A to Z DIY off grid powerwall

PowerLio

PowerLio

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Hello everyone !

I need your help understanding something.

My project is to make a 1 kwh powerwall short term and to expand it later to get 10 kwh. This will power a small house (just lights, small inverter and a 600w water pump which will be used max 30 min/week). Powered by solar panel.

The idea is to do it entirely myself (the led lightind using 12v led strip, the powerwall using laptop batteries, the solar panel using cells and connecting them to create a solar panel).

So I was planning to make a 7s system to get 24v but the problem is that most of the solar cells I can buy are 4.5w 0.5v so to build 24v solar system I would need to wire 72 cells together...

My question is: even if the li ions battery are not suited for 12v can I make a 4s system and then use adjustable buck converter to have 12v output (because if I make a 24v powerwall I will use anyway a 24v to 12v buck converter for the lights) ?

Then with a 4s powerwall, will a 12/24 pwm solar charge controler will work ? even if the 4s is a little higher than the regular 12v batteries ?
 
Short answer: No, you can't and it won't work.

Solar panels are always made from individual cells in series. If you buy a panel then the company has done that for you. This is technically necessary and there is no shortcut. If you want to build this yourself then you have to wire a lot of cells together, yes!

4s and a 12V buck converter might work. You should set it to 11V output since you always need a gap between input and output. As the input will drop to 12.0V as the cells discharge you can't efficiently put out 12V.

You should use a MPPT controller if possible, PWM only if it needs to be really cheap. But either way you can't use a 12V nor a 24V controller obviously. The 12V one won't provide the necessary charging voltage and the 24V will always provide too much voltage. And there aren't small and cheap solar charge controllers compatible with lithium cells as far as I know.

Go for 7s, buy panels if necessary, watch carefully when charging with a 24V controller, a MPPT one preferably, and/or use a BMS, power as much as possible directly from 7s / 24V and use a 12V DC-DC converter for the rest.
 
And don't forget, you can't charge a 7s lithium battery from a cheap 24V controller just like that. You need adjustable voltages at least and better also have configurables phases of lead acid charging. You need some way of CCCV within the limits of the lithium cells with a proper cutoff.
 
If you need 12V for your lights, then just use the buck converter for the lights. The lights use very little current, so your buck converter should be able to handle the load easily. Aside from that, you could also you resistors to bring your voltages down to acceptable ranges. Though, this could cause the resistors to get hot in the process. It's cheap to do it this way, but not the best way.
 
You have to do the related math. With a random online solar calculator, I plugged in the numbers. I assumed 3 days of storage, using a rule of thumb for lithium solar battery banks, so 1kWh/3=333Wh/day. Plug in 24V for the battery, and it gives you about 42Ah.

This particular calculator assumes 16V nominal solar panels (12V panels). Plug in 100W for the solar panels, and itsays you need 4 panels, with 2 in series and 2 series strings. So even if you made 12V panels, due to ease of assembly, you still need four 100W panels for a 1kWh battery bank. That's a rough estimate. It wasn't clear from a quick glance how many derating factors were included in the calculation.

There's no other way to break it down.

Oh, I didn't factor in the ccharge-discharge curve for the lithium chemistry, so you might actually need at leastthree 12V (16V nominal)panels in each series string. I'm using LiFePO4cells, so I forgot that factor, because the voltage range of thecharge-discharge curve of LiFePO4 more closely matches that of Lead Acid, for which standard solar panels are suitable. If you're making panels from individual cells, you could factor that into your series count, so that you would need only two in series.

Also, I looked into making my own panels and actually bought a rather large batchof cells that I found for sale privately. I found that properly glazing the panels might be challenging, and I never found the time to assemble the panels. The cells are still in the hardware closet.

OOPS! I looked again, and the result of thecalculation was 2 panels, in one string.
 
Well, thank you all for your time so, what I'm going to do is make 2x 18V solar panel and wire them in series in order to test the process with a small Li-ion battery. By the way I will receive laptop batteries next week hopefully!

I've bought the material in the morning to make it and I will begin to asseble the parts in 2 weeks I think so maybe I'll post photo of my progress :)

Thank you again for all the help!

DarkRaven said:
And don't forget, you can't charge a 7s lithium battery from a cheap 24V controller just like that. You need adjustable voltages at least and better also have configurables phases of lead acid charging. You need some way of CCCV within the limits of the lithium cells with a proper cutoff.
Click to expand...

Do you mean that a mppt charge controller is mandatory or have I misunderstood ? For now I bought a 10 pwm 12/24V 20A (This one: https://aliexpress.com/item/10A-20A...troller-with-LCD-Dual-USB-5V/32818868189.html)
 
PWM works but you should keep the output voltage of the panel closer to the battery for it to be more optimal. Neither less you can save all between 10-50% going MPPT.

Making your own panels: Only do it because you want to test. Its not really worth it today. You can buy complete panels for nothing today :) So the time it takes to make your own tend to get to expensive unless you got plenty of time :D
 
daromer said:
PWM works but you should keep the output voltage of the panel closer to the battery for it to be more optimal. Neither less you can save all between 10-50% going MPPT.

Making your own panels: Only do it because you want to test. Its not really worth it today. You can buy complete panels for nothing today :) So the time it takes to make your own tend to get to expensive unless you got plenty of time :D
Click to expand...

That's noted ! thx
 
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