Before I got lithium my 2x 100Ah AGM batteries once deep discharged and I lost like 80% of their capacity. Because they were useless to me I took them to the scrap dealer and maybe got R100-R200 per battery, I can't recall but I kept thinking it was a huge waste. The guys who worked there all looked like they were eyeing a pot of gold when I took it in, so I thought in hindsight I should have put it on Facebook marketplace for a few hundred bucks more and I'm sure someone would have picked them up.