Tesla owners who were in Hurricane Irma's path in the Southeast recently got an unexpected boost to help them, after the carmaker remotely upgraded vehicle batteries to their highest capacity.

The boost gave customers' cars an extra 30 to 40 miles, but it's also temporary: The batteries will lose their extra juice this weekend.

The move came at the request of a customer who was worried about traffic and the range between charging stations during a massive evacuation that saw millions of Americans leave their homes in Florida, Georgia and neighboring states.

The battery boost applies to Model S and Model X vehicles that were built and sold with 75 kWh battery packs — but were software-limited to 60 kWh or 70 kWh capacities. Customers who bought the cars got them at a lower price, with an offer of paying to unlock the remaining battery power later.

News of the upgrade emerged Saturday, as Florida residents made their final preparations for Hurricane Irma — which hit the state as a Category 4 storm Sunday morning.

"There are a lot Tesla owners in Florida and they are also escaping north using the Supercharger network," Fred Lambert wrote for Electrek. "Now Tesla has even facilitated travels for some of them as the automaker remotely unlocked the full battery pack capacity of Model S/X 60/60D vehicles with 75 kWh battery packs."

The news prompted a number of questions, including: How can Tesla unilaterally (and remotely) boost battery power? And: Should car companies change vehicles unilaterally after they're sold?

After they're sold, Tesla cars stay in touch with the company's network — and the vehicles get a free software upgrade every few months. In August, for instance, new Model S and Model X 75 kWh vehicles got an over-the-air update that increased not their batteries but their performance — allowing them to shave a full second of their zero-to-60-mph times.

In the battery boost, the range of Tesla's Model S 60 was increased from 210 miles to 249 miles, for instance, and the Model X 60D could travel 237 miles on one charge, rather than its standard 200 miles.

Tesla's use of over-the-air software updates was initially meant as a way for the company to offer content, such as streaming music or maps, to drivers. That's what NPR's Aarti Shahani found out in 2015, when she reported on hackers who were able to infiltrate the cars' network (and shut a car down). The company then used auto-updates to fix the vulnerability.

Science writer Brian Fung of The Washington Post called the temporary battery boost "an example of the perks — and tyranny — of software in cars."

The tyranny, Fung and other observers say, comes in the ability to change the terms of what a consumer has bought. When cars are remotely updated like a cellphone or other device — and without a customer's consent — problems can arise.

David Friedman, the director of cars and product policy and analysis at Consumers Union, says Tesla is "really a great example" of a carmaker that can give to customers with one hand while removing features with the other.

"This is a case where they gave consumers the ability to get greater range, but we also just heard this week that Tesla took away automatic emergency braking, at least temporarily, as a feature for some of the cars," Friedman says. "So, there's sadly two sides to this."

The former interim head of the National Highway Traffic Safety Administration, Friedman now works for the policy arm of Consumer Reports. This week, the site published a story about Tesla disabling AEB systems on recently manufactured cars — that had the feature as standard equipment when they were purchased.

Friedman says he's not trying to criticize Tesla for helping its customers get out of the way of a deadly hurricane. And he says electric and hybrid vehicles have proved their worth after natural disasters — serving as backup power sources, for instance.

Still, Friedman says, "It's important for government to have a clear role in pushing automakers to provide consumers with clarity on what their cars can and can't do, and to not allow car companies to at a whim change the functionality of a vehicle, especially if safety is involved."

More questions arise, Friedman says, when it comes to how companies communicate changes to a car's capabilities.

"Maybe they notify the owner, but what about the owner's spouse, or child, or friend who borrows the car?" Friedman asks. "Or what if it's a company car? It creates some uncertainty."

Like phones, modern cars have increasingly become mobile computers. But as Friedman notes, when your phone crashes, it usually has less disastrous consequences than does your car crashing.

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