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Powerwall using 50 AH cells
#11
OK so i didn't post this to have a discussion on fuses but to demonstrate the practicality now of a reasonably cheap prismatic type  alloy cased cell as opposed to a cylindrical cell. This saves the very drawn out process of obtaining 18650 cells ,constructing them into a suitable bank with fusing and the voluminous wiring and connections that can go wrong. The cost too if you are paying $5 a cell for 18650's is a lot less.

I offer my experience in buying and using them and I have found they work well. They are now available on amazon and ebay and other suppliers are importing them.

A note of caution though . Prismatic cells have false cells just like 18650's and weight should be checked.The technology has not changed so discount any statements of "our technology is better".
The cells should be confined lightly on the large flat sides to avoid cell expansion under heavy loads or temperatures but the alloy case does a pretty good job of stopping that anyway.
My preference is to have a BMS on each string of cells so that each cell in the string is watched. When you order make sure the specs are for LFP,LiFePo4 which has a cut off voltage of 3.6V and no more or the cell will be damaged.

The BMS has back to back MOSFETS so current may only flow if both are switched on. If one blows the BMS will not pass
current.
This is a good FB page for knowledge. There is a text file of suggested weights for AH sizes on it.

https://www.facebook.com/groups/271980786862023/
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#12
(06-17-2019, 01:43 PM)daromer Wrote: But a bms is not a fuse or dead short protection. If you get a dead short a bms potentially can fail in open state.
And what do you mean with same for you?
My appologies . I missed the above points .
The same for you meant the same answer was meant for you .
I will have to be careful how I put this next bit ..You being a super moderator and all.

I am intrigued though by how you think a BMS can fail in an open state?
That is how it can fail but still pass a current is what I assume you mean.
I will put a typical BMS circuit here for referral.

There are two back to back Mosfets usually ,each is turned on only if certain conditions defined by the chosen control chip are met. Otherwise it is off .
If both are on current can flow across the mosfets plates. If either one is off no current flows.
In my experience and from some hours of research on the net I can find no mention of a shorted mosfet being able to pass current. If that situation can arise I would like to know about it .

The best I can find so far seems to indicate a blown mosfet would act exactly like a fuse in that there would be no material left between the source and the drain to pass current?

Can you explain why you think that is the case please and perhaps offer some backup info I may study ?
I also attach a diagram of a simplified Mosfet .



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#13
(06-19-2019, 10:25 PM)tytower Wrote: I will have to be careful how I put this next bit ..You being a super moderator and all.

this is of no consequence. We aren't that type of forum where we throw our moderatorship power around. We only act on spammers and people who are trying to purposefully start an argument. I haven't seen either of the above on this thread.
I'm actually intrigued by this thread and have been watching it to get answers myself. Please continue Wink
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#14
I also think it's good idea to have a dumb but pretty much fail proof fuse inline with any BMS. For one in case a MOSFET should fail in a closed state, however remote/impossible that may be. But also other components of the BMS could fail and not shut off the MOSFET in the first place. The firmware could hang. Or a dead ant could short the circuit and make it do unintended things. Corrosion, bad soldering, lightling strike (even if not a direct hit), strong radio signal, static discharge... There are hundreds of reasons why it might fail.
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#15
We wont kick ur nuts just because we dont agree Smile dont worry.

The bms can "häng" in the same way as an inverter can. Mine blow Up last year in shorted state. The mosfets of some reason was shorted. Same happens with amplifiers and other Gear i have repaired.

Is it worth taking a chance ? I dont think so. I have a Main fuse for everything and There is a reason houses or any other systems do have Main fuses before the electronics.


I dont have any Links to scienimtific pappers and have never thought that should have been needed. I have personally seen it happen. Perhaps not on bms as such but on electronics.

What do a fuse cost? Most likely fraction of what rest of the Gear cost Smile
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#16
Yes ... this is a very sensible idea ... main differences between li-Fe and li-ion is that Fe weigh about twice as much for the same storage ... not an issue for static applications ....

Also Fe life cycles are not influenced by how high you charge ... with Li-ion it's paramount to keep top voltage low .. charging to 4.2V gives a life of 400 cycles ... charging to 3.93V gives 3,000 cycles .. Li-fe doesn't care about top voltage and has a naturally high life expectancy of over 2,000 ...

New cells and battery prices continue to drop(slightly) .. around 6WHrs/$ now ... that's equivalent to a price of $1 for a 2200mAHr 18650 cell new including delivery ... ( prices for buying over 100 cells )

This makes recycling less attractive unless you have a good supplier and like pulling things apart.
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#17
(06-20-2019, 01:39 PM)ozz93666 Wrote: Yes ... this is a very sensible idea ... main differences between li-Fe and li-ion is that Fe weigh about twice as much for the same storage ... not an issue for static applications ....
Also Fe life cycles are not influenced by how high you charge ... Li-fe doesn't care about top voltage and has a naturally high life expectancy of over 2,000 ...
New cells and battery prices continue to drop(slightly) .. around 6WHrs/$ now ... that's equivalent to a price of  $1  for a 2200mAHr 18650 cell new including delivery ... ( prices for buying over 100 cells )
This makes recycling less attractive unless you have a good supplier and like pulling things apart.
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?
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#18
Agree on the top voltage for LiFePo4, I run my banks at max 3.45 or so per cell = full voltage.

Re failed MOSFETs, I've seen lots of failed short circuit ones, mainly in power supplies. Test a spare one by putting >20V on the gate eg 40V (via 1k resistor), see what happens ;-)
Ironically they're tolerant to voltage spikes across D-S & work like a zener diode mostly there.
Also seen plenty where they are really "open circuit" too eg the body blown apart, literally legs loose.
What happens depends on the energy source when they fail.
There's pics somewhere on this site of blown FETs from inside an inverter.
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#19
(06-20-2019, 09:35 PM)tytower Wrote:
(06-20-2019, 01:39 PM)ozz93666 Wrote: Yes ... this is a very sensible idea ... main differences between li-Fe and li-ion is that Fe weigh about twice as much for the same storage ... not an issue for static applications ....
Also Fe life cycles are not influenced by how high you charge ... Li-fe doesn't care about top voltage and has a naturally high life expectancy of over 2,000 ...
New cells and battery prices continue to drop(slightly) .. around 6WHrs/$ now ... that's equivalent to a price of  $1  for a 2200mAHr 18650 cell new including delivery ... ( prices for buying over 100 cells )
This makes recycling less attractive unless you have a good supplier and like pulling things apart.
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.

...

I think you misunderstood that. ozz was not referring to exceeding the maximum voltage, but there being no real gain to under charging LiFe batteries, as can be had with normal Li-Ion batteries. By reducing the full charge voltage of a Li-Ion battery you can drastically increase the number of charge cycles the cell will have.
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#20
Over the last 1.5yrs, I bought 5,400 18650 cells in 20 different purchases to build a 780ah @ 48v battery bank. These were mostly used cells at the 85% original capacity / 5yrs old / low cycles. 1/3 were not so good price and 2/3 were good price (I got more knowledgeable over time). Overall I paid between $1.70 - $2.00 per cell. 5400 * $1.85 = $9,990.00

Looking on ebay just now I see things like Lot of 16 pcs 3.2v 86Ah 90Ah LiFePO4 prismatic Lithium Li-ion battery for $1,000. To build a 780ah @ 48v battery I would need 8.7 (8.7 * 90ah = 783ah) sets of 16. So 8.7 * $1,000 = $8700.00

So prices do seem competative BUT.... I have yet to find a source of TRUSTED LifePO4 Prismatics for the ebay price I mention above. In the 18650 world, I had several experiences of complete misrepresentation. For example, an advertised 90ah battery might only have 60ah once you get it.

If you look at a more mainstream site - I see 100ah for $155 + shipping. Say 100ah for $165. To build that 780ah @ 48v battery I would need 16 of these * 8 = 128 of them. 128 * $165 = $21,120.00. That's new of course and 18650's were 85% lower cycle.

I'd love to do LifePO4 as processing 5,000+ 18650 cells took me over a year part-time - but the key thing for me is reliable purchase to know if they are new or 90% etc before I could make the leap. And I'm not about to risk $5000 or $10000 dollars on a single purchase, so it has to be a source or sources where you can get equivalent, reliable prismatic over a year or 2 so that when you space out the purchases you can still get equivalent matched cells... and in prismatic case, since there are *fewer* cells you can't distribute them to mix manufactures as easily as you do with 18650.

There are pros/cons in either direction as far as I can tell Smile
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