You do need batteries, BUT it won't be eating at your wallet. You just need 1 AAA.
When I was traveling on my flight to London and back, more than 10 hours each way, I had no problems with the battery. I even used it for more than a few hours before even traveling. The avg time for the battery is 35 hours.
If you really are concerned about them, buy a four pack and keep it in your Bose Case, there is a sleeve in there perfect for batteries. Or you can invest in rechargeable batteries. I still haven't changed my battery, but I know I will need to do it soon, but at least you'll know when you need to change it. Cause you won't hear anything. It's very smart way to know when you need a change if you ask me.