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What voltage pack to build?
#11
I too started at 24v (7s). But over time realized that for many reasons (shear amps / wire size / inverters / PV array+controller) had to go to 48v system to produce 45Kwh per day (in summer) and I'm only powering 40% of my house!. For example, my home has 240v @ 200amp main panel service. A 12,000watt inverter = 240v @ 50amp - e.g. 25% of grid service.
To supply a 12,000 watt its 48v @ 250amps. At 24v system, that would be 500amps - and you have trouble (and more expense) finding circuit breakers etc at 500amp range. My PV array to supply this 12,000watt inverter is 7Kwh - e.g. I have 2 strings of 48v @ 70amp (and 2 Midnite Classic 150 MPPT controllers). You won't find it convenient to have a 7Kwh PV array at 24v level as you'd need double the controllers (and wiring).

Bottom line - IF you are shooting to power a home, AND that means 240v@50amp or larger... I would research the overall chain of parts starting with PV Array -> Controller -> Circuit Breakers -> Battery Bank -> Inverter and associated things like wiring, transfer switches etc.

I can attest that 48v system WILL get you 240v @ 50amps.
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#12
(05-05-2019, 03:26 PM)OffGridInTheCity Wrote: I too started at 24v (7s).   But over time realized that for many reasons (shear amps / wire size / inverters / PV array+controller) had to go to 48v system to produce 45Kwh per day (in summer) and I'm only powering 40% of my house!.     For example,  my home has 240v @ 200amp main panel service.   A 12,000watt inverter = 240v @ 50amp - e.g. 25% of grid service.
To supply a 12,000 watt its 48v @ 250amps.    At 24v system, that would be 500amps - and you have trouble (and more expense) finding circuit breakers etc at 500amp range.      My PV array to supply this 12,000watt inverter is 7Kwh - e.g. I have 2 strings of 48v @ 70amp (and 2 Midnite Classic 150 MPPT controllers).    You won't find it convenient to have a 7Kwh PV array at 24v level as you'd need double the controllers (and wiring).  

Bottom line - IF you are shooting to power a home,  AND that means 240v@50amp or larger...   I would research the overall chain of parts starting with PV Array -> Controller -> Circuit Breakers -> Battery Bank -> Inverter  and associated things like wiring,  transfer switches etc.

I can attest that 48v system WILL get you 240v @ 50amps.

If You need that kind of power, You may even aim higher, say 96 Volts. Smile

ChrisD
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#13
But 96V is not something you find for home consumers. Its 48VDC max as of today. That also falls under the max voltage legislation in many countries.

I would love to have a 320VDC system but not doable for a proper cost.
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
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#14
A lot higher than 48V you end up choosing between death by internal burns or death by external 3rd degree burns from embeded molten copper. HV DC faults and shocks are not nice, compared to AC. If had been working with DC at the same voltage rather than AC in the past few decades i would not be writing this.

Cheapest way to go 'large' is with a used commercial UPS system, 10kVA £700 (240V battery/20 cells / +/- 120V)
https://www.ebay.co.uk/itm/10-kva-UPS-si...3810606334


In terms of starting off with 24V that is a good strategy if you dont have enough cells and want to build packs first, moving up to 7x2 parallel pack arrangement then 7x4. Once you have enough packs made up you can just reorganise and put 14 in series and you then have your 48V. If you have a modular BMS then gong 24V to 48V is no issue or just keep the balancing at 24V and split.

The only advantage with starting at 48V is your inverter as this can be the most costly part of the build and replacing it later on is an added cost... charge controllers, buy 48V capable ones....
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If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#15
(05-05-2019, 08:23 PM)daromer Wrote: But 96V is not something you find for home consumers. Its 48VDC max as of today. That also falls under the max voltage legislation in many countries.

I would love to have a 320VDC system but not doable for a proper cost.

Also the price of circuit breakers starts to really climb once you go over 48v. It's really not practical or safe for home use. 28s is going to be expensive and potentially difficult to keep balanced as well. Especially if you are using second hand cells.
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#16
This thread started with a new user that have gathered a few hundred laptop cells.

Now we are talking 25 KW  Huh

If anyone is serious about building a system capable of 48V @ 500A, I suggest You use something beefier than second hand laptop cells.

ChrisD
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#17
Chris I use 2nd hand laptop cells and my system is capable of 500A easy...And yes it started with a new guy but most people dont think enough ahead when they plan and that tend to get more expensive in the end. Its important to know where and what limits are out there.

I started with LA some years back and then i started with a LiIon bank that was capable of only 1kW continues. But with that said I started with a base line that allowed me to expand without swapping out to expensive gear along the way. And now I have a system that run my whole house Wink
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The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#18
48 for sure!
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#19
(05-06-2019, 07:08 AM)ChrisD5710 Wrote: This thread started with a new user that have gathered a few hundred laptop cells.

Now we are talking 25 KW  Huh

If anyone is serious about building a system capable of 48V @ 500A, I suggest You use something beefier than second hand laptop cells.

ChrisD

Yep I am serious and am building my system with measly old second hand laptop cells. Almost at the 2800 cell mark and soon it will begin.

At my average mAh results I should have 482.33 Ah which translates to 27.01KWh at 56V. Can't wait. Big Grin Big Grin Big Grin

Oh and while staying on topic yes 48V nominal of course.
Wolf
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 5/16/2019
Total Number of Cells                          5198
Cells  >80% of Capacity                      3675
Cells <80% of Capacity                       1483
Cells ≥2200mAh & ≥ 80% & ≤75mΩ    2539
For Info Google Drive
Not your average Wolf       
            Cool



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#20
(05-09-2019, 03:32 PM)Wolf Wrote: Yep I am serious and am building my system with measly old second hand laptop cells. Almost at the 2800 cell mark and soon it will begin.

At my average mAh results I should have 482.33 Ah which translates to 27.01KWh at 56V. Can't wait. Big Grin Big Grin Big Grin

Oh and while staying on topic yes 48V nominal of course.
Wolf

I am really impressed by Your spreadsheet. You must have put an immense amount of work into this (hats off).
Now I can not wait to see You get started. Building cell-banks, connecting it all up, and see how it all works out.

I wish You all the best, and hope You keep us posted about how You get along Smile

ChrisD
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