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is there a "standard" size diy powerwall
#1
hi
I have experience with 18650s building while batteries but I have never built a powerwall. I have an opportunity to buy new 18650s at a good price. I am trying to figure out how many I should buy to build a standard size powerwall. What I mean is that I am sure there is well know go to equipment that would limit the size, otherwise you have to buy crazy expensive speciallized equipment. Also I imagine there is a standard size of module you build of parralell cells. could someone please give me an idea of module size and number of modules ( I imagine 13 to make 48 volts?) so I can estimate how many cells I need.
thank you
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#2
Really depends on your situation. What you want to power and for how long. Generally 48V is 14 cells. Please read the FAQ sections as it will answer most of your questions
******Hi My name is Jason and I have SOCD (Solar Obsessive Compulsive Disorder)*******
Current Powerwall - 1400 Cells 7s200p (modular 40p packs) ~ 12kWh of storage     4x 315W Canadian Solar Panels

Working on the next 7s40p packs     ~2.5kWh

Waiting on 2000 Cells of unused Sony vt4 (2000mah 30A) ~ 15kWh      hehehehehe  More Power
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#3
I find that you can extrapolate a design that balances 1) PV Array input,  2) Battery size (life expectancy), 3) Power consumption, and 4) Goals. 

For example, last year I ran a 7kw PV array from which I got 8,700kwh of power to my home.   In the middle, I had a 48v@780ah battery bank (5,000 cells) with a 32% DOD.    My goal is to consume all the power produced within a 24hr period.    

There are many details -  but in general I think you can do a kentucky windage 'sizing of things' proportionally from these kind of stats.   For example - I needed an average 32% DOD (with ahigh of 46%) as a buffer in my goal to consume all the PV power in a 24hr period for each day of the year, I could function (in theory) with 1/2 the battery size (2,500 cells - 2167mah average) with a 92% high / 64% average DOD.      But of course a higher DOD will shorten the battery bank life...    there are many trade-offs.

Maybe you could share your planned PV array size, inverter (power output) size, and your goals (off-grid, grid-tie, ...), the average mah per cell you plan to buy, and I think this forum could give you some ideas Smile
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#4
(12-11-2019, 02:58 PM)heynow999 Wrote: hi
Hello

(12-11-2019, 02:58 PM)heynow999 Wrote: I have an opportunity to buy new 18650s at a good price.
What's considered a good price? And from where?


(12-11-2019, 02:58 PM)heynow999 Wrote: I am trying to figure out how many I should buy to build a standard size powerwall.  What I mean is that I am sure there is well know go to equipment that would limit the size, otherwise you have to buy crazy expensive speciallized equipment. Also I imagine there is a standard size of module you build of parralell cells.

This really is based on "your" application. You can make any sized parallel pack. Starting out, I would suggest smaller packs (<=80P) until you get the hang of it. Mike has packs that are 400p (I think is the number). Just depends on how many cells you have available at once, as you don't want to build them out of balance or take too long between builds. You want to try to build a full string together if possible to minimize to much variance in cell quality/life/degradation.

There are some that have a powerwall that's only 5kWh. Others have 20kWh. And I think there's at least 2 that have over 50kWh of storage. Just depends on how many kWh you need to store based on your consumption, how much solar/hydro/wind collection you can put up (or if you will be charging off-peak times)

(12-11-2019, 02:58 PM)heynow999 Wrote: could someone please give me an idea of module size and number of modules ( I imagine 13 to make 48 volts?) so I can estimate how many cells I need.
13s is too small for 18650 cells. Reason being you are killing the cell life cycle by pushing them to 4.2V; or beyond depending on charger. 14s or 15s is more applicable to "48V" systems. The base system is based off of the "nominal" voltage, not the "max" voltage. So for a standard li-ion cell, the voltage range is 2.8 - 4.2V. The nominal voltage is between, or about 3.5V. 48 / 3.5 = 13.7. As we can't have a partial cell, we round up to 14s.
At 14s we have:
2.8V Bottom Voltage:   39.2V
4.2V Top Voltage:      58.8V

If we compare to a lead acid battery:
11.1V Bottom Voltage:  44.4V
14.5V Top Voltage:     58.0V

All multiple of 12 based systems are based off of lead acid as that was first. Until lithium becomes the standard, this will always be the comparison.

(12-11-2019, 02:58 PM)heynow999 Wrote: thank you
You're welcome
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#5
Powerwall to me is a generic term… I don’t feel there is a standard size, shape, or form factor… it really just dependent on your needs / application.

You could be sizing a powerwall to run a shed, garage, or a whole home. You could build a portable powerpack/solar generator in a box.

Start with some basics… if your application uses 4 kWh a day then use that as your jump off point. Add 30-40% for cell health, efficiency loss, and degradation over time.

So you need 5.6 kWh of 18650s… if you get 2900 mAh 18650s, you need approximately 1931 cells.

The arrangement will depend on your load requirements and equipment. You might end up at 24VDC (7s) or 48VDC (14s to 15s) depending on your inverter and charge controller choices.
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