Why?

So, I stumbled across this forum and after reading a bit I soon had in my hand an old laptop battery... I crack it open and proceed to rip out the cells. "Neat!" I think to myself as I start to open up the second battery. I start envisioning a nice array of these cells hanging from my wall in the garage and soon am on the phone with a local PC recycle shop and sure enough they have roughly 50 old batteries I can have free.

Now I have these batteries and have opened up a dozen or so... Ive got a DMM and checked voltages and the next thing I know a new Opus charger shows up at my door. I toss 4 cells in it and it starts charging away.

Then I realize... Why? Why do I need this? What purpose do I have for building this?

I do have one thought, the thought that brought me here to begin with. A battery backup for my hot water heat. My new house is heated by a natural gas boiler that pumps hot water through out the house. The only electric in the system is the pump, valves and thermostats.. I could easily power that with battery and if we lose power in the winter we can keep warm if nothing else.
So I think thats what Im going to build and why. What are your reasons for what you have built?

Using it as backup for the pumps is awesome use! My first experience startig with this 25 years+ ago was just that. Backup for pumps and the wood-boiler. Lead acid though but still

Currently i have mine to power the whole house... So no specific area

Just in case of power loss?

I read somewhere that part of the idea behind the PowerWall was to be able to charge/store power at night when the rates are lower and then draw from batteries mid-day when electric rates are higher. Does anyone do this? Where I live we have the same flat rate 24/7 so that is no use for me.

Some do but its not worth it in many places. I have like 0.5% difference in price during night compare to the day....

Yes, here in Florida, USA, the difference in on-peak and off-peak isn't worth it at the moment. However, my plan is to be off-grid 99%. The reason I keep the 1% is because I will still have grid connection, but, they will serve as my backup in case something happens with my setup (solar panels need to be reworked, lightning blew out something and needs replacement, moving equipment for what ever reason, etc, etc). For me it's only about $25 to stay connected to the grid. I think that's worth the insurance of emergency backup if needed.
Ultimately, I hope to fully disconnect as I'd have enough redundant systems to keep my house operational with what may come.

Initially just going to convert some UPS to Li-Ion - give them a longer run time. Later when I have tested more cells time shifting off peak power until I can afford a solar setup.

Being able to isolate my electronics from the grid will be handy. Power quality out here can be pretty bad.