That's 60kwh of energy.
One of the things I've been thinking of doing is setting up some old car batteries as fast-charge station for an EV. Trickle-charge them all day long at whatever rate, but if I pull in and want to juice up quick, well, they're lead acid starter batteries, I could yank power out of them fast.
And blah blah, they'd be starter batteries and not ready for deep discharge, but, a single starter battery can put out 10,000 watts. A rack of 20 or 30 will certainly be able to fast charge without straining them much at all.
If you do go to sell them, remember the scrapper principle of not turning a table into toothpicks. Items have the most value the more you preserve their complexity.
That is... if you can sell them as batteries, you'll get the highest price.
If you can only sell them as scrap, you'll get a lower price.
And one thing I found when selling batteries, more than any other scrapyard material, prices varied. You want to dump steel or aluminum at a scrapyard, you can call around but they'll all be pretty close.
I called probably 10 places the last time I took batteries in, prices varied by 500%, and that one that was 500% was double the next closest. I almost didn't even call them to find out their price that day, because I thought it was starting to be diminishing returns on making more calls.
If you're looking at ~$0.33/lb, and you've got something like 6000 lbs no? That's $2,000 of scrap. Make lots of phone calls, and tell them you're bringing in 6000lbs and shopping for the best rates. They'll give you better rates.
I really need better suggestions of how to keep these things charged/balanced until such a time that I can afford the solar/wind charge controller + inverter equipment to take advantage of these.
Good news! You're already doing it!
If you keep them all in parallel they'll all stay balanced. If you keep a charger on them, they'll all stay topped up and not sulfate.
If you have some bad cells in there, they may get hot (the charger will be dumping all it's energy into a high-self-discharging cell to keep it maxed out, making the cell hot). Those ones are probably junk, so disconnect any that get hot and keep the others topped up.