How Charging Smart Phone Batteries Works and How to Keep Them Charged

These days, everyone has a smart phone connected to their hip at all hours of the day. These smartphones are our personal assistants, sources of entertainment, and friends – all conveniently meshed into one small device. Being one of our most powerful devices, doing just about everything we could possibly imagine and more, the strain on them is immense; as expected. The constant demand placed upon them through endless usage drains their batteries, and a charge or two every day is just about a given. You may even carry your favorite charger with you everywhere you go in order to be sure that you don’t experience any downtime with your phone.

Even with your charger always next to your side, you may not fully understand how charging your smartphone battery works.

Are you curious to know if it’s safe to use a higher rated charger when charging? We’re going to dig into the intricacies of smart phone charging to better understand what the numbers written on your charger mean. We can then better learn how chargers operate.

Reviewing charger and battery specs

On a typical phone charger, you may have noticed that the output is rated 5V (for 5 volts) and 1A (for 1 amp), and another charger is rated 5V and 2A, while your battery input is rated 5V and 1A. So what’s the deal? Is it safe to use both chargers to charge this battery? And what does this all mean anyway when it comes to charging a phone? We’ll answer these questions further down in this article.

Background on phone batteries

To better answer these questions, let us first go over the type of battery predominately used in our smart phones today, and their implications. Lithium ion batteries are what you will more than likely find powering your smart phone. Lithium ion batteries are charged with a constant current. This charge continues until the battery voltage hits a set level, and then the charge controller switches to constant voltage – charging until the drawn current by the battery drops down to zero. The question then becomes how the current is held constant.

The latest smart phones have charge controller chips inside that control the amount of current that can be drawn into the phone battery. Think of this charge controller as a gate, which limits precisely what can enter through. And that is the secret to why it’s very difficult to damage your phone by using a high rated charger, as the amount of current let through is always regulated; thus, the phone is never overloaded.

Let us now take a look at a summarized list of advantages and disadvantages of lithium-ion batteries:

Advantages and disadvantages of lithium-ion batteries

How does a phone regulate the flow of current?

To regulate current, the charge controller regulates the flow of current into the battery using one of two methods. The first method is with the linear charge controller, where the controller’s integrated chip uses a transistor to operate as a switch or variable resistor. The resistance between the battery terminal and charger input is varied to only allow a set amount of current through. The second method is with the switching charge controller, where usage of a DC to DC converter in the charge controller is used to transport charge into the battery. The DC to DC converter uses inductors and capacitors for energy storage, and switches (typically a transistor), to vary the input voltage into the battery.

Of the two methods, the switching charge controller is the more efficient, but more expensive, of the two options. The linear charge controller is typically inefficient – as the controller leads to power dissipation somewhere, which then creates excess heat that is not good for your phone.

With the charge controllers described, the controller takes command from the software in the phone to dictate the amount of current that the battery can draw.

Let us now pull everything that we have learned together and see what it all means for your phone

1. Fully charged phone

Once your phone is fully charged, the charge controller stops all current from entering your phone in order to prevent damage. In other words, the charger is shutoff once your phone is fully charged. So if you forget to unplug your phone, your phone will not be damaged from ‘overcharging’.

2. 95% charge

When your phone drops to 95% charge, your phone will resume charging until it is once again fully charged; then again, the charger will be shut off.

This graph shows the charging stages for a lithium ion battery, which is the battery type found in most phones as we learned. We can tie this graph back to how a phone regulates the flow of current – we see at the end of stage 3 the charge ends. Then, at stage 4, the current flows once again as the phone charge slightly dips.

Charge stages of lithium-ion batteries

3. Time of charge

Charging your phone can be done at any time and for any amount of time.

4. Usage when charging

It is safe to use your phone while it is charging.

5. Deep discharge

Do not let your battery charge get too low. Deep discharge on a lithium battery is not healthy for battery life. It’s a good idea to always charge your battery when you see the charge dropping too low. When discharged below the set safe voltage that varies between manufacturers, a battery may experience a short circuit inside.

Smartphone deep discharge then charging

It’s good to know though, however, when a battery shows 0% charge, the charge typically is not really 0%. Newer phone models have a circuit inside that shuts the phone down before any battery damage is caused with a fully drained battery. But it’s still best not to allow your battery charge to decrease too low. Batteries simply need to be used and charged in order to prevent premature deterioration.

Now, back to our original questions and their answers

Charger output #1 is rated: 5V, 1A

Charger output #2 is rated: 5V, 2A

Battery input is rated: 5V, 1A

Clearly, charger #1 will work as its rating matches the battery input rating. Yet, with charger #2, the ampacity rating of 2A is doubled; so what happens? Using Ohm’s law, which gives us the relation between current, voltage, and resistance, we can see what happens.

I (current) = V (voltage) / R (resistance)

Since the voltage of 5V is held constant, the remaining variable that will determine the current draw on the phone is the resistance, which in our case is the load that the phone places on the charger. As we now know, the load (resistance) can be varied using the charge controller. Thus, the current will be held at a set level by varying the resistance in order not to harm the battery. Therefore, it is completely safe to use charger #2. In fact, using the capabilities of the charge controller and phone software, some phones may even safely be able to charge up faster using charger #2.

Colorful smartphones on desk


A smart phone, or any new device, will only draw the amount of current it needs – not more. This will ensure the battery is not harmed, and your device remains in operating conditions by your side; especially in times when you need it the most.

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