Your dad’s old RadioShack universal adapter probably had a slide switch on it for adjusting voltage. And the reason for that was simple: A mismatch between the power needs of the Sony Walkman it was feeding and what the adapter actually delivered could have stopped Rod Stewart in his tracks—perhaps for good.

Today’s smartphones have various energy requirements, too. In the past two years, smartphones have been coming with fast-charging technologies that let you bring an almost dead battery up to 50 percent in as little as 15 minutes.

In 2014, fewer than a half-dozen phones in Consumer Reports' smartphone ratings came with that technology; today 20 do, and it's fair to assume that, with the exception of iPhones (at least so far), it will be a standard feature on all but the least expensive phones. Which raises a question: Is it safe to plug one of the new, fast-charge phones into an old, slow charger? And, conversely, can you use a new charger with an old phone? 

As an example, can you use the rapid charger from a new Google Pixel phone you buy this fall with an old LG G3 smartphone? After all, many people would like the convenience of using whatever charger's at hand. But the fires caused by all those Samsung Galaxy Note7 phones could make them fearful of taking risks with mobile-phone batteries.

In brief, the answer is yes, according to the engineers and safety researchers we asked. There's no reason to fear a fire, explosion, or even premature death of your smartphone if you mix and match charging devices. 

How Rapid Charging Works

There are several types of rapid phone chargers. Some work by feeding the phone at a higher voltage (9 volts instead of the 5 volts put out by standard USB chargers). Others, such as the DASH system used by the OnePlus 3 smartphone in our ratings, keep the voltage at 5V but raise the amperage (4A vs. 2A for standard phone chargers).

But although the new phone chargers are built for speed, they default to a slower charging setting that's safe for all smartphones (5 volts at 500mA to 2A) when they sense they've been plugged into an unsupported device.

Qualcomm, which is known for its Snapdragon smartphone processors, is also behind the rapid-charging Quick Charge tech found in more than 100 smartphone models, including flagship phones from Samsung, Motorola, HTC, and others. (On some Motorola smartphones, Quick Charge is branded Turbo Charge.)

“There’s a lot of intelligence built into the charging process, with algorithms that help the adapter determine what the phone can take,” says Geoff Gordon, a marketing manager at the company.

It doesn't matter if that phone charger’s amp rating is higher than the one shown in your phone battery’s specs. The phone takes only the current it can handle. Consequently, when you plug a fast-charging phone into a conventional phone charger, it will charge at a slower rate.  

Onboard microprocessors monitor and manage the battery’s state of charge and health to ensure that they get what they need at a rate they can handle. And that works across ecosystems. In other words, it makes no difference that iPhones use Apple’s lightning connector and Android phones use micro USB or the new USB Type-C connectors. If you plug the cable that came with your phone into the USB port of any phone charger, it should work safely.

Check the top smartphone cameras in our reviews and the smartphones with the best battery life. And see our reviews of phone plans and carriers.

What You Give Up

Some websites have claimed that using a rapid charger with a non-rapid-charging phone may speed charging by at least a little. But that's pure myth, according to the experts we consulted.  

“Don’t expect faster charge times for a rapid charger on a non-rapid charging phone," says Isidor Buchmann, the CEO of Cadex Electronics, a developer of battery-testing and management technology based in Richmond, British Columbia. “For the charger to work faster than normal, both charger and phone need to harmonize. If they don't, the charge will take the usual time but the service will be safe.”

Another thing to understand about smartphone battery charging is that the rate of charge actually drops off sharply after about 80 percent and further reduces to a mere trickle once you reach 85 to 95 percent. That's also true of rapid-charging systems, which is why manufacturers often brag about things like "8 hours of talk time after a 15-minute charge" instead of the time it takes to fully charge the phone.

This charging slowdown is necessary to prevent overheating and other conditions that may bring the battery to a volatile state.

For this reason, you might find impaired phone performance if you play video games or engage in other processor-intensive activities while charging your phone because they, too, can generate quite a bit of heat. You should still be safe, according to our sources, thanks to sensors on the phone charger and the phone itself that monitor the battery temperature. But the phone might deliberately slow down the processor, dim the display, and slow down or even halt the charging process to keep the heat under control. 

That's also why an overheated phone might shut down completely and refuse to restart until things cool down.