Battery failing on a Bose Soundlink Revolve

I’ve got a two-year-old Bose Soundlink Revolve portable speaker with a 12-hour battery. I always keep it plugged in; it’s in my office on a shelf. It occasionally announces, “The battery is at 100%” or “The battery is at 90%,” etc. Recently, this percentage has been dropping rapidly and is now down to 30%. The Bose manual, website and other sources just say to “Replace the battery.” But they don’t say how. There’s no obvious route into the insides of the speaker — entirely apart from the fact that I don’t get why I should have to replace a battery that’s always been plugged in. What gives? I’ve switched to several different wall outlets with no change. Nothing in the speaker or cable LOOKS damaged either. Any ideas? Thank you — George S. (Harpswell, Maine)