My battery keeps dying quickly; what can I do ?

That's a fairly drastic drop.

You have not told us what model iPhone you have, or how old it is, or what version of iOS it uses.

See this article ...

About the battery usage on your iPhone, iPad, and iPod touch - Apple Support

... although it does sound like you should visit an Apple Store or Authorized Apple Service Center for assistance.

Oh, OK, so you have the latest operating system version (excellent!) and a fairly recent iPhone.

As the article suggests, check Settings > Battery and see if some app is malfunctioning (using a whole lot of battery to your surprise). If not, a trip to the Apple Store or Authorized Apple Service Center seems warranted. Make an appointment first.