what is fast charging and how does it work without blowing up your phone?

Just a few years ago, your average smartphone typically came with a 5W charging pad and would typically charge your phone from 0% battery to 100% in about two or two and a half hours. Fast forward to today, 40-65W fast charging is a very common feature in most phones and some even go as high as 80.

What is fast charging and how does it work

The OnePlus 10R and Realme GT Neo 3 150W edition support 150W charging, which can fully charge a 5000mAh battery in almost 17 minutes. It’s impressive. So how does fast charging work exactly?

Not all phones are created equal

Now different manufacturers use different fast charging protocols. iPhones and Pixel phones use USB-PD, while most Android phones use Qualcomm’s latest standard or, as in the case of Realme and OnePlus, use their own proprietary charging protocols. That said, most of these protocols work the same way. The difference is in the current that is allowed to pass between the charging block and the device itself. This is done by using a specific quality of equipment, called controller chips, and calibrating the device’s charging ports to operate only at maximum capacity with a selected type of charger. This is also the reason why a particular fast charger, says OnePlus, may not work on a phone from another manufacturer, such as Samsung.

A little chemistry

In a given battery there is a positive terminal and a negative terminal. Most phones use a lithium-ion or lithium-polymer battery. When a battery drives a device, lithium ions flow from the negative side to the positive side through a liquid electrolyte solution. This flow is what drives the device. When there are not enough lithium ions on the negative side, ie. the flow becomes low or stops completely, the battery is discharged.

Recharging the battery causes the ions to flow from the positive side to the negative side again through the liquid electrolyte solution. The power of the charger and the charging protocols determine the speed at which this flow takes place. The higher the power, the faster ions flow to the top.

Handling the heat

During the charging process, it is this electrolyte solution that heats up. Sometimes this heating of the solution can even cause the battery to explode, especially if it is not handled properly. This is why you will see that when a battery starts to reach its maximum storage space while charging, e.g. 75% or so, the charging speed is reduced, this is done to reduce heat generation and extend battery life. Another thing that causes the battery to heat up is that after a number of charge cycles, these ions lose their ability to hold a positive charge. This means that they can not cross the solution. Therefore, the battery capacity also decreases after a few months or years of use. The more “dead” ions there are in a battery, the faster it heats up. This also leads to expansion of batteries. Basically, with fast charging, a device’s battery drain is also higher.

What is fast charging and how does it work

Expandable batteries

Now the cells inside a lithium-ion battery expand a bit when charged. It’s because of the heat, and it’s a completely normal process. It regains its shape when the electrolyte solution cools, provided it is not charged with dead ions. To address this and give devices and their batteries a longer life, manufacturers have now started using separate batteries. Instead of one huge battery with a capacity of e.g. 5000 mAh, some manufacturers use two batteries of 2500 mAh each.

What is fast charging and how does it work

Fast charging is a double-edged sword for most manufacturers and users. This is why most of the research and development around batteries today is focused on managing the time it takes to charge and on ways to optimize battery health. With 240W fast charging on the way, it will be interesting to see how manufacturers improve ways to extend battery life and how to maximize battery health.

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