Does charging a phone for a long time damage the battery?
Not in terms of what most people would consider “damage”. But read on!
Know Who Your Charger Is!
That thing everyone calls a “charger” when you have your phone plugged in is just a power supply, like a battery or other DC adapter. The charger is an integral part of the power management circuitry of the handset.
The Power Control circuitry would first use an external power to power the phone while plugging power into your phone. It has to, since a battery can not be charged and unloaded simultaneously. And if the phone doesn’t suck up all the electricity, the charging circuit on the phone will start charging the battery.
Under ordinary circumstances the charging cycle continues until the battery is complete anyway. The power management system then simply stops charging the battery at that level. Your phone is still working from input control. If there are any power surges that your wall wart can’t handle, most phones can combine power from input power and battery power. But if the phone’s only always idle, most battery dongles can battery the whole phone quickly.
The phone’s power management circuitry is still monitoring the battery. If that charge should drop below around 95%, the charging circuit will probably top it off again. Technically, this charging point is just a “simple matter of software”, but phone manufacturers don’t necessarily allow you to adjust it, though increasingly, some do (see notes at the end).
Actually this is how all USB powered systems work, as the USB power dongle is not a charger, but a power supply. This allows your phone to run the charging cycle its battery needs perfectly, without worrying about who supplies the power. And it also makes the charging from the same “absolute” socket of different battery chemistries. It makes the power management system of the phone more complex, as it can not rely on USB power to power the phone at peak loads entirely.
It is not recommended, as others have mentioned, that a phone battery be stored under full power for the long term. Full charged or near zero charged batteries wear out faster than if stored half full. If placed in a hot environment, they often wear out quicker, pretty much everything above average room temperature. If you are storing a battery for a month or more, putting it away with maybe 60 percent charge is better, but enough to compensate for around 3 percent self-discharge a month.
Daily Wear and Potentially Avoidable Damage
I’m trying to distinguish regular usage from “harm.” Theoretically, if you charge your battery, some of its useful life is expended, so theoretically it’s “harm,” and yet it’s a fairly pointless task not to use the battery. Many phone batteries are rated at 500 full charge / discharge cycles, which is a scale of 100 percent to 0 percent, but the power manager also allows the battery to actually go up to 0 percent, so it is possible to stop a true 100 percent to prolong the battery life.
So you’re going to use it, of course, and most users need their phone every morning at or near 100 per cent. If you want the absolute longest battery life, keep it charged between 20 percent and 80 percent or so … that’s what hybrid cars are doing to make their batteries last for ever. Why 80 percent of them? In a modern Li-ion battery, there are several ageing mechanisms, and one of them starts only at about 3.92V, or just around 80 per cent charging on a new battery. If the software of your phone makes a “longevity” mode, the charger may be restricted to 80%, which would be perfect.
And if you leave the phone all night plugged in it won’t kill your battery short-term by overloading, if you leave it partially charged through the night and charging in the morning, or maybe on the way to work, the battery will possibly last a little longer. I never practiced this with my old LG V10, but then again I had to swap in and out with multiple batteries. I actually do with my new phone … but it helps me to get 2–3 days or more of power on a single charge (4000mAh battery) right now. In the days of locked-in batteries, that’s another thing to think about: a larger battery doesn’t just give you a better run time every day, it will actually let your phone last longer. Don’t tell Apple!
But wait … Apple already knows, so they’re trying to really help you out here. No, Apple won’t trust you to set a clear cap of 80 per cent. So as with iOS 13, they do adaptive charging based on an AI agent that follows the trends of using your devices, something that Apple calls Optimized Charging of Batteries. You must configure this mode in the Preferences panel for iPhone Battery Safety. The phone will stop or delay charging past 80 per cent based on your history of activity.
In conclusion, modern devices do not suffer from any sort of damage due to overcharging to a battery left charging.
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