Or will using the wrong cable fry your phone?

Liudmila Chernetska/Getty Images

It's a common occurrence: You have your fully charged laptop plugged in before you, but your smartphone is seriously lagging behind. So, you unplug the charger from your laptop and use it with your phone. Both have USB-C ports, so it should be OK, right? That's the big question that we've all asked ourselves, especially as USB-C is now common across phones, tablets and laptops. It's easy to assume every charger works the same way, and, in practice, it's a safe bet to make. Modern charging standards are designed to make it possible to mix and match your chargers and devices safely.

Something you should know is that a USB-C laptop charger does not simply force maximum wattage into a phone. In fact, modern USB-C charging is governed by the USB Power Delivery standard, which is a protocol that enables a charger and a device to communicate before any significant power is transferred.

That said, just because using a high-power charger is considered to be generally safe doesn't mean there aren't situations that come with a risk. Charging results can vary depending on the charger, cable, phone model and charging protocol involved. A high-quality USB-C laptop charger from a reputable manufacturer is usually fine for a modern smartphone, but older devices, cheap accessories and damaged cables can still create issues.

How does USB power delivery actually work?

USB Power Delivery (USB-PD) is a charging standard developed by the USB Implementers Forum (USB-IF) that governs how power flows between a charger and a device. The key thing to remember here is that it's not the charger pushing power into your phone, but rather the phone pulling exactly what it needs.

When you plug in a device, there's a back-and-forth happening. The charger will transmit its full range of power levels to the smartphone, and the smartphone will accept the highest level it can safely use from the list. Because of this negotiation process, a high-wattage charger cannot fry your phone's circuits. The phone will never actually absorb more power than its internal components are designed to handle.

When is it safe to use a laptop charger on your phone?

In short? Almost always, provided both devices were made in the last decade or so. All recent iPhones, Pixel phones, Galaxy devices, MacBooks and most Windows laptops these days support USB-PD. When both your charger and phone are USB-PD compatible, the negotiation process described above kicks in automatically and you can use that combination every day without issue.

So what happens if one or both devices lack USB-PD support? It's still generally fine because a USB-PD charger connected to a non-USB-PD phone will simply default to a safe output. Even if the negotiation steps are skipped, the phone will charge slowly but safely. Similarly, a non-USB-PD charger connected to a USB-PD phone will cap out at 5V/3A, which is more than enough for a slow, safe charge.

Will a laptop charger charge your phone faster?

Cronislaw/Getty Images

The answer to this question really depends on your phone's fast-charging capabilities. If your phone supports higher-voltage charging and the laptop charger can deliver it, then yes, you'll likely see faster charging speeds than with a standard wall adapter. For example, Apple states that fast charging for many recent iPhones requires a compatible 20W or higher USB-C power adapter. A 65W laptop charger clears that bar quite easily, so your iPhone will negotiate the power it needs and charge accordingly. You should also be aware that some smartphone brands use their own proprietary charging protocols that require their specific cable or power brick to achieve advertised speeds.

However, if your phone is limited to 18W or 20W input, plugging it into a 100W laptop charger won't make it charge any faster, as the phone will only pull what it's rated for. More watts on the charger label doesn't mean more speed; it just means the charger has headroom for other devices. Since you'll most often see 100W, 140W or higher on multi-port chargers, those maximum outputs are designed to "cover" all plugged-in devices. The reverse, namely using your phone's charger with your laptop, won't deliver the same quick charge you are used to achieving. Oh, and you shouldn't expect impressive results if you use your car's USB ports to charge your laptop, either. 

You should still exercise caution when charging your devices, especially if you have cheap, uncertified chargers. Counterfeit or poorly engineered chargers often bypass essential isolation barriers between the AC mains and the DC output side, so a surge in that type of device can pass damaging voltage spikes to your phone. Low-quality cables are also a hidden risk, as a non-certified USB-C cable may lack the internal pull-up resistor required by the USB-C spec. As a rule of thumb, before you use any charger, check it for safety certification markings such as Underwriters Laboratories (UL), Intertek (ETL) or the CE mark.

Since we've focused on whether or not it's a good idea to charge your smartphone with your laptop brick, you should know that other smaller USB-C devices may not come with the same microcontrollers needed to properly communicate with a high-power charger. Most fast chargers start at 5V and only step up after negotiation, so the risk is usually low, but you should still stick to the charger that came in the box, just in case.