Another interesting possibility is nickle-iron batteries. Very old technology. Seems that they were invented somewhere in the 1890's, and were made for many years. GE made quite a few of them in various sizes. They weren't as efficient as lead-acid batteries, and didn't hold a charge well; in a month they'd lose about 50% of their charge. However, they had an exceptional lifespan; they could last 30-40 years and more.
One of the big drawbacks of these batteries was the large amounts of hydrogen and oxygen they emitted, particularly if their temperature exceeded 115°F. However, since in this case that's the "holy grail" that's being sought ... it could be worth re-investigating.
One of the big drawbacks of these batteries was the large amounts of hydrogen and oxygen they emitted, particularly if their temperature exceeded 115°F. However, since in this case that's the "holy grail" that's being sought ... it could be worth re-investigating.