Careful with that Dick Tracy routine on your new Apple Watch — you won’t be able to keep it up all day.

Apple has quietly admitted that using its new gadget as a phone — a nifty feature that CEO Tim Cook gushed about in a splashy presentation Monday — drains the battery in as little as three hours.

That’s far less than the watch’s 18-hour battery life touted by Cook as he showed off the device to reporters before the show.

“I’ve been wanting to do this since I was 5 years old!” the 54-year-old exec told the crowd. “The day is finally here.”

Gabbing on the phone isn’t the only thing that will send you back to the device’s magnetic clip-on charger well before 18 hours are up.

A music-listening session will last just six-and-a-half hours, according to a product page on Apple’s Web site.

But perhaps more importantly for some users, the Apple Watch’s health-monitoring mode drains the battery in just seven hours.

The battery-life figures were given for the smaller, 38 mm model of the Apple Watch, which has a smaller battery. Figures weren’t given for the larger, 42 mm model.

Apple said the battery will last 18 hours in a session that includes five time checks per hour, 90 notifications, 45 minutes of using apps and a 30-minute workout while playing music from an iPhone via Bluetooth.

“Battery life varies by use, configuration, and many other factors; actual results will vary,” according to the Apple site.

Charging the watch requires taking it off and connecting it to the magnetic charger. The watch will be 80 percent charged within 90 minutes and take two and a half hours to get fully charged.

In a Tuesday note to clients, Deutsche Bank analyst Sherri Scribner was cautious on her outlook for Apple Watch sales, saying the gizmo’s 18-hour battery life struck her as “somewhat limited.”

Apple shares dropped 2.1 percent Tuesday to close at $124.51.