Leave your phone on the dashboard on a 35-degree day and it might greet you with a black screen and a warning that it needs to cool down. Take it out early on a frosty morning and it might switch itself off with charge still showing on the battery icon.
In my day job in IT, I see the long-term results of this constantly. Phones barely two years old come back with batteries that hold a fraction of their original charge. When you ask where they have spent their lives, the answer is usually a windscreen mount or a hot car.
Phone battery drain in heat and cold comes down to one thing: lithium-ion batteries are chemical, and temperature changes how fast the chemistry runs. Cold slows it down, so the battery cannot deliver its charge properly, but it recovers once it warms up. Heat speeds up the reactions that wear a battery out, and that damage is permanent.
| In the heat | In the cold | |
|---|---|---|
| What happens inside | Unwanted chemical reactions speed up and wear the battery out | Lithium ions move sluggishly and internal resistance rises |
| What you notice | Phone slows down, dims, stops charging or shows a temperature warning | Battery percentage drops fast, or the phone switches off with charge left |
| Is the damage permanent? | Yes, lost capacity does not come back | Usually not, it recovers once warm (charging below freezing is the exception) |
| Biggest risk in Australia | A phone left in a parked car or on a dashboard | Early mornings, the snowfields and cold outback nights |
A quick look inside your battery
Almost every phone today runs on a lithium-ion battery. Inside, there are two electrodes separated by a liquid or gel called the electrolyte. When you charge your phone, tiny charged particles called lithium ions move through the electrolyte from one electrode to the other and are stored there. When you use your phone, they flow back again, and that movement is what powers it.
It is a chemical process, and like most chemical reactions, it is very sensitive to temperature. Too cold, and everything slows down. Too hot, and things start happening that should not.
Apple says iPhones are designed for use at ambient temperatures between 0°C and 35°C, with 16°C to 22°C as the ideal comfort zone. Most other phone makers give similar ranges. The trouble is that an Australian summer can easily push your phone well outside that window.
Does heat damage phone batteries?
Yes, and permanently. Apple says that using an iPhone in very hot conditions can permanently shorten battery life. Its battery advice goes further, warning that ambient temperatures above 35°C can permanently damage battery capacity.
Why? Heat speeds up the unwanted chemical reactions that slowly wear a battery out. Over time, these side reactions use up some of the lithium and break down the electrolyte. A protective layer inside the battery also gradually thickens and starts to clog things up. The result is a battery that holds less charge than it used to and struggles to deliver power when you need it.
A laboratory study by researchers at Hamad Bin Khalifa University in Qatar shows how big the effect can be. Small lithium-ion cells cycled at 55°C lost about 36% of their capacity after 350 charge cycles. That matched the wear seen after 1,000 cycles at 25°C, so the heat aged the cells roughly three times faster. Battery University gives a gentler rule of thumb: a battery used at 30°C instead of 20°C loses about 20% of its cycle life, and at 40°C it loses about 40%.
That is also why a hot phone seems to struggle in the moment. When it gets too hot, your phone deliberately protects itself by dimming the screen, slowing down its processor and pausing charging. If it gets hot enough, it shows a temperature warning and stops working until it cools down. Meanwhile, every hour spent hot chips away at the battery’s long-term health.
The Australian summer problem
In Australia, the real risk is less the weather than where phones end up sitting. A parked car heats up astonishingly fast. Frankston City Council, working with Kidsafe Victoria, warns that a parked car can be 20 to 30 degrees hotter than the air outside. It can easily climb above 60°C on an Australian summer day, and most of that rise happens within five minutes of parking.
A phone sitting on the dashboard in direct sun can get even hotter. That is far beyond the 45°C upper limit Apple gives for even storing an iPhone switched off.
Using your phone for navigation while it charges on a sunny windscreen mount is a common sight in utes and work vehicles. It combines three sources of heat at once: the sun, the charging and the processor working hard.
Why does my phone die in the cold?
Cold works very differently. Instead of speeding up damaging reactions, it slows down the normal ones. In the cold, lithium ions move more sluggishly through the electrolyte, and the battery’s internal resistance goes up. Internal resistance is simply how hard it is for current to flow.
The battery still contains its charge, but it cannot deliver it as easily, so its usable capacity drops. According to Battery University, a battery that delivers its full capacity at 27°C typically delivers only about half of it at -18°C.
That explains the strangest thing about cold phones: why they can switch off while the battery icon still shows plenty of charge. Apple explains that a battery’s resistance rises temporarily in the cold and when its charge is low. When the phone asks for a burst of power, such as when you open the camera, the voltage dips. If it dips too low, the phone shuts itself down to protect its electronics, even though there is charge left inside.
The good news is that, unlike heat, this is usually temporary. Battery University notes that the lost capacity comes back once the battery returns to room temperature, and Apple says performance returns to normal once the battery is back in its operating range.
There is one exception worth knowing. Charging a lithium-ion battery below freezing can damage it permanently, so if your phone has been out in the cold, let it warm up before you plug it in. An iPhone that is too cold may not charge at all anyway.
Australia is not known for its freezing weather, but cold catches people out more than you might think. Think of the snowfields, early-morning drives in the high country, or a clear desert night in the outback, when temperatures can drop sharply after sunset.
How to protect your phone battery
These are the habits I recommend to anyone who asks me why their phone battery is dying early.
- Never leave your phone in a parked car in summer. Take it with you, or at least put it somewhere out of direct sun, such as the glovebox, rather than on the dashboard or seat.
- Be careful with windscreen mounts. If you navigate while charging on a sunny day, point an air-conditioning vent at the phone, or charge it in a shaded spot instead.
- Take off thick cases when charging if the phone feels hot. Apple notes that charging inside some styles of case can trap excess heat.
- Turn on battery protection features. iPhones have Optimised Battery Charging, and iPhone 15 and later models also let you set a charge limit anywhere from 80% to 100%.
- On Android, look for the same idea under a different name. Samsung Galaxy phones have a Maximum battery protection setting that stops charging at 80% (older software offers an 85% limit), and Google Pixel phones have a Limit to 80% option.
- Do not fight the phone when it is hot. If it gets too warm, charging can slow down or stop altogether until it cools, which is the phone protecting its battery.
- Do not cool a hot phone in the fridge or freezer. Let it cool down naturally in the shade or near a fan, because rapid cooling can cause condensation inside the phone.
- In the cold, keep it close to your body in an inside jacket pocket, and let it warm up before charging.
How to check whether heat has already hurt your battery
If you suspect your battery has had a hard life, you can check. On an iPhone, go to Settings, then Battery, then Battery Health (called Battery Health and Charging on some versions). Look at Maximum Capacity, which compares what your battery holds now with when it was new.
Apple says batteries in the iPhone 14 and earlier are designed to keep 80% of their original capacity after 500 complete charge cycles. For the iPhone 15 and later, that figure is 1,000 cycles under ideal conditions. A complete cycle is 100% worth of use, so charging from 50% to full twice counts as one.
If your phone has dropped well below 80% long before it reaches those numbers, heat is a likely culprit. On Android, battery health information varies by brand. Many newer phones show it under Settings, then Battery, and Samsung owners can run a battery check in the Samsung Members app.
Warning signs of a heat-damaged battery
Occasionally, heat and age do more than reduce capacity. A badly degraded lithium-ion battery can swell as gas builds up inside it.
If your phone’s screen starts lifting away from the frame, the back bulges, or the phone no longer sits flat on a table, act straight away. Stop charging it, stop using it, and take it to a repairer or the manufacturer. Do not try to press it back into shape or puncture it. A swollen battery is far more likely in a phone that has spent a lot of time hot.
What about storing a phone?
If you are putting an old phone away in a drawer, or storing a spare work phone, do not leave it fully charged or completely flat. Apple suggests storing devices at about half charge, because a battery stored fully charged for a long time can lose capacity. Keep it somewhere cool and dry, not in a hot shed or garage.
Hot is worse than cold
Heat and cold both drain your phone, but in completely different ways. Cold slows the chemistry down: your phone may die early, but it recovers once it warms up. Heat speeds up the chemistry that wears the battery out, and that damage does not come back.
In a country where a parked car can top 60°C, the single best thing you can do for your phone’s battery is to keep it out of the sun. And if you enjoy everyday physics like this, my post on static electricity explains why dry days give you a zap.
Sources
- Apple: If your iPhone or iPad gets too hot or too cold
- Apple: Battery life and lifespan
- Apple: iPhone battery and performance
- Apple: About Charge Limit and Optimised Battery Charging on iPhone
- Samsung: Galaxy battery protection feature in One UI 6.1
- Google: Get the most life from your Pixel phone battery
- Engadget: Apple says the iPhone 15’s battery has double the promised lifespan
- Battery University: BU-502 Discharging at high and low temperatures
- Battery University: BU-410 Charging at high and low temperatures
- Hamad Bin Khalifa University: Thermal challenges in lithium-ion battery technology
- Frankston City Council and Kidsafe Victoria: Kids in hot cars