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why 80p for powerwall?
#11
(01-23-2020, 03:19 AM)Oz18650 Wrote:
(01-22-2020, 08:17 PM)OffGridInTheCity Wrote: I've stumbled into the idea of building to a specific ah per pack rather than specific number of cells.

I shoot for 130ah per pack.    In the early days I bought 2100mah cells it took 62'ish to make 130,000mah.   Lately I've been buying 2600mah cells where it takes 50 to get to 130,000mah.  But all my packs are the same physical size - so in terms of mah they are interchangeable.      
By referring to the packs as interchangeable, it sounds like you are saying that you put the 62 cell packs into strings along with the 50 cell packs?

If so, the current per cell (stress on cells) will be higher for the 50 cell packs.

Common design is that each cell should not be providing more than 500ma of current.

Using that number, your 62 cell packs could comfortably deliver 31A of current.
If you have a string of 62P cells and you substitute in a 50P cell pack and draw 31A from the string, you will be taking 620ma per cell from the 50 cell pack.

>Common design is that each cell should not be providing more than 500ma of current.
I wouldn't disagree with this - but its not a fixed rule. This is more about testing on the cells you put into your packs. If you test at 1,000ma (and cells are designed for this) then 1,000ma is OK. I (like many others I'm sure) use 500ma discharge tests because that's what OPUS supports easily Smile

>If so, the current per cell (stress on cells) will be higher for the 50 cell packs.
*Removing my original comments here about different packs discharging at sightly different ma rates/cell as this is 'incorrect logic'. Thanks to @Daromer for correcting the discussion in following posts.
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#12
All the cells are in parallel so the 50p pack doesnt get more current per cell than the 62p pack gets unless you fucked up...
Think someone missed a lesson in ohms law there Big Grin

Once again dont overcomplicate things here. There aint no magic behind or hidden things ongoing Smile
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#13
(01-23-2020, 06:14 PM)daromer Wrote: All the cells are in parallel so the 50p pack doesnt get more current per cell than the 62p pack gets unless you fucked up...
Think someone missed a lesson in ohms law there Big Grin

Once again dont overcomplicate things here. There aint no magic behind or hidden things ongoing Smile
You're saying that all cells (50 + 62 = 112 in this example) deliver the same ma/cell to meet the overall load on the battery - correct?   I'm asking so I can correct my own thinking Smile
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#14
Yes you are correct. As long as you didnt do something wrong with wiring so the resistance in busbars and wires causes issues.

those 50+62 is just like having a larger pack of 112... "basically" because once again there is of course resistance everywhere but for a normal user here that really doesnt change that much. The internal resistance in the cell causes larger differences in the same pack than between packs.
NOTE! My links supplied in this message may be affiliated with Ebay and by clicking on them you agree on the terms.
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
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#15
WOW! I'm so glad y'all mentioned amps per cell - I totally forgot to double check my amps per cell calculations. I was going to build a quick & dirty 14s30p powerwall (3 kWh, 1+ hour of run time - simply because I'm so eager to try things out) and that would of been a disaster!!! Based on our average energy usage of 2.5 kWh that would of equated to 1.8 A per cell continuous draw and based on a "better safe than sorry" guesstimated surge/spike/burst of 8 kWh that would of equated to 5.6 A draw per cell. And as y'all said laptop batteries are low drain so they would not have been able to manage those draws.

I guess I'll stick to my original configuration of 14s300p (30 kWh, 10+ hours run time). This will give me 180 mA draw per cell continuous and 560 mA draw per cell during a surge/spike (refrigerators, furnace, AC unit, etc.)... are these safe draws or should I add more cells in parallel to get the surge/spike draw under 500 mA too?

Also, would a 1s300p config (20 cells deep x 15 cells wide) be physically unmanageable and/or a b*tch to troubleshoot/maintenance or should I simply throw four 1s80p packs together in parallel and call it a day??
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#16
There is no real reason for the 80p pack size or 4x10 design, other than that's how Peter/HBPowerwall did it and this is his group/forum. Most people just see the endless videos and pictures of others doing it and assume it's the best way. I'm doing 300p packs in a 15x20 layout.
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#17
(01-23-2020, 11:28 PM)crashintoty Wrote: Also, would a 1s300p config (20 cells deep x 15 cells wide) be physically unmanageable and/or a b*tch to troubleshoot/maintenance or should I simply throw four 1s80p packs together in parallel and call it a day??

Just all depends on what you need for you space. If you can put 300p on a shelf and you can handle it just fine, go 300p. If it won't fit on the shelf and/or you can't manage it, moving it around, lifting it, etc, then go smaller and put two strings in parallel instead of making 1 large one.

Personally, even if I can manage it, and it does fit the space, I'd go with 2 strings. This allows you to bring a string down to work on it if need be and your system is still operational. As daromer mentioned, N+1 configuration. This also allows you to add a 3rd string easily as you've already built for 2 strings to be connected in parallel.
The down side is, you'll need more bms monitoring (longmons if going Batrium) doing the N+1 route. But in the end, worth it, imho.
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#18
(01-23-2020, 11:28 PM)crashintoty Wrote: Also, would a 1s300p config (20 cells deep x 15 cells wide) be physically unmanageable and/or a b*tch to troubleshoot/maintenance or should I simply throw four 1s80p packs together in parallel and call it a day??

No, not at all. This is exactly what I'm doing and it works GREAT. I even wrapped mine with heat shrink and the compression of the shrink creates a rock solid pack.

There's some info in this video...

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#19
(01-23-2020, 11:54 PM)mike Wrote:
(01-23-2020, 11:28 PM)crashintoty Wrote: Also, would a 1s300p config (20 cells deep x 15 cells wide) be physically unmanageable and/or a b*tch to troubleshoot/maintenance or should I simply throw four 1s80p packs together in parallel and call it a day??

No, not at all. This is exactly what I'm doing and it works GREAT. I even wrapped mine with heat shrink and the compression of the shrink creates a rock solid pack.

There's some info in this video...


45g/cell * 300cells = 13,500g = 30lbs.    30lbs is doable (my 'units' are 20lbs)  but reaching an upper limit (for me) if you have to stoop over or do any kind of awkward placement/hookup.    Another consideration is BMS balance - Batrium is perfectly capable of 1s300p balancing (in a reasonable time frame) but I'm not sure how many other BMSs are beefy enough.
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#20
(01-23-2020, 06:14 PM)daromer Wrote: All the cells are in parallel so the 50p pack doesnt get more current per cell than the 62p pack gets unless you fucked up...
Think someone missed a lesson in ohms law there Big Grin

hmmm....
I think we are talking about different scenarios.
--
There was the section which talks about physical size and mah being the same for the packs and the word "interchangable".

"But all my packs are the same physical size - so in terms of mah they are interchangeable"

Where the word "interchangable" was used, I took this to mean that a 50P pack could and would be swapped into a string instead of a 62P pack (with the justification that this is ok, because each packs capacity is the same).

eg for 7S

-- 62P -- 62P -- 62P -- 62P -- 62P -- 62P -- 62P --

becomes

-- 50P -- 62P -- 62P -- 62P -- 62P -- 62P -- 62P --

In this "interchanged" scenario the 50P pack is delivering more amps per cell than the 62P packs.

I think you were talking about adding 50P packs to the 62P packs effectively making 112P packs.
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