A couple of years ago, your average smartphone would typically come with a 5W charging brick and would usually charge your phone from zero percent battery to 100 percent in about two or two and a half hours. Today, 40-65W fast charging is ubiquitous in most phones, and some even go up to 80.
OnePlus 10R & the Realme GT Neo 3 150W edition support 150W charging, completely charging a 5000-ish mAh battery in almost 17 minutes. Now that’s impressive. So how does fast charging work exactly?
Not all phones are created equal.
Now, different manufacturers use different fast-charging protocols. iPhones & Pixel phones use USB-PD, whereas most Android phones either use the latest standard from Qualcomm or, as with Realme & OnePlus, use their proprietary charging protocols. With that being said, most of these protocols function similarly. The difference arises in wattage allowed to pass between the charging brick and the device itself. This is done by using a specific grade of equipment, called controller chips, and by calibrating the charging ports on the device to work at its maximum capacity, only with a selected type of charger. This is also why a specific fast charger from, let’s say, OnePlus, might not work on a phone from some other manufacturer, like Samsung.
A little chemistry
There is a positive terminal and a negative terminal in any given battery. Most phones use a lithium-ion or a lithium polymer battery. When a battery is powering a device, the lithium ions flow from the opposing side to the positive side through a liquid electrolyte solution. This flow is what powers the device. When there are insufficient lithium ions on the negative side, i.e., the flow becomes weak or stops altogether, the battery runs out of charge.
Charging the battery again makes the ions flow from the positive side back to the negative side, again, through the liquid electrolyte solution. The wattage of the charger, and the charging protocols, determine the speed with which this flow takes place. The higher the wattage, the faster the flow of the ions at the peak.
Dealing with heat
During the charging process, it is this electrolyte solution that heats up. Sometimes, this heating up of the answer may even cause the battery to explode, mainly if it isn’t managed correctly. You will see that as a battery starts reaching its maximum storage while charging, say, 75 percent or so, the charging speed slows down; this is done to reduce the heat output and extend the battery’s life. Another thing that causes the battery to heat up is that these ions lose their capacity to hold a positive charge after a certain number of charging cycles. This means they cannot flow through the solution. This is why, after a few months or years of usage, the battery’s capacity also dips. The more “dead” ions in a battery, the quicker it heats up. This also leads to batteries expanding. The wear and tear on a device’s battery are also higher with fast charging.
Now, the cells inside a lithium-ion battery will expand a little when being charged. This is because of the heat and is an entirely normal process. When the electrolyte solution cools down, it goes back to its shape, provided it is not loaded with dead ions. To counter this and give devices and their batteries a longer life, manufacturers have now started using split batteries. Instead of one massive battery with a capacity of, say, 5000mAh, some manufacturers are using two batteries of 2500mAh each.
Fast charging is a double-edged sword for most manufacturers and users. That is why most of the research and development around batteries is now around managing the time taken to charge and ways to optimize battery health. With 240W fast charging on its way, it will be interesting to see how manufacturers improve ways to make batteries last longer and how to maximize battery health.