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advise on design
#1
Hi guys,

I am after somwhere between 40-60kwh for my pack (aiming for the moon) and after some recomendation on the size of the banks.

have a solax 5kw inverter and charger. seems 14s is the way for a 48v system but what is the advantages/disadvantages of 100P vs 80P vs 150P??

keen on your advise.

Also thinking about storing them with cells in vertical position (to aid in cooling) with a side by side pack configuration (will upload a pic of my thoughts later)

Bus bar terminals will need the same wire length where they connect? if so some funky chicken wiring will need doing to go side by side. I have the space and wonder if the passive cooling advantage would make it worthwhile?
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#2
The total "P" term is dictated by your bank capacity as you've fixed your system voltage at 14S (48v).

So 60kWh / 48V = 1,250Ah.
1,250Ah / 2.6Ah = ~480 cells. Assuming 2.6Ah (2600mAh) per cell.

14 x 480 = 6,720 cells. About 300KG worth, plus connections and mounting, so perhaps 350KG all in.

You can arrange this in one large bank so you only need one BMS, effectively giving you 14S480P, or you could split this up into strings, each with their own BMS system (at greater cost). Ie 4 strings of 120 cells as 14S120P with 4 BMS systems. The 4 strings only connect at the very top and bottom of the pack.

Multiple strings would give you redundancy allowing you to take a string out of service but leaving the others in the system. Single string is cheaper and simpler.

Either way, making 14 physical blocks of 480 cells wouldn't be much fun, so you will make smaller packs and then electrically connect them with wire to make a 480P pack.
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#3
(11-19-2018, 10:41 AM)Scottietheyoung Wrote: seems 14s is the way for a 48v system but what is the advantages/disadvantages of 100P vs 80P vs 150P??

Also thinking about storing them with cells in vertical position (to aid in cooling) with a side by side pack configuration (will upload a pic of my thoughts later)

Bus bar terminals will need the same wire length where they connect? if so some funky chicken wiring will need doing to go side by side. I have the space and wonder if the passive cooling advantage would make it worthwhile?

The larger the packs, the harder to move and the more difficult the work to assemble. You also need the space for it, obviously.
The smaller packs are easier to manage and quicker to build.
Mike actually has a really large P arrangement. I just don't recall how many. I know it's well over 100.

The wire lengths generally need to be about the same, yes. But they don't have to be "exactly" the same length. Difference in a few inches is fine. The thing to make sure of is that the connections on the packs are balanced. The central bus bars are equal distance from cells. This will come into place when large current draws are applied.
Mike and Pete both have adopted a variation, where they parallel 5 cells together, then fuse that small pack to the bus bar. This is fine as long as the connection from cell-cell in that small pack is large enough to keep them equally balanced as well. So you wouldn't use a 26awg wire to connect them as it would be too small.

Passive cooling is fine. Generally speaking, if you have enough cells together, you really shouldn't have a heating problem. If you have 100A charge, and 200 cells in parallel, then each cell is only getting 500mA. If you multiple strings together, say 5 strings total, then at a 100A charge, each cell is only getting 200mA per cell. As long as you keep each cell below 1A, you really shouldn't have a cooling issue.
Depending on where you live, you might have a 'heating' issue, though. Cells don't like being charged/discharged below 0C/32F and can shorten their life span. So if you are in a climate where this can occur, some have actually designed their sheds/boxes/etc so that the inverters/chargers while running actually keep the area warm enough for the cells to operate safely.
Proceed with caution. Knowledge is Power! Literally! Cool 
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#4
When you are trying to get up in the terms of capacity I would personally look at larger batteries than an 18650. Something like a Leaf pack or like electric car that has been wrecked can get you up there in kWh and not be really tough to manage.

When I go properly off grid when the kids leave the nest I will be looking at minimum 100Ah Single cells There are Calb cells or the LTO cells too.

I currently have about 5kWh of build 18650's and that's 560 cells (7s80p) then I have another 1120 cells tested and ready to build for an additional 10kWh and that is about as big as I want to go on such small cells. too many places for things to go wrong.
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#5
As above .... I don't think it would be wise to build such a high capacity battery using such small format cells such as the 18650, especially given how cheap ex EV packs are now. There's just to many points of failure.

https://www.ebay.co.uk/itm/2013-CHEVROLE...ect=mobile

https://www.ebay.co.uk/itm/2016-BMW-i3-A...ect=mobile
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#6
(11-19-2018, 01:19 PM)Korishan Wrote:
(11-19-2018, 10:41 AM)Scottietheyoung Wrote: seems 14s is the way for a 48v system but what is the advantages/disadvantages of 100P vs 80P vs 150P??

Also thinking about storing them with cells in vertical position (to aid in cooling) with a side by side pack configuration (will upload a pic of my thoughts later)

Bus bar terminals will need the same wire length where they connect? if so some funky chicken wiring will need doing to go side by side. I have the space and wonder if the passive cooling advantage would make it worthwhile?

The larger the packs, the harder to move and the more difficult the work to assemble. You also need the space for it, obviously.
The smaller packs are easier to manage and quicker to build.
Mike actually has a really large P arrangement. I just don't recall how many. I know it's well over 100.

The wire lengths generally need to be about the same, yes. But they don't have to be "exactly" the same length. Difference in a few inches is fine. The thing to make sure of is that the connections on the packs are balanced. The central bus bars are equal distance from cells. This will come into place when large current draws are applied.
Mike and Pete both have adopted a variation, where they parallel 5 cells together, then fuse that small pack to the bus bar. This is fine as long as the connection from cell-cell in that small pack is large enough to keep them equally balanced as well. So you wouldn't use a 26awg wire to connect them as it would be too small.

Passive cooling is fine. Generally speaking, if you have enough cells together, you really shouldn't have a heating problem. If you have 100A charge, and 200 cells in parallel, then each cell is only getting 500mA. If you multiple strings together, say 5 strings total, then at a 100A charge, each cell is only getting 200mA per cell. As long as you keep each cell below 1A, you really shouldn't have a cooling issue.
Depending on where you live, you might have a 'heating' issue, though. Cells don't like being charged/discharged below 0C/32F and can shorten their life span. So if you are in a climate where this can occur, some have actually designed their sheds/boxes/etc so that the inverters/chargers while running actually keep the area warm enough for the cells to operate safely.

(11-19-2018, 02:31 PM)jdeadman Wrote: When you are trying to get up in the terms of capacity I would personally look at larger batteries than an 18650.   Something like a Leaf pack or like electric car that has been wrecked can get you up there in kWh and not be really tough to manage.  

When I go properly off grid when the kids leave the nest I will be looking at minimum 100Ah Single cells    There are Calb cells or the LTO cells too.  

I currently have about 5kWh of build 18650's and that's 560 cells (7s80p)  then I have another 1120 cells tested and ready to build for an additional 10kWh and that is about as big as I want to go on such small cells.   too many places for things to go wrong.

Thanks heaps for the input guys.

I have an internal garage for the battery system and feel confident in the fire safety of these systems after seeing how they are running in other applications. The garage never gets below 5C in winter as it’s fully insulted (better than our house actually.

I have a really good supply at  no cost of 18650's so hence the desire to build that big with them. the leaf packs are a poor lifecyle chemistry option compared to other larger cell options. even still the used EV pack route is very expensive ultimately.
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#7
Modular is the way to go with a big system.
That way you can take a pack off line without taking the whole system off line with it.
Figure out your plans & space to build it & this will help you work out pack size.
Running off solar, DIY & electronics fan :-)
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#8
So I am thinking of running 14S 120 P and have 4 strings to make up the size required.

My question I have is around the connection of the strings? Has anyone else got a thread with this many strings attached and pics of their design/ build?
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