By Jack Nicas And Greg Bensinger 

Companies hoping to use drones to deliver small packages are confronting technical hurdles such as battery life and weather that are at least as vexing as proposed U.S. regulatory limits.

Retail and shipping companies including Amazon.com Inc., Alibaba Group Holding Ltd., and Deutsche Post DHL AG have been among the most enthusiastic supporters, seeing drones as potentially transforming their businesses.

But hurdles including short battery life and unreliable location data suggest it could be years before armies of drones replace FedEx and UPS vans. Companies also face obstacles such as bad weather, aggressive birds and gun-toting drone opponents

Delivery drones "are absolutely viable, but there are a lot of technical hurdles that have to be crossed," said Nicholas Roy, a robotics professor at the Massachusetts of Institute of Technology and the former head of Google Inc.'s drone-delivery project. "We are very much in the prototype stage."

The top two U.S. private package delivery companies think drone delivery won't fly soon. FedEx Corp. and United Parcel Service Inc. say the technology remains far from being market ready. "There remain numerous reasons why drones are not a feasible delivery technology at this time," UPS said last month.

Experts say the most pressing challenge to deliveries is drones' battery power. Like makers of laptops, smartphones and electric cars, drone makers still need to pack more energy into smaller batteries. The issue is especially acute for delivery craft: the heavier the package, the more power is needed to fly.

Amazon wants its drones to be able to carry 5-pound packages on a 20-mile round trip route. That performance is almost certainly impossible today with the eight-rotor prototype Amazon demonstrated in late 2013, said Raffaello D'Andrea, a robotics professor at the engineering university ETH Zurich and co-inventor of the robots that help sort Amazon's warehouses.

An alternative is the approach Google unveiled last August--a hybrid drone with propellers and wings that takes off like a copter and glides like a plane, increasing efficiency. Yet that design is generally less wind-resistant, agile and reliable than a copter drone. Indeed, Google this month said it scrapped that initial design because it was too difficult to control.

Amazon this week received approval from the Federal Aviation Administration to test its drones outdoors. The company has developed nearly a dozen aircraft as part of its Prime Air project. "We're testing a range of vehicle capabilities," Gur Kimchi, Amazon Prime Air vice president, said in a statement. "We won't launch Prime Air until we are able to demonstrate safe operations."

Experts say a trickier challenge is ensuring drones can make deliveries without incident almost 100% of the time. Delivery drones must be cheap to be commercially viable, developers say, and exceptionally reliable to pass muster with regulators and the public.

"A sea change is going to be required in how these vehicles are designed and manufactured to support moving from a hobbyist flying on the weekend in a park to a 24/7 delivery service flying over your highways," said Mr. Roy.

Then there is how to drop off a package. Some companies have tested landing on a customer's doorstep while others have tried lowering packages down on a line. But global-positioning-system data can be inaccurate--enough to put a drone at the wrong house or over a swimming pool.

A person familiar with Amazon's thinking said that to simplify delivery, its drones could deliver to storage lockers, accessed through a code sent to customers.

And delivery drones would have to fly autonomously, which requires sensors and software that can three-dimensionally map the environment and navigate it on the fly. Such technology isn't yet ready, though a handful of companies, including chip makers Intel Corp. and Qualcomm Inc. say they're getting closer to solving it.

If and when these technical limitations are overcome, regulators might not allow drone deliveries. Last month, the FAA proposed rules that would require one pilot to monitor each drone, limiting flights to within sight of the operator, and barring flights over bystanders. The agency said the proposed rules, which are expected to be completed next year, wouldn't allow drones carrying an "external load."

The FAA said it would likely allow drone flights beyond operators' eyeshot if companies demonstrate reliable collision-avoidance technology. It is unclear if the agency would consider authorizing large-scale autonomous drone operations overseen by just a few humans, as companies currently envision.

Still, companies are zooming ahead. Amazon and Google are working on drones that could deliver small packages in less than 30 minutes. A DHL drone has delivered medicine to an island in the North Sea, and Alibaba, the Chinese e-commerce giant, last month delivered tea with drones.

"The technology is challenging, but totally doable," Amazon Chief Executive Jeff Bezos said last year.

Mr. D'Andrea, the robotics expert, predicts the technology for large-scale drone deliveries would take about five years to develop.

"People underestimate the technical difficulties.... Folks watch videos on YouTube and think, Wow, this is great. Why isn't somebody delivering my pizza?" Mr. D'Andrea said. "It takes time and effort to make something the public doesn't have to think twice about."

Write to Jack Nicas at jack.nicas@wsj.com and Greg Bensinger at greg.bensinger@wsj.com

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