Why a full charge is a stressful state
Most phones, laptops, and electric cars use lithium-ion batteries. Charging moves lithium ions from one electrode to the other; near full charge, the battery holds them at a higher voltage. That high-voltage condition can encourage slow side reactions inside the cell, gradually consuming usable battery material and making it harder for the battery to deliver power.
The wear is cumulative, not a sudden penalty for touching 100%. The important factor is how long the battery spends near full, especially when it is warm. A device left plugged in overnight may remain close to its upper charge limit for hours, though its charging controls can reduce the strain.
Why the middle of the range helps
At a moderate charge level, the battery operates at lower voltage, so those unwanted reactions tend to proceed more slowly. This is why a charge limit—often available in a device’s settings—can help preserve capacity when the device is usually plugged in. “Capacity” is the amount of energy the battery can hold; as it declines, a charge that once lasted all day runs down sooner.
There is a tradeoff: stopping short of full means less energy available before the next recharge. If you need the range, charging to 100% is a normal use of the battery. The useful distinction is between occasionally starting the day full and routinely keeping a warm battery topped off at its maximum. For a device that spends most of its time at a desk, lowering its charge limit can avoid hours in the battery’s most stressful state.
