claim)

The charge current also influences cycle life and lesser lasts longer.

On the cost ,its about USD$ 45 for a 50Ah cell and I have been offered 200 AH cells for USD130.
I only have experience with the 50AH cell 3,2V atm  so thats 160 watt hrs for USD$45 or 2.80c/Wh if my maths are right.
The larger cells are my pathway forward and that cost is down near 640 watt hrs for USD$130 or 0.49 cents per Wh.
I'm not used to calculating it out in watt hour cost. Somebody will correct that I am sure.

On the weight side a 50AH cell weighs 1.3Kilos . How does that compare to a similar 18650 capacity/ weight?

On your first point , see Geek's post 

On limiting depth of discharge for Fe , there is no great advantage , we must remember if you discharge to 50% you have only processed half the WHRs ... so you would expect to get double the cycles , It's all about how many WHrs are processed over the life of the battery , there is a slight benefit in reducing D of D for Fe , but not comparable to keeping top voltage down on Li-ion ... charging only to 3.93 will give 65% full capacity , but the gain in Whrs processed over the battery life is about 5 fold !!

I calculate in WHrs/$  so you got 3.55Whrs / $ .... the larger 640WHr cells are 4.02Whrs /$    the very best deal of the reviews thunderheart has done is on a cell two and a half times larger than 18650 , it works out at 6Whr/$  ... but then you have the hassle and added cost of connecting them all , I think it's much better to do what you have and buy large units ...

So your 160 WHr cell weighs 1.3Kg , that's ............................. 123WHrs/Kg 

A 3500mA 18650 weighing 47gms holds about 10.5 WHrs  .......225WHrs/Kg
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#22
>On the weight side a 50AH cell weighs 1.3Kilos . How does that compare to a similar 18650 capacity/ weight?

18650s weight between 40g (2000'ish mah) and 45g (3000'ish mah) depending on mah. So let's go with 2000mah / 42g per 18650cell. If you divide 1.3Kilos by 42g my math says 1300/42 = 31cells @ 2000mah. This would be 62,000mah or 62ah. Therefore, if 1.3Kilos for 50ah is accurate - 18650 is a bit lighter. Are we sure that those 50ah ebay prismatics are really a full 50ah of capacity?
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#23
(06-21-2019, 02:03 PM)OffGridInTheCity Wrote:   Are we sure that those 50ah ebay prismatics are really a full 50ah of capacity?
To be honest i doubt it now . When new they lasted much longer so its going to be a trial and error job I think but worth giving a small bank a try in your system . Thats what I did so I could get the system worked out without great cost.
Buying and building a 50AH system is straightforward and should not cost more than $500 all up plus an inverter. .

I include this link to Will Prowse DIY that some of you may find enlightening . Hes doing a 75 Ah cell bank for 12V here but many of his videos are informative especially the cost comparison video for LFP to Lead acid.


https://www.youtube.com/watch?v=8aDSKJ84rOU
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