How Fast Chargers are able to Charge your Phone Faster?

by Mayank

Battery capacity and charging speeds are two of the most important factors that people consider when purchasing a smartphone these days, along with screen size and camera quality. The most obvious choices that manufacturers must make in order for your phone’s battery to last a full day of heavy usage are:

  1. Provide a bigger capacity battery in their smartphones.
  2.  Provide a fast charging solution that will charge your phone for the full day of usage while you plug it in for 10 to 20 minutes.

Even if manufacturers choose the first option, a larger capacity battery will require more time to charge. As a result, quick charging is almost a requirement these days.

With manufacturers boasting fast chargers that can charge your device from empty to full in under 30 minutes at an insane wattage of 120W, it is vital to answer some questions such as:

Is it true that all fast chargers are made the same? Is it possible to use a different fast charger to charge your device? And how exactly does fast charging work?

In this post, I’ll unearth the actual facts concerning quick charging and your phone’s battery in general.


Here are some basics you need to know before you dive in:

Voltage (or Volts) is the pressure that moves current from a low potential to a high potential.

The flow of electrons is represented by current (or amps), and power (or watts) is the product of voltage and current.

How does your phone’s battery work?

Before we get into the charging process, it’s critical to know how a Lithium-ion battery works. Most consumer devices, including smartphones, laptops, and even electric cars, use lithium ion or Li-ion batteries.

Here’s what happens inside the battery when you are using the phone

When you use your device or your device is on standby, the phone’s battery is being discharged, lithium ions migrate from a negative electrode (anode) to a positive electrode (cathode) via an electrolyte medium as a result of the oxidation process.

The electrons released from the lithium ions flow through the electronic components of your phone such as screen, antenna, the processor and memory. This flow of electrons is also known as current (or Amps).

The electrons then return to the cathode and mix with the lithium ions that were deposited on it. The lithium ions sit on the cathode as stored energy.

Because of the difference in charge concentration between the two electrodes, current continues to flow. This is referred to as potential difference or voltage.

Credits: Battery University

This process completes the battery’s discharge cycle.

Over time, all of these lithium ions are deposited on the cathode, and there are no more electrons to flow, rendering the battery unusable (this is when we need to replace the AA cell on a TV remote).

Phone batteries, thankfully, can be recharged.

Here’s what happens when you plug-in your phone for charging

Charging your phone effectively implies that you are giving a current externally to the battery. As a result, the process explained in the previous section is reversed.

Lithium ions now begin to move back to the anode, where they mix with external electrons (given by the current supplied by your phone’s charger) and are stored as electrical energy in the anode.

Credits: Spectroscopy

This completes the charging cycle.

The 3 Stages of Charging

It may appear that the charging procedure is simple, but it is not. A Li-ion battery is charged in three stages.

Consider a conventional charger with a 5V, 2A, or 10W output and a maximum rated or peak voltage for your phone’s battery of 5V, which is common among most smartphones.

The 3 stages of charging depicted through the respective battery percentages.
Credits: Battery University

Constant current stage

When you connect your phone to the charger, the voltage of the battery immediately rises to roughly 4.2V and continues to grow.

The provided current will also increase to its maximum (2A in this example), and the maximum amount of charges will flow to the battery.

This is why you may have noticed that your phone charges really quickly from 0% to 60%.

This is also the stage where fast charging kicks in. In the upcoming section, we’ll learn more about fast charging.

Saturation stage

This is a crucial stage, where your phone’s charging circuit must carefully control the flow of charge. The stage begins when your battery percentage is around 80% and the charger’s supply voltage is nearing its maximum value of 5V.

The voltage regulator slows the voltage rise, and the supply current begins to fall as the voltage rises slowly but gradually.

The decrease in current is necessary so that the battery can absorb the extra charges floating around. If not carefully controlled, the excess charges can induce a short circuit in the battery, which could damage it.

This explains why your phone charges slowly from 80 to 100 percent.

Trickle charge stage

This is the final stage of the charging process, and it begins after your phone’s battery has reached 90 or 95 percent capacity. When your battery is fully charged, the voltage and current begin to fall, and the current eventually reaches zero.

This is also the stage at which your phone’s charging circuit stops the current flow from the charger in order to prevent overcharging.

Tip: The best way to visualize this is to think of the battery as a sponge. When you initially soak a dry sponge in water, the pores immediately begin to absorb water (just as the constant current stage). The sponge will slowly absorb water once it has completely soaked water near to its capacity (similar to the saturation stage when in order for the anode to absorb the extra charges, the voltage increase slows down).

How does fast charging work?

Smartphone makers use the term fast charging to signify that their phones will be able to charge from empty to full faster than conventional chargers.

There are brands that provide 25W fast charging on their devices, such as Samsung, and also brands that provide anywhere between 55W and 120W of fast charging support on their devices, such as Oppo, OnePlus, and Xiaomi.

The Mi 11 Ultra supports up to 67W of fast charging.
Credits: Mi

NOTE: We usually define the charging capabilities of a phone based on the power rating or Watts. A 10W (5 Volts * 2 Amps) charger means that the maximum current of 2 amps can be supplied by the charger and can be successfully accepted by the battery at the voltage of 5 volts.

Generally, any charger supplying greater than 10 Watts (in case of standard Micro-USB) or 15 Watts (in case of USB-C) is termed as a fast charger.

Fast charging merely improves the first stage of charging (constant current stage) by allowing higher current values to flow to the battery before the phone reaches the saturation stage (where the voltage nears the peak value).

How manufacturers implement fast charging on their devices

Although fast charging may sound a recent development, the foundation for rapid charging was laid several years ago.

Today, smartphone makers implement fast charging in a variety of ways, but the principle remains the same: increase the current supplied by the charger to the battery while maintaining a steady voltage.

Brands such as OnePlus, Oppo, Xiaomi have implemented their own fast charging technologies while other brands use Qualcomm’s quick charge technologies on their devices.

The OnePlus Warp Charger (65W)
Credits: OnePlus

Frequently Asked Questions (FAQs)

Many phones nowadays incorporate dual batteries. What significance does it have?

When you charge a single battery at high voltages and currents, the device’s overall temperature rises, stressing the battery and affecting its long-term health.

Hence, manufactures put multiple batteries inside their phones in parallel to allow charging at insane rates like 65W and 120W.

The OnePlus 8T has 2 batteries that split the power delivery from the charger.
Credits: JerryRigEverything

Using dual batteries reduces the strain on a single battery by splitting the incoming power between the two batteries.

My phone charger is rated at 5V/9V/12V. What do these voltage ratings mean?

Certain chargers can operate at multiple voltages, such as 5V, 9V, and 12V. If the phone can accept voltages higher than the standard 5V, these higher voltages can help charge it faster.

An 18W fast charger (9V, 2A)

You may wonder how the charger knows whether your phone can handle 9V or 12V.

The idea is easy: the charger and the device communicate with one another and agree which voltage is appropriate for charging. This conversational feature is also known as Power Data Objects (the charger’s question) and Request Data Objects (the response from device).

Tip: Keep in mind that temperature is a battery killer. Higher voltages will put more strain on the battery and thus increase the temperature of the overall device. As a result, it is always advised to charge your phone near proper ventilation.

Can you charge your device with a different brand’s fast charger and power cord?

You can charge your device with a separate charger and power cord. The general rule is that if your device supports the same voltage and current values as the charger, you should be good to go.

NOTE: Some power cords, on the other hand, are designed to work with the charger for which they were designed. For example, OnePlus’ Warp charge technology is dependent on the power cord supplied by OnePlus. This is because the copper inside the power cord is thicker (or has a larger cross-section) to allow maximum current flow while avoiding overheating.

I have a 120W charger, and the charger that came with my device was only rated for 50W. Will a 120W charger allow my phone to charge faster?

The ability of a smartphone to charge quickly is partly determined by the device itself. For example, if your phone has a charging circuit that can handle a maximum wattage of 50W (5 Volts and 10 Amps), using a 120W (6 Volts and 20 Amps) charger will not allow your phone to charge faster because the overcurrent and overvoltage protection circuits built into your phone will not permit these high values to charge the battery.

I have addressed most aspects of fast charging in this post. If you believe I have missed any crucial information, please let me know in the comments and I will gladly add it.

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