How to care for your phone’s battery
Smartphones get their power from their batteries. Every time you charge and discharge (i.e. use) your phone, the battery dies a little. Battery life is measured in charge cycles. Most modern smartphones will last for around 500 charge cycles. The number of charge cycles is an average — typically, you could expect your phone to last that number of cycles. You might be lucky and get more (maybe 510) or unlucky and get less (maybe 480). But the typical phone’s battery can expect to stop working around 500 charge cycles. More expensive phones sometimes get more.
Not so long ago, batteries were replaceable — you could take off the back cover and simply replace the battery. Those days are gone. Modern smartphones are slabs of glass (front and back), glued to a metal (sometimes plastic) frame. Taking off the back is no mean feat and batteries are meant to be replaced by service centres. The cost of replacing a battery varies but you could expect to pay £50-£100 (perhaps more) and be without the phone for a week (perhaps longer). It’s better to take care of your battery. There are two ways to do that. Reduce battery cycles and avoid heat.
A battery cycle is the number of times the battery can do a full charge/discharge cycle (0–100%). It is not the number of times you charge your phone. To complicate things, battery cycles are not linear. Charging from 0% to 50% will not use half a cycle (more likely 0.1 cycles) and charging from 50% to 100% will not use half a cycle either (more likely 0.9 cycles). That’s because the last 50% is a lot harder to push into the battery than the first 50%. In fact, the last 20% is hardest of all. Charging your phone from 80% to 100% will use around 0.6 cycles. You can see from this why it’s best not to keep topping-up your phone. Most manufacturers recommend keeping your phone battery between 20% and 80%. Charging from 20% to 80% will use around 0.2 cycles — which means you could charge your phone 2,500 times (assuming your phone is rated at 500 cycles) before expecting problems. Or, to put it another way, charge it every day for almost 8 years. On the other hard, if you constantly charge your battery, for example charge from 50% to 100% twice a day, it would use around 1.8 cycles a day — and your battery would last a year.
Heat is the other factor. Irrespective of the number of charge cycles, heat reduces your battery’s capacity. Phone batteries are measured in milliamp hours (mAH). A typical phone battery is 4,000 to 5,000mAH and is designed to operate in temperatures between 25–35C. Regularly exceeding that upper limit (35C) will reduce your phone’s capacity. Poor heat management could see a 4,000mAH battery reduce to 2,000mAH within a couple of years, which means that it will last half as long as it did when new. So what causes over-heating? Charging is the main cause. Charging your phone produces internal heat. Fast charging produces more heat. So does leaving your phone in the sun or gaming for extended periods of time. All of these activities could see your battery temperature exceed 40C. A little bit of heat is fine. All phones heat-up during normal use and warm weather is inevitable. But if you regularly game on your phone for long periods or live in a hot climate the capacity of your battery will suffer.
Wireless charging is worth a special mention since it can exacerbate both problems. Wireless charging makes it easy to “pop your phone on the charger” (which will eat battery cycles) and generates more heat than wired charging because of the relatively poor connection (and therefore high resistance ) between phone and charger.
The combination of the two is the killer. If your charging regime is to keep your battery at 100% and you regularly game and edit videos on your phone and live in a hot country you could kill a new smartphone in a year. One (regular charging) will eat up the battery cycles and the other (heat) will reduce your battery capacity. Let’s take an example. Say you buy a new smartphone with a 5,000mAH battery and like to keep it topped up by throwing it on your wireless charger at every opportunity. You live in a hot climate (where it regularly bakes in the sun) and like to play games on it. After 18 months you could expect your capacity to have fallen to around 3,000mAH and have used about 300+ charge cycles. The reduced capacity means more charging, which means more cycles. Give it another six months (two years after purchase) and you’ll be looking at a phone with half of the original capacity (2,500mAH) with most of its battery life (cycles) exhausted. Alternatively, you could cable charge up to 80% only when needed, avoid intensive apps, keep it out of the sun — and finish up at the same point (two years) with 90% of the original capacity and most of the cycles unused.
In reality, things are a bit more complicated. My own phone is a OnePlus 9 Pro, which comes with a 4,500mAH battery and a fast charger (65W) in the box. It’s a “flagship” phone with a flagship battery- which means it has a superior battery in terms of charge cycles (750) and heat resistance (due to thermal cooling). This phone will stand up to poor battery habits much better than a cheap phone. Many Oneplus 9 owners only use the supplied fast charger that in theory should reduce battery capacity but report than battery health is largely unaffected.
It’s not worth obsessing about your battery. Replacing it isn’t the end of the world. OnePlus (my brand) makes it relatively cheap (around £50) to professionally replace the battery (although that’s not true of all brands). But you can see the difference that good charging habits make. Avoiding really bad battery habits is the vital thing. My advice is to try to limit charging to something less than 100%. Say, 80% or 90%. The occasional full charge (100%) does no harm. Neither does occasional wireless charging. Neither does occasional gaming or anything else that stresses your phone. Your battery will still last longer than your phone. If you have a fast charger, use it. I slow charge my phone (using a standard 5W charger and cable) most mornings to 80%. If I have a long day ahead of me, I charge it to 100%. If I’m in a hurry, I throw it on the fast charger. If I need extra capacity late in the day, I use the fast charger for five or 10 minutes. When I’m away from home I only pack the fast charger. After six months of use, my screen on time is higher than when I bought it (around 8 hours) and I’ve used 48 charge cycles. At this rate, I can expect to have used 300 cycles and have 4,000mAH capacity after three years. The battery should last seven years, which means that the battery will outlast the phone.
So it’s not worth obsessing about your battery but it is worth caring about it. Battery degradation isn’t binary — it doesn’t suddenly die overnight. Doing what you like with the battery (overnight charging, wireless charging at every opportunity, leaving it in the sun, gaming for two hours at a stretch, etc.) will see it degrade within months of purchase. Your six hours of screen on time will rapidly become five— then four then three. Unless you plan to replace the battery every year, it’s best to take care of it.
