Mr. Lahey wrote:...
Most likely I'm to frugal to pop for a lithium.
Frugal is my middle name! I built a couple 100 aHr LiFePo4 batteries, thanks to the urging of Lauren, and they both cost under $300 each. It is amazingly simple if you break it down to baby steps. At the time a "good" factory built 100aHr LiFePo4 battery was over $800. There were some at around $500 and though it was tempting I just couldn't get by the negative reviews of some of them. At today's prices to replace my 144 aHr Mighty Max AGM bank would cost me $360 resulting in only about 72 Ahr useable. The first bank lasted less than five years.
As flboy states the lead acid, sealed lead acid and AGM batteries should not be drained down below 50% of their capacity. And my experience with AGMs is they don't have a great lifespan. I treat my LiFePo4 banks with TLC by not charging over 90% of their capacity and not draining below 20% capacity. I'm seeing "opinions" on diysolarforum that taking care of these batteries like that may result in 15 to 20 year lifespans. Not sure there is enough evidence of that claim but I'm hopeful.