The hidden challenges and limitations of fast charging in smartphones
That’s how we end up with all the different races between phone makers: whose phone has the most processor cores or amount of RAM, the biggest battery, and most recently, the highest charging speed.
But improving charging speeds isn’t as easy as putting a massive power adapter in the box and calling it a day. Just like you can’t just change the engine of your car with one that has 500 more horsepower without also changing the gearbox, brakes, suspension and so on, you also have to beef up the whole power infrastructure of a smartphone to shave off those extra minutes of charging time.
Let’s look at all the major parts of that puzzle and what obstacles manufacturers face when trying to bring down charge times. Starting with…
The charging adapter
The challenge in designing a modern power adapter lies in enabling it to deliver lots of power safely and efficiently. That's because modern smartphones come with bigger and bigger batteries, and these need more powerful chargers to charge quickly. But a more powerful charger has to be either physically larger or made using a more efficient circuit design. Thankfully, things have greatly improved on the latter front. Had the Galaxy Note 10's 25W power adapter – which is about as big as two TicTac containers – been built using the tech inside a Nokia 3310's adapter, it would have been the size of a soda can.
Speaking of evolution, a modern power adapter is much more sophisticated than a counterpart from only a decade ago. No matter what fast-charging technology it supports, specialized chips inside of it allow it to communicate with the phone throughout the charging process. This allows the phone to request the optimal amount of power, depending on the battery's charge level and temperature. All of this is done in the name of speed, efficiency, and safety.
Cable and connectors
Power equals voltage times current. Therefore, transferring more power from the charger to the phone can be done by increasing the voltage, the current, or both. For example, 30W can be pushed through in a 10V, 3A configuration (high voltage, low current) or using 5V and 6A (low voltage, high current). There’s no right or wrong way. In either hypothetical case, there’s a problem to be solved.
Increasing the voltage allows the cable to be kept thinner and cheaper to make. There’s also greater compatibility: it is much more likely for the USB cable that came with one of your gadgets to charge all your other electronic devices just fine.
However, once it reaches the smartphone, the higher voltage has to be lowered to about 3.2 to 4.3 volts in order to charge the battery safely. This transformation isn’t 100% efficient, and some of the power is lost as heat – heat that may slow down your phone, hold back the charging process, and shorten the life of the battery.
In scenario two, a low voltage of about 5V can easily be handled by the phone and its battery. There’s also less heat generated inside the phone. However, the higher current requires thicker wires and special cables. That’s why OnePlus’s Dash Charge tech works only with the company’s own cables. If an incompatible cable is detected, the phone limits the charging speed since it doesn’t know if the wire can handle the current. And a wire that can’t handle the current will literally melt if subjected to excessive power.
But the biggest challenge comes from the batteries themselves...
How fast a lithium-ion battery can safely charge depends on its design. Physical size is a factor here. Bigger batteries do have the ability to better handle faster charging rates. But if you want to redesign that same cell to charge even faster without increasing its physical size, you have to increase the size of the elements inside the cell, and this will reduce its energy capacity. A real-world example of this relationship is the 5000mAh battery inside the Asus ZenFone 6, which charges at up to 18W. A battery of the same physical size capable of 40W charging would have been only able to hold 4000mAh of charge, the company’s engineers shared with us.
But no matter how large the battery is, the fastest charging rates can only be sustained up until the battery is about 70% full – and that is only if the battery cell does not exceed set temperature limits. The more you go beyond that threshold, the more incoming current is reduced for safety reasons. We’ve explained some of the issues with fast charging and its diminishing returns in our post about the 45W charging.
The rarity of battery-related incidents is a true testament to the technological achievement that are smartphone batteries and they deserve more credit than we’re giving them. While they’re not owe-inspiring like stunning OLED displays or camera lenses, the engineering behind them is just as admirable as that of any other smartphone part.