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1st PW build- w/ solar, EV, and TOU considerations... (in San Diego)
Hi all-
Home tinkerer here, I've built e-bike packs in the past, this is my 1st powerwall build.

Parts so far-
APC smart ups 750 (24v setup)
~160 voltage and capacity tested 18650 (used laptop cells from local re-cycler)
12ga copper household wire
20 cell holders
7s BMS (cheap non-manageable Chinese one)
large "security system" cabinet- about 2ftx3ftx5in with locking door- this would hold the cells.

Reason for PW build-
I've got the cells anyway.
In in San Diego w/ a 4kw solar panel setup. Because I also have an electric car (Leaf), I'm on the EV-TOU2 plan with SDGE.
so- at night, I pay ~19c/kwh, and between 12pm and 6pm, I pay (or get credit for) 57c/kwh.
This allows me to get a smaller solar panel setup than I use, only because I get 57c/kwh credit when the solar system is crankin'.
But it's not quite enough...  I want to charge the bank when paying 19c, and grid-tie back at 57c between 12 and 6. I'm sure most here are in a similar boat.

I'm aware I can adjust the APC UPS to set a correct float voltage for a 7s setup. I'm investigating what grid-tie inverters to use (more on that later).

so my plan is-
Use modified 24v APC UPS to charge at night, and provide emergency power to small appliances when power is out
Make individual 3.7v packs with a bolt connector at the top (looks like this is pretty common). Use resister wire for cell fuses.
Mount the 20s packs with 1 cell removed and the frame notched so I can push-mount it onto 1/2in pex (like this)
managed though the cheap BMS.
The cabinet can fit 7s all next to each other. I'll start the install at the top. This will allow me to pull a pack, add another 20 cells when I get them, and re-insert the pack.
I have 8ga and 4ga wires from my stereo install days- this would go to the grid-tie inverters.

Grid tie inverters- 
I have a cheap sun-600g inverter that would operate in the range the 7s puts out. It cuts off at 22v, and 21v would be the lowest safe discharge voltage for a 7s setup, so I'm cool there (I have a BMS anyway). I could use this and hope it doesn't set on fire.... or....
My solar setup uses Enphase micro-inverters, so I have the monitoring station, etc.
This project uses an Enphase inverter w/ Leaf batteries
I'll pick up an enphase m215 (or similar) with operating voltages in the same range os the 7s build. I know 215w isn't much, but I could still discharge it fully during the 6hr peak usage window.

Ideas? Comments? am I crazy?
Korishan and chuckp like this post
Are you crazy? Most definitely. But then again, most of us are Wink

I suppose you are going 24V as that's the UPS you have available to you or you are able to get a hold of cheaply, correct? What model UPS are you going to use?
completelycharged likes this post
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I'm only going 24v right now as I already have an APC SMT750US. I can remotely monitor battery voltage and UPS output (when the power is out). . I'm also at 24v as that seems to be what many grid tie systems accept. Because this is a modular setup, I can grow it to a 14s setup when I get a bigger inverter.
Wow, 57c credit vs 19c overnight gives a lot of margin... install 50kWh charge it every night !!!

Forgot to add, it's like jail here, nobody is crazy... woop woop...
Direction change- 
not much will change, but I picked up a 2200kva ups that uses 48v, so I'll be building a 48v powerwall now.
completelycharged, jdeadman, Korishan And 1 others like this post
That sounds like a better inverter. I have a 1500VA which is only a 980W inverter and I max it out under peak loads from my internet devices 1 computer, fridge and freezer. but it normally runs at 10% except when the freezer kicks on. then it OL it but it takes it in stride. The issue I have is I have been unable to adjust the Charging voltage so My system I have to externally charge to 28V (any more and the UPS gets angry).

Good luck with your unit.

Eventually I will be going for a Split phase inverter for my house so I can get 220V AC for a welder
If it is an APC unit, first job is to take the sounder out... saves messing around trying to reset any battery counter and anoying beeps when the UPS thinks it has 30 seconds left instead of 30 hours... just been running the 800W vacuume with a 2200VA unit to clear sawdust, spiders and a part that dissapeared up the pipe I did not know what it was from.. it won't run my saw though, freaks out, overloads and shuts down.
you can turn off the Alarm in the software.
Resets each power cycle and needs you to login each time... I also have an older MGE unit, which seems to need windows 1866 and java to run the stupid interface to change any settings (yep it is that old...).. pliers, rip, done.
Any updates with you? Let me know how you're doing. I'm curious.

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