Fast charging: Understanding speed, power, and battery life

Nowadays, electronics manufacturers increasingly implement fast charging features in devices. This is especially useful in devices that require frequent charging, such as smartphones. Many of us are familiar with the scenario of needing to leave the house quickly when the phone's battery is almost dead. In such moments, the fast charging feature proves to be very useful. Below, we explain how charging power affects the charging time of a phone battery.

The charging power of smartphones depends on the technology used and the USB standard. To better understand this process, we need to look at charging power, expressed in watts (W). In simple terms, it is the amount of energy we can deliver to the device.

Charging smartphones is most often done via a USB cable. Initial standards predicted a charging power of 2.5 W, which allowed the battery to be charged in 2-3 hours. However, back then, the batteries were much smaller.

The introduction of the USB 3.0 standard increased charging power to 4.5 W. Currently, even basic USB chargers offer a charging power of 10 W, providing a voltage of 5 V and a current of 2 A. Today, it's hard to find a smartphone that couldn't be charged with such power.

USB Type-C cables increase the charging capability to 15 W, thanks to a current of 3 A (at 5 V voltage). With increased power, fast charging technology becomes necessary. Even smartphones with a maximum charging power of 18 W, 20 W, or 25 W (like the Samsung Galaxy S24) use fast charging technology.

The most advanced smartphones have a charging power of 45 W, 67 W, 80 W, or sometimes over 100 W. Some models even reach over 200 W. However, as the power increases, so does the temperature, which can lead to faster wear or damage to the device.

Different fast charging technologies ensure safe charging at high power by using advanced protocols and optimizing current parameters. The most popular systems include:

• Power Delivery (from 10 to 100 W), developed since 2012
• Qualcomm Quick Charge (up to 27 W)
• SuperVOOC (up to 240 W) used by brands such as Oppo, OnePlus, realme, Vivo
• Huawei SuperCharge
• MediaTek Pump Express

It is also possible to combine different technologies to increase total charging power.

Charging power is important for estimating how quickly we can charge the battery. However, we must also consider other factors, such as battery capacity—with the same power, a larger battery takes more time to fully charge. The state of charge of the cell is also important—manufacturers often provide the time required to charge it from 0 to 100%, from 0 to 50%, or from 1 to 100%.

It's also important to ensure the compatibility of all the charging accessories used. The smartphone is not the only one responsible for the charging speed; the charger and cable must also support the required charging power. It's also worth noting that the power source, such as a power bank, laptop, or USB port in a car, may have its own limitations.

The charging speed is limited by the element with the lowest maximum charging power. Sometimes it is the smartphone itself that decides—if its battery supports charging at 25 W, a charger with 50 W or 100 W won't speed up the charging process; it depends on the weakest link in the charging ecosystem.

Let's move to practice. To determine how long it will take to charge a battery of a certain capacity, we can use a simple formula: charging time in minutes equals battery capacity (Wh) divided by charging power (W) and multiplied by 60 minutes.

For example, assuming a battery capacity of 5000 mAh (which at a voltage of 3.7 V gives 18.5 Wh), the results for different charging powers will look as follows:

• 18 W - about 62 minutes
• 25 W - 45 minutes
• 33 W - 33 minutes
• 45 W - 25 minutes
• 67 W - 17 minutes
• 80 W - 14 minutes
• 120 W - just over 9 minutes
• 240 W - 5 minutes

These values show impressive charging times at higher powers. In practice, charging takes longer, even with fully compatible devices and accessories. Power losses or limitations due to rising temperature can affect charging time.

It's worth paying attention to the manufacturer's data to get more realistic results. For example, the Xiaomi Redmi Note 9 (5000 mAh battery, 18 W charging) charges in 153 minutes from 0 to 100%, while the Xiaomi Redmi Note 11 (5000 mAh, 33 W) in 60 minutes. Models using 67 W power, like the Xiaomi Redmi Note 12 Pro, take about 46 minutes. The fastest models, such as the Xiaomi Redmi Note 13 Pro+ with 120 W charging, reach full charge in 19 minutes.

As you can see, charging time varies even within one manufacturer's range, despite similar specifications. The final result is the outcome of many factors, such as the charging technology used, battery capacity, and the quality of components.

It's also worth considering how fast charging affects battery life. It's hard to clearly state whether it shortens battery life. Higher power generates more heat, which can negatively affect components. However, manufacturers assure that fast charging is safe and should not drastically decrease battery capacity. After years of use, capacity may decrease to about 80%, which still allows for comfortable use of the device. If necessary, replacing the battery is an option and is not very costly.

In conclusion, fast charging is a great convenience, especially in emergencies. When using this technology, use compatible accessories and be aware of its limitations.