Fast Charging Is Useful, but Comes at a Cost

Feel the heat

Key Takeaways

  • Chargers can now fill your phone in minutes, not hours.
  • Heat is the enemy of batteries, and "wireless" chargers create a lot of it.
  • If you don't need a full charge, keeping it below 80% will help your battery stay healthy.
Contactless charging of smartphone at home

Yagi-Studio / Getty Images

Two wild new phone-charging technologies showed up this week: Oppo's 150-Watt firehose of a charger and Honor's 100 Watt wireless charger. But can this speed really be good for your phone?

Currently, regular Qi "wireless" charging can only manage around 7.5-10 Watts of power, and a lot of that goes to waste as battery-damaging heat. Meanwhile, fast-charging via a wire is becoming more common—the iPhone has done it for a while—but Oppo's 150 W tech is more powerful even than pro laptop chargers. 

"Fast charging a battery is more than just dumping as much voltage and current as possible at it," Akshay VR of Solar Labs told Lifewire via email. "Instead, battery charging is divided into two phases: constant current and constant voltage."

Fast and Loose

Lithium-ion batteries, the kind found in almost all our rechargeable devices, from laptop to phone, hate heat. You can charge them as fast as you like, but if they get hot while doing it, that’s when the big damage occurs, and that’s what shortens battery life.

But fast-charging itself isn’t necessarily bad. The trick is to pump the electricity into the empty battery until it reaches around 80% of its capacity. Then, you switch to trickle charging to go the rest of the way.

"Fast charging a battery is more than just dumping as much voltage and current as possible at it."

"Fast charging systems take advantage of the constant current phase by putting as much current into the battery as possible before it reaches its peak voltage," explains Akshay. "As a result, rapid charging technologies are most efficient while your battery is less than half full, but their impact on charge time diminishes once the battery reaches 80 percent. In addition, steady current charging is the least harmful to the battery's long-term health. Higher continuous voltage, combined with heat, is more damaging to battery life."

Think of it like filling up a water bottle. You can crank the faucet to full to start, but as that bottle gets fuller, you turn down the flow, so it doesn't spill. OK, it's not the best analogy, but you get the gist. 

But that's fast charging. When used properly, it lets you get a useful charge in minutes. By staying out of the danger zone, AirPods, Apple Pencil, and smartwatches manage to grab a few hours of charge from a few minutes of charge time. 

Wireless World

Wireless is a whole other game, though. The same principles apply to the battery, but the problem is in the delivery method, which is absurdly inefficient. A wireless pad is really an induction pad. A coil in the base creates a magnetic field, which then induces (hence the name) a current in a coil inside the phone. 

This transaction loses power in all kinds of places. For a start, the coils need to line up perfectly to work well, or at all. That’s why Apple’s MagSafe charger uses magnets to line things up. 

This inefficiency turns electricity into heat, which is like Kryptonite to phone batteries.

"Wireless chargers aren't great for either your phone or the planet," electrical recycling expert Eloise Tobler told Lifewire via email. "Some tests have found that wireless chargers actually need around 45% more power than a cable to charge a device. Generally speaking, when using these pads, your phone has to work that bit harder, which in turn generates more heat and can actually shorten the overall lifespan of your battery. Wireless charging pads also have a far bigger environmental cost and are quite difficult to recycle."

Take Care

So how can you protect your phone? First, use the manufacturer's charger. If not, buy something decent, not the first cheap model you found on Amazon. 

"When customers are shopping for a charging accessory, make sure the product is Qi and FCC certified," Igor Spinella, CEO of wireless charging technology company Eggtronic, told Lifewire via email. "Ensure that it has high-quality materials and thermal management. [Avoid] extremely cheap devices sold on the market [as they] might be dangerous or limit the battery life of a smartphone." 

And remember, you can always be patient and use a low-powered charger or simply wait to charge the phone overnight.

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