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13s229p... wait, what?!!!
#1
So I was running numbers trying to calculate how many 18650's and which config I need to get 8 hours of battery runtime (500 Ah x 48 v = 24 kWh) for my whole home. Using harvested laptop batteries and assuming the lowest capacity will be 2200 mAh I'd need a 13s229p config (about 3000 cells). I imagine putting 229 cells together is not recommended or even physically sensible, so how does one configure a 48 v, 24 kWh 18650 based battery?
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#2
Simple, run several parallel strings. Instead of 13s229p (lets round up to 300p), you could do 3 strings of 13s100p. This allows you to bring a string down and work on it if needed. Also allows for easy expansion if needed.
Curious, why 13s instead of the more common 14s?
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#3
(06-10-2019, 08:40 PM)crashintoty Wrote: I imagine putting 229 cells together is not recommended or even physically sensible

Why not? I am running 300p packs. I have 28 of them...
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#4
Even 300P needs reinforcement, as you have found out, mike. I test fitted a 500P pack yesterday for my 2P7S500P setup, and it's not really manageable. Maybe with reinforcement, but I wonder if the bottom cell holders can handle the weight over time. I barely moved the pack around without it falling apart. I wish I would have weighed it, I bet it was over 50lbs. Anyway, I'm rethinking doing 500P packs. I think 200P is probably the max for these cell holders without some kind of reinforcement like mike has done, but I haven't done any real research into it, yet. Parallel strings are good if your BMS is expandable or you use multiple BMSs.

Formerly known as Dallski
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#5
I put a giant heat shrink around my 300p packs. The compression of the heat shrink adds an incredible amount of structural support.
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#6
Note that "48V" systems with 18650's are usually 14s, ~52V nominal.

To avoid structural issues, why not build like Owitte & make multiple smaller packs eg 40p units & parallel several to get each 1s section
Eg, round up to 240p & use say 6x 40p modules in parallel
So you'd have 14s x (6x 40p modules)p
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#7
(06-11-2019, 10:46 AM)mike Wrote: I put a giant heat shrink around my 300p packs. The compression of the heat shrink adds an incredible amount of structural support.

Makes sense. The 5 bolts hold it together in the center and the shrink wrap pulls in the outsides. That's all you really need. Could probably do 500P that way with a bigger shrink wrap and more bolts. You could make smaller packs, but 1 big block is less wiring, more compact, semi-portable, and easier to monitor via BMS.

And I would go 15S instead of 13S. Can do 15S200P to get to 3000 cells. Just restrict your top-end voltage and you should be fine. It's better for the cells. And a higher voltage pack will be slightly more efficient.
Formerly known as Dallski
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#8
Vote +1 on Korishan's comment.
Having a big pack (say 300p) is a nightmare when you have leaking or dead cells and have to remove all the fuses to find it/them. The whole system will be completely offline for days perhaps.
With 3x batteries each 100p system, you can keep 2x batteries running while doing maintenance on that 1 pack. And you only need to remove 100 fuses.
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#9
(06-11-2019, 01:40 PM)Dallski Wrote:
(06-11-2019, 10:46 AM)mike Wrote: I put a giant heat shrink around my 300p packs. The compression of the heat shrink adds an incredible amount of structural support.

Makes sense. The 5 bolts hold it together in the center and the shrink wrap pulls in the outsides. That's all you really need. Could probably do 500P that way with a bigger shrink wrap and more bolts. You could make smaller packs, but 1 big block is less wiring, more compact, semi-portable, and easier to monitor via BMS.

And I would go 15S instead of 13S. Can do 15S200P to get to 3000 cells. Just restrict your top-end voltage and you should be fine. It's better for the cells. And a higher voltage pack will be slightly more efficient.

What does it mean to "restrict your top-end voltage"? Also, if I go with a 14s or 15s setup, will a 48v inverter handle a fully charged battery at 58.8 or 63 volts or will I then have to get a 60v inverter?
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#10
Restrict top end voltage means dont charge to 4.20V. General consensus is to charge max to 4.10V and a lot of people are even charging only to 4.00V. 14S at 4.2V is the same as lead acid (14.7V charging) at 24S (4 12V batteries in series). 15S means only charging to 3.92V per cell at 58.8V, greatly extending the life of your cells. The better 48V inverter/chargers can run over 60V, so you can go to 4V or higher if you need to.
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Formerly known as Dallski
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