Tim Cook introduces the health and fitness tracking features of the Apple Watch at the event in September / Getty

Customers to keep expectations reasonable about what the watch can do, say those working on apps for it

Apple is warning developers to limit features on apps made for the Apple Watch, according to reports.

Apps won't be able to be too complex to save on precious battery life, developers have told Business Insider. The time that the watch can go without needing a charge is set to be central to selling the watch, which is likely to need plugging in at the end of every day.

“Apple is limiting all this nice stuff, all these sensors, NFC, haptics, heartbeat sensor, gyrometer — everything,” Sumit Mehra, CTO of app studio Y Media Labs, told the news websites. “Apple is only going to allow developers to do the basic stuff to just get the Apple Watch out there, because either it's not ready, or they don't know what the implications of something like this could be.”

Sensors like those built in to the watch to track fitness data and other movements take up lots of battery. If all the apps built for the watch made heavy use of them, the already short battery life would be quickly depleted.

Apple released its design rules for the smartwatch in November, giving an initial look at what developers will be able to do with apps made for the watch. Initially, all apps will be extended versions of those for the phone, though Apple hopes that standalone apps will come eventually.

The watch will require the iPhone for processing power, sending messages constantly between the two. Most of the apps that are set to be released along with the watch, which is set to be announced on Monday evening, will probably give users access to notifications and only very simple features on their wrist.