Technology now allows a quick charge during the limited free time we find in our busy day-to-day lives. Who wouldn’t want a smartphone that charges in minutes instead of hours?

However, it is not that simple. Charging standards are a complicated mix of chemistry and physics, and each has its own sets of limitations, with incompatibility an issue as well.

Here are some of the main popular charging standards on the market.

The USB Implementers Forum specifies four types in total, one for each corresponding USB specification:

USB 1.0, 2.0, 3.0, and 3.1.

A typical USB 1.0 and 2.0 plug can deliver up to 5V/0.5A (2.5W).

That’s the charging rate of a typical normal phone, and it doesn’t deliver a lot of power. An iPhone charging at 2A over USB uses 5V x 2A = 10W. By comparison, the average incandescent light bulb, draws about 40W of power.

By default, USB 3.0 ports push 5V/0.9A (4.5W).

USB-C, the oval-shaped reversible plug on newer smartphones, is completely different. It’s technically capable of carrying the USB 2.0 spec, but most manufacturers opt for USB 3.1, which can potentially deliver a much higher voltage.

Many USB 3.1 devices take advantage of the USB Power Delivery (USB-PD) spec, which has a maximum power output of 20V/5A (100W). Smartphones don’t usually need that much power, so manufacturers commonly stick with a lower amperage (like 3A), but it is perfect for USB-C laptops like the MacBook Pro, Google Chromebook Pixel and other new laptops.

Fully compliant smartphones and chargers respect the limits of USB 2.0 and BC1.2, but not all phones and chargers are compliant. That’s why, generally speaking, smartphones always default to the lowest charging speed.

The USB specs are more like guidelines. Fast-charging standards like Qualcomm’s Quick Charge and Samsung’s Adaptive Fast Charging exceed the USB spec’s voltage parameters, but on purpose — that’s why your phone is able to recharge in minutes, rather than hours.

The USB Power Delivery (USB-PD) standard was developed by the USB Implementers Forum (USB-IF) and it’s a standard that any manufacturer can use on any device that has a USB port.

It can deliver up to 100W, so it’s suitable for use with all kinds of devices beyond smartphones including some laptops if they have a USB-C charging port. USB-PD brings other benefits too. The direction of the power is not fixed, so you will find portable battery chargers, for example, that have a USB-C port that can be used both to charge another device or to charge the battery pack itself.

USB-PD also only provides the power the device can support, so the same USB-PD charger could charge a smartphone at top speed, but then also charge a laptop at its top speed.

However different manufacturers employ the USB-PD standard slightly differently.

Qualcomm’s Quick Charge is one of the most widely implemented charging standards on the market. It's an optional feature of Qualcomm technology that can power phones like the Samsung Galaxy S10, Google Pixel 3, and LG V40 ThinQ. But the technology isn’t tied to Qualcomm’s processors. Any smartphone manufacturer is free to license the Quick Charge’s power controller technology.

Quick Charge achieves fast charging by upping the charging voltage, which in turn boosts the wattage. Quick Charge 5 is the latest standard in the series, can fully recharge smartphones in 15 minutes. It only supports limited recent phones such as the Samsung Galaxy S20.

Quick Charge 4.0+ works with phones like the LG G8 ThinQ, Razer Phone 2, and Xiaomi Mi Mix 3 and it can deliver up to 27W of power. It can recharge smartphones to 50% in just 15 minutes

The more widely used Quick Charge 3.0’s voltage range is 3.6V minimum and 20V maximum, it can identify the most efficient voltage at any giving point during charging. At its peak voltage, Quick Charge 3.0 can deliver 18W of power. It can deliver about a smartphone's 50% capacity in half an hour

Quick Charge 4 and newer have the added bonus of compatibility with USB-PD chargers, but Quick Charge 3.0 and older only work with Quick Charge-certified accessories.