They are Long-Life Lithium Batteries.
Yes, the batteries will need to be replaced eventually because they are long-life lithium batteries. And lithium batteries do need to be replaced eventually. They are also recharged by a small photo cell that is on the front of the display unit. I forgot about this. The question that I would have is: What if they are mounted under a dodger and away from direct sunlight? Will they charge enough where they are located?Also, they are not permanently mounted. Only the base is. This allows you to move them about the boat or remove them and put them away when they are not being used. I think it is also a theft-deterrent since there is nothing "fixed" in place, such as wires. Each unit can be programmed to show either a multi display or single display of data, thus the flexibility of moving them about. This also gives the skipper the flexibility to mount one down below where he can keep an eye on it while underway at sea.