I have been getting core batteries from a junk yard for $20. each and reviving them with a small stick welder using this technique posted on YouTube:
https://youtu.be/VYtkn-N_p4s?si=7tu6E_17PfVjVOFX
So far, 95% of the batteries have come back to full voltage and amperage with some of them holding more amperage than stated on the battery. Right now, I have 10 of them ($200) wired in parallel with a single, 300 watt solar panel and an inexpensive basic charge controller I got on amazon. They have held their voltage and amperage for months and have provided me with lights, TV, small appliances, and small tools like drills, skillsaws, etc. via a 3000 watt inverter. They are still working, holding full voltage and amperage, and providing my off grid camper with power. I do not try to run anything that draws a lot of power like air conditioning or any appliance that has a heating element like coffee pot or toaster, etc. I use a generator when i need lots of power. My plan is to monitor the batteries weekly and if any in the bank goes bad, pull it out, scrap it, get another 20 dollar core and repeat the revive process and re insert it into the bank. I've read that it takes 3 car batteries to equal 1 deep cycle battery. Hope this helps.
