Solar power panels continue to drop in costs but the batteries that can store the power they produce are NOT coming down at the same rate. Most of us do not have unlimited cash to purchase batteries with.
Here are a couple of ideas for batteries taht are not the run of the mill ways to buy them. First, look at automotive salvage yards. Hybrids get wreaked too, and when that happens youy can buy a single battery that will run your entire house, at salvage rates! Local salvage yards are the most cost effective, and some will not want to fool with unfamiliar technologies. So start there, then try some online salvage sources. Here is one I have found, that I know almost nothing about, except they willsort by which is closest to you by zipcode!
Image from this web page
http://www.car-part.com/
I found a Chevy Volt battery a hundred miles away on this site, sso this is a good option. If you find others, post them here. I have no affiliations with this website or business.
http://www.car-part.com/
But try it, it is always free to look, LOL!
The second source is your local computer repair store. Go in and buy up the failed battery packs for recycle prices. Bring them home, and open them and recover the good LiIon cells from the packs. They are 10 watts apiece, and I have purchased 85 good cells for $10; this give fourteen 24 Volt batteries at 850 watts (35 amps) for $10. The computer stores will be happy to see you, as recycling battery packs is a pain for them.
Note: LiIon batteries can burn when you mess with them, so keep a metal gallon can half full of sand close when you work with the (in case you short out a battery) as a safety bucket, in case one does ignite.
If you build your own, there are Battery Management System (BMS) on Ebay that will bring professional design to you DIY battery system. I am using this second method myself, it is less expensive, but more labor intensive. You can stack cells inside plastic plumbing pipes to make any voltage you want to work with. Cells are 4 volts each, so you can design from there
https://www.ebay.com/itm/PCB-Charger-for-4-Packs-3-7V-Li-ion-Li-Lithium-18650-Battery-Rechargeable-2-3A/261191600168?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649
The battery packs do take a little work to open, but each one has a free thermistor in the battery pack, you should save. Batteries that show no voltage can usually be recovered by reaching into the empty hole on top, and pushing down the built in thermodisk. When it snaps, the battery will be usable again.
Here is a picture of a batch of recovered cells from failed laptop battery packs:
Here was my first tube battery prototype:
I have fun and think outside the box. You may be rewarded for the good battery system for your alternate energy power systems!
Since lead acid batteries are good for about 5 years, and Lithium-ion batteries are good for about 30 years, this makes a lot of good sense!