Best answer: Unfortunately, there's no way to replace the QC35's battery. Your manufacturer's warranty should have you covered, but if it's expired, your best bet is to buy a new pair.
Amazon: Bose QC35 (Series II) ($299)
The battery isn't user replaceable
If you were hoping to buy a new battery for the QC35 and replace it yourself, we've got some bad news.
There is currently no way to replace the QC35's battery on your own. Bose doesn't sell replacement batteries and tampering with the headphones to get to the battery will absolutely void your warranty.
However, there is a 1-year warranty
As long as you bought your QC35s brand-new, they come with a 1-year manufacturers' warranty.
You can read through the full thing here, but it essentially protects you from any defects you encounter so long as you aren't misusing the headphones. Bose notes that it will, "repair or replace (using new or refurbished replacement parts) any defective parts within a reasonable period of time and free."
So, if you're experiencing a defect with your battery, contact Bose support, have your warranty info handy, and you just might be able to get your headphones replaced/repaired at no cost.
Once your warranty is up, you'll need to pay for a replacement
If you're experiencing battery issues with your QC35 and the warranty is expired, Bose does still offer a helping hand.
Non-warranty QC35 replacements are rated at $259 and that doesn't include the cost of shipping.
You can certainly contact Bose and talk about pursuing a replacement for that price, but once you factor in paying for shipping and the time it'll take to send your current headphones in and wait for replacements to arrive, you might as well just buy a brand-new pair.
You've got options
Getting help with your QC35s should be easy if you're covered under the warranty, but if it's expired, you might want to look into buying a new pair of headphones to avoid the hassle of an expensive replacement process. Considering they're some of the best noise-canceling headphones money can buy, it might be worth the price.
Warranty expired? Just buy a new pair.
The Bose QC35's battery is backed by a 1-year warranty. If you experience problems after that time, treat yourself and buy a new pair.
Get the Android Central Newsletter
Instant access to breaking news, the hottest reviews, great deals and helpful tips.