I'm looking to build an underwater glider robot that will need to remain autonomous for long periods of time, perhaps even months. Power draw should be minimal, and I'm thinking of including some form of charging device (such as a solar charger) however I'd also like the battery capacity to be large enough so I don't hugely need to worry about this. Large current draw isn't really needed, but the battery does need to hold its charge effectively for long periods of time. Considering this is an underwater vehicle, weight and size are also a concern.
Cost isn't too much of an issue, as long as it's within reason of a hobbyist project.
I am looking to understand the pros and cons of each technology (Lead acid, LiPo, NiCad, fuel cell?), so I can decide what type of battery would be best suited to my purpose. As such, I'm looking at battery technology rather than looking for a specific shopping recommendation.