How Smartphone Batteries Charge

All smartphone batteries need to be charged within a day or at most two, depending on individual usage patterns. Charging your device can be a pain sometimes. Some persons simply can not wait for it to charge so that they can get back to what they were doing. Some others may not be too bothered, they simply plug in their chargers and walk away till it fills up. Regardless of the different attitudes towards charging, how smartphone batteries charge is still a mystery to a lot of people.

Rechargeable batteries are a blessing. I mean imagine having to buy disposable AA batteries every time you needed to use your phone. Think of how much you would have spent by now.


How do smartphone batteries charge?

Charging a battery is simply the process of replacing electrons spent by a battery during use. This is done by plugging in a charger to supply electrons into the battery. This causes the electrons in the cathode to migrate back to the anode.

Image credits: Australian Academy of Science


The charging process

There are two main phases in charging a battery. They are the rapid phase and the trickle phase. For the sake of simplicity, I will like to extend the phases to three: rapid or constant current phase, slow charge and trickle charge.


Read: Voltage, Current, Resistance and Power


Think of charging a battery like filling a bucket with tap water (or water from a faucet). When the bucket is totally empty, you can open the tap all the way to the end for maximum water pressure (voltage). This would increase the amount of water (current) that falls into the bucket. When the water has filled up over half of the bucket, you could then slowly reduce the water pressure (voltage) by closing the tap head progressively till the water runs at a trickle. When the bucket is almost at the brim, you turn it off. The same logic works with charging as well.

Rapid charge or constant current phase: is where electrons start to enter into an empty or almost empty battery. The battery’s voltage increases quickly from 0V to about 4.2V (max charging voltage). During this phase, the battery can accept a lot of current and the charger will pump more electrons into it. This will cause the battery to charge from 0% to about 80%.

Slow charge: When the battery hits 78-80%, the voltage is gradually reduced and the battery charges slowly. This is done to protect the battery from overcharging, high temperatures and subsequent explosions as well as to increase their lifespan. As the battery charges, electrons move from the cathode back to the anode. This creates a lot of pressure (electric potential or voltage) on the anode side. Pushing the electrons at high speed into such a high pressure environment is a recipe for disaster. Hence slow charging is effected. The battery will charge slower and slower till it gets to about 90-95%.

Trickle charge: At around 95%, the voltage drops drastically and current only enters the battery in trickles (or drops). If you use a smart charger, as soon as the battery hits 100%, it would cut off power to the battery. Even if you do not use a smart charger, some phones have an internal circuit that should cut off power supply to the battery and use the power directly like a laptop.


How to charge your phone

Now while most of us cannot help it, charging your battery all the way up to 100% and draining it completely is bad for Lithium batteries. This is because moving all the electrons in a battery to one side (either below 10 or above 90) puts the battery under a lot of stress. It also uses up your charge cycles faster.


Read: Battery life explained


The best way to keep the battery balanced is by keeping the percentage between 20% and 85%. That means you should not let the battery run below 20% and not charge it above 85%. You can use partial charging to achieve this.

A lot of misinformed people argue that full charging (0-100) is the best way to charge phone batteries. I would say it is best way to use up your charge cycle and age your battery. Top up your batteries as often as you can without letting it run down. Partial charging i.e. 15% now, 30% in two hours time, 10% in three hours time etc. is the best way to charge lithium batteries.

Some companies also implement a software trick to help consumers manage their batteries better. Here’s how they do it. Whilst charging, your phone’s software is designed to display 100% as soon as the battery gets to 85-90%. So most of the time, that 100% full charge that you see is an illusion designed to protect your battery. In this same vein, your battery never runs down to zero. Even when it shuts down, there is always some reserve being kept so as to save the battery’s lifespan.


Should I use my phone while charging?

Using your smartphone extensively while charging gives rise to what is commonly called a parasitic load. When a battery is being used and charged simultaneously, it reduces battery life and increases heating. This simultaneous charging and discharging can distort the charging cycles of the battery and reduce its life. Furthermore, if the battery is in the constant voltage phase, this might lead to extra stress on the battery, which can impact the battery’s life and even increase the chances of spontaneous ignition or explosion. (XDA Developers, 2021).



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