You Can’t Overcharge an iPhone
Assumptions that you can “overcharge” an iPhone are based on a fundamental misunderstanding of how electricity works. Electrical charging current is never pushed to a device. In other words, your charger doesn’t “force” power into your iPhone. It supplies power, which the iPhone draws on, or “pulls” from the charger. This means that an iPhone can never get more current than it needs, regardless of what charger you’re using, since there are power regulation chips in the iPhone itself to determine how much power is being drawn from the charger and fed into the battery.
Once an iPhone is fully charged, it simply stops drawing anything more than a trickle of power to run from. In fact, even at about 80 percent capacity, the iPhone slows down charging, and with iOS 13’s Optimized Battery Charging feature, the iPhone will actually cut off charging at the 80 percent point if it knows it still has time to get there later in order to prolong the life of your battery, since lithium-ion batteries don’t like to be continually kept at full capacity.
Contrary to a popular misconception, however, with standard USB none of this is based on a “negotiation” with the charger — there is no intelligent back-and-forth communication involved where the iPhone “tells” the charger what to do. Instead, it’s simply electricity 101 — it’s the same reason that the light bulbs in your house don’t explode even though they’re on the same circuits that are capable of delivering enough power to run a block heater, air conditioner, refrigerator, or a microwave oven. The light bulbs draw the power they need from the circuits, and nothing more.
This is also why you can always use a higher-current charger. For example, for many years, Apple’s iPhones could only charge at five-watt (1A) speeds, yet you could safely use a 12-watt (2.4A) iPad charger to charge your iPhone. The iPhone draws only the power it needs from the charger.
Keep in mind, however, that there is a slight risk in going in the other direction — using a lower-current charger to charge an iPhone. Although Apple’s power regulation chips try to determine how much power they can draw from a charger, if the charger or cable are improperly designed, the iPhone can try and draw too much current, resulting in the charger overheating as it tries to keep up with the iPhone’s demands for more power. This will definitely cause the charger to get hotter, and depending on how badly designed the charger’s circuitry is, could even cause a fire.
To be clear, however, this kind of voltage regulation isn’t an Apple thing — it’s part of the USB battery charging specification. All USB-certified chargers devices are supposed to be able to let the device on the other end determine what level of power can be supplied, and only draw that amount of power and no more. So in other words, you don’t need to buy an Apple-certified charger — any USB-certified charger will handle this just fine.