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why 80p for powerwall?
#1
I notice a lot of DIYers online (including YouTube) make 80p cellpacks/modules(/strings?) for their 24v or 48v power wall batteries. Why is 80p so common?
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#2
There is no hard or fast rule regarding this and its 100% preference / customizable.

Your parallel count should be determined by your capacity requirements and max load of the systems design.

Your series count should be determined by your equipment operating voltage range.

To answer your question, I suspect people just like to copy other people who have made systems and don’t really take much of the above into consideration.
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#3
If you figure max amp draw of about 500mA per cell, then 80p would be 40A. This is usually the max most ppl would draw from the strings. However, there are many who make larger packs. Mike has 400p, I think. There are several with 120p.

Also, it's usually based on pre-defined holders. If you get the 4x5 holders, then 1 would be 20p. Then 4 of them together is 80p. The next would be 100p. So this is probably another reason for this common arrangement.
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#4
The 80p comes from that it basically is 10kwh. A round and even Number based on the cells available att that time. The size also did fit a bench om the depth. It hvae nothing todo with average or max current.

Then many People followed us when we started.
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#5
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.      

Another reason for pack size is the physical space that will house them.    As you can see in the picture below - my shelves are only so large.

I also used the 4 x 5 cell spacers as protection for the + and - terminals...  so a few spaces a 'wasted' for this as well...

As you can see in this picture - the center pack (grey) is one of those 50cell packs and the one above (pink) and one below (blue) are of the 62'ish cell size.


However, as mentioned above - this totally up to you and I wish you good building Smile
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#6
(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.
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#7
Interesting. I was starting to think 80p was the go to because it's as big as you can physically go before the pack started suffering structural issues.

Good point about the physical location of the battery - I don't even have shelves to hold 80p packs (16" to 18" deep).

(01-23-2020, 03:19 AM)Oz18650 Wrote: Common design is that each cell should not be providing more than 500ma of current.

Why is 500mA the suggested limit? Is this for continuous or for burst/surge/spike? If the former, what is the suggested max for burst/surge/spike?
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#8
(01-23-2020, 04:17 AM)crashintoty Wrote: Interesting. I was starting to think 80p was the go to because it's as big as you can physically go before the pack started suffering structural issues.

Good point about the physical location of the battery - I don't even have shelves to hold 80p packs (16" to 18" deep).

(01-23-2020, 03:19 AM)Oz18650 Wrote: Common design is that each cell should not be providing more than 500ma of current.

Why is 500mA the suggested limit? Is this for continuous or for burst/surge/spike? If the former, what is the suggested max for burst/surge/spike?

You can also "stack" the common 4x5 holders.
E.g. 3 deep and 2 high,
Or stand them so they have the 5 cells high and the 4 cells deep.

I did not easily find reference to 500ma per cell just now.
I believe there are several reasons to aim for ow current draw
- longer run time. if the cells can supply 2000mah and you draw 1A from each cell, then your pack will be flat I 2 hours, if you draw 500ma then the pack will last 4 hours
- less stress which equals longer lifespan for the packs
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#9
There's a Battery University link that shows that <1A draw increases life cycles. I don't recall the link, or which post(s) has it. This has specific relation to low drain (LD) cells, like those used in laptop packs and such. They are not designed to run at or greater than 1C for very long (1C = rated mAh capacity / 1000, generally speaking). A lot of LD cells are rated at .5C. So if a cell is rated at 3000mAh, then 1C = 3A, and 1.5A is .5C.
Generally speaking, the less aggressive the current draw, the longer the cell will last. 500mAh is a pretty good balance between usable power and size. Some DIY builds go as low as 250mA, and I think there are a few even lower. However, spike current could still be around 1A.
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#10
The 500mA comes from a generic saying. If you test charge/charge at 500mA thats also your max current. If you test at 1A thats then your max. Its not based on any university page stating it prolongs life span either

Once again 80p choosen because of 10kWh and that was what Peter among others had in total cell count. The size of the pack was also workable to move around. Nothing special.

The max current to draw on above depends on the tested current.

There is no magic over this Smile
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