I would not try to buy a product with a built-in battery for life. If you want a battery-powered speaker and want to keep it for a long time, I'd go for something that has standardized, removable batteries.

EDIT: It looks like a lot of the products out there (a) are Bluetooth, which I don't know if you want or not (but I wouldn't consider that something that will be a buy-it-for-life feature either, since Bluetooth has steadily seen protocol change over time and I doubt that twenty years down the road, the state-of-the-art in Bluetooth will be what it is today) and (b) have built-in lithium batteries. What you might consider doing is getting a radio with a built-in speaker with aux-in, as it's easy to find those with removable batteries and without Bluetooth.