The hidden challenges and limitations of fast charging in smartphones

The hidden challenges and limitations of fast charging in smartphones
Smartphone manufacturers love to compete with each other and that’s especially true when it comes to hardware specs. While design, user experience and build quality are things that are hard to measure exactly, specs usually come with nice, specific numbers that you can compare and confidently say “X is larger than Y” which obviously means X is better, end of story.

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 humble power adapter is the unsung hero of every fast-charging technology. Regardless of its make or model, your phone will never reach its peak charging speed unless it is paired with a capable power 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


But a powerful, efficient charger would be of no use if it has to squeeze all that power through a cable that’s not meant to handle it. While cables are undoubtedly the simplest part of the charging infrastructure, they’re just as important as any other. The challenge here depends on how each manufacturer decides to transfer the power from the adapter to the phone. 

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...

Battery


The inconspicuous design of modern Li-Ion batteries does a great job of concealing their volatile nature. If you’ve seen any of the countless videos of a smartphone battery being punctured and releasing a fury of flames and smoke, you know what we’re talking about. On one hand, the highly reactive chemicals allow the battery to store great amounts of energy for its size. On the other, they make mandatory the use of specialized circuitry preventing the battery from short circuits, overcharging, and overheating.

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.

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6 Comments

1. OneLove123

Posts: 1180; Member since: Aug 28, 2018

I like fast charging with a big battery.

2. Poptart2828

Posts: 423; Member since: Jan 23, 2018

I learned nothing from this article. From my personal experience, fast charging is a godsend and BBK has been doing it for many years now. None of my batteries have exploded or detoriated so much that has been rendered useless yet. Even my oppo find 7 still works great with vooc charging

3. Brewski

Posts: 710; Member since: Jun 05, 2012

So the article is about the fast charging techniques used by OnePlus, Asus, Samsung, and how they must use the special power adapters/cables they include with that phone or it won't work. But PhoneArena can't resist including something Apple in every picture which does not include anything special in their box...

4. Subie

Posts: 2386; Member since: Aug 01, 2015

Good article Georgi! Please pay no attention to members of the complaint department.

8. XyAzario

Posts: 53; Member since: Mar 15, 2019

Insightful indeed

* Some comments have been hidden, because they don't meet the discussions rules.

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