Why does my phone heat up when I charge it?

It’s common for smartphones to get warm when charging. But why exactly does this happen? There are a few key reasons that explain the temperature increase.

Faster Charging Generates More Heat

Modern smartphones support fast charging technologies like Quick Charge or USB Power Delivery. These can deliver power at 10W, 15W, 18W or even 30W. The higher the wattage, the faster your phone can charge. However, more power going into the battery also creates more heat.

Let’s break this down. Power is measured in watts, which is determined by multiplying volts and amps. More volts or amps equals more watts delivered. Current smartphones have lithium-ion batteries with a voltage of 3.7V. So to increase power, manufacturers boost the amps. Higher amperage causes electrons to move quicker through the phone’s circuits, generating more heat.

For example, a standard 5V/2A charger provides 10W of power (5 x 2 = 10W). But a Quick Charge 3.0 charger can deliver up to 18W (5 x 3.6A = 18W). So at maximum output, Quick Charge generates nearly double the heat of a regular 10W charger. And upcoming standards like Quick Charge 5.0 will go even higher to deliver up to 100W to charge large batteries quickly.

Faster Charging Standards

Charging Standard Maximum Power
Standard USB 10W
Quick Charge 3.0 18W

Higher wattage allows phones to charge faster, often reaching 50% in 30 minutes or less. But all that extra power generates more heat in the process.

Inefficient Charging Also Plays a Role

The heating effect is amplified if your phone’s charging circuitry is inefficient. Power conversions create some energy loss in the form of heat. An inefficient charger will convert more power to heat rather than charging the battery.

Part of the problem comes from resistance in the phone’s wiring and components. Resistance impedes the flow of electrons and converts some of their energy into heat. Poorly designed circuit boards, low-grade components, and damaged cables/ports can all add resistance.

Additionally, cheaper or older chargers may not deliver stable current and voltage. Fluctuations force the charging circuit to work harder regulating the power flow, again generating excess heat.

Using the charger that came with your phone is your best bet for efficiency. The stock charger and charging circuitry are designed in tandem to work optimally together.

The Battery Gets Hot Internally Too

It’s not just your phone’s exterior that heats up. The battery also generates internal heat as part of the electrochemical reaction during charging. Here’s a quick primer on how batteries work:

A lithium-ion battery has a positive and negative electrode (cathode and anode). The cathode is made of a lithium compound like lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4). The anode is made of graphite (carbon).

The electrodes sit in an electrolyte solution. Lithium ions flow between the cathode and anode through this solution, causing oxidation and reduction reactions that generate electricity.

When charging, an external current forces the lithium ions to move from the cathode to the anode. This intercalation process energizes the battery. Unfortunately, it’s not 100% efficient – some power gets dissipated as heat due to electrical resistance and chemical reactions.

Studies have found the internal battery temperature can rise by over 18°F (10°C) during fast charging. Faster charging also degrades the electrodes over time, making this heating effect worse in older batteries.

Lithium-Ion Battery Diagram

Image source: Wikimedia Commons

Heat Dissipation Plays a Role Too

Another factor is that smartphones aren’t very efficient at dissipating heat. Many phones have metal or glass builds, which do not readily conduct heat away from internal components.

Batteries and processors generate the most heat, but it can build up inside the confined space of the phone. The lack of airflow in an enclosed case makes cooling even harder. As internal temperatures rise, the exterior casing warms up too.

Some manufacturers add heat pipes, vapor chambers and graphite sheets to help transfer heat away from critical components. But there are limits to how much active cooling you can pack into a slim phone profile.

Charging Habits Can Exacerbate Heat Buildup

Certain charging habits can worsen overheating issues:

  • Using thick phone cases traps in more heat.
  • Charging in hot ambient environments like under sunlight.
  • Plugging into chargers with a coiled cord can cause electromagnetic interference that makes the phone work harder.
  • Using a wall charger that is higher wattage than the phone is designed for.
  • Continuing to charge at 100% instead of unplugging.
  • Using an old charger in poor condition.

Try to avoid these practices whenever possible to keep your phone running cooler.

Is the Heat Harmful for My Phone?

In most cases, the heat from charging is not a cause for concern. Smartphones and lithium-ion batteries have upper temperature limits before damage can occur:

  • Batteries – Typically 60°C (140°F) max operating temperature.
  • Processors – Around 95-100°C (200°F) max junction temperature.
  • Displays – Around 60-65°C (140-150°F) max.

During normal charging, phones rarely exceed 40-45°C (105-115°F). This is well below thresholds for immediate damage.

However, there can still be long-term effects. Repeated heat cycling stresses components over time. Higher temperatures also accelerate the natural capacity loss of lithium-ion batteries. So while not dangerous per se, it’s still best to minimize heating when possible.

Typical Safe Temperature Ranges

Component Max Temperature
Battery 60°C / 140°F
Processor 95-100°C / 200°F
Display 60-65°C / 140-150°F

How to Keep Your Phone Cooler When Charging

Here are some tips to prevent excessive heating during charging:

  • Use the original OEM charger and cable designed for your phone.
  • Don’t use bulky phone cases that trap in heat.
  • Avoid wireless charging which generates more heat.
  • Charge in a cool room at normal ambient temperatures.
  • Don’t charge under direct sunlight.
  • Avoid using the phone intensively like gaming during charging.
  • Disable fast charging when temps get too high.
  • Don’t overload power banks with multiple devices.
  • Replace any chargers/cables that are damaged or frayed.
  • Turn off the phone or limit screen brightness when charging for a long time like overnight.

Following these guidelines will help keep your phone’s temperature lower during charging sessions.

When to Be Concerned

As a rule of thumb, you should start to be concerned if your phone feels uncomfortably hot to the touch. Some key warning signs include:

  • Cannot be held in your hand due to high temps
  • Display or apps start malfunctioning
  • Battery charges very slowly
  • Odd smells emit from the phone
  • Phone shuts down unexpectedly

If you notice any of these issues, discontinue charging immediately. Persistent overheating may indicate a bad battery, faulty charger, blocked cooling vents or other internal faults. Get your phone inspected by a repair tech if the problem continues.


It’s normal for smartphones to heat up a bit during charging. But excess temperatures can accelerate wear on components over time. The main factors causing heat are faster charging wattages, inefficient power conversion, internal battery chemistry and limited heat dissipation.

Use best practices like the original OEM charger, no bulky cases and room temperature charging to keep your phone running cooler. Avoid damage by being alert for any warning signs like extreme overheating. With proper care, you can minimize heating issues and extend the lifespan of your device.