A firmware update from Apple reveals the company is working on new MacBook Air batteries that should offer a better battery life:
Apple confirmed that the firmware update was to work with a new series of internal MacBook Air replacement batteries. Like the previous batteries, the new ones would not be user replaceable, and are only able to be officially replaced by Apple repair depots and other Apple authorized service repair centers.
The company has said it is increasingly using advanced chemistry and adaptive charging to prolong battery life. The battery design from the MacBook Air has been replicated in the new MacBook Pros. This in turn has transition the MacBook Pros from having user replaceable batteries, to having much harder to replace internal batteries. The tradeoff gain has been a substantially improved battery life.