Lithium-ion batteries slowly shrink in capacity; a replacement may be covered under warranty.
The lithium-ion battery in your Mac laptop will lose its ability to charge to its maximum factory-made capacity over time. But you don’t have to take it lying down. Depending on the level remaining, you might be able to get a free replacement from Apple.
Lithium-ion batteries transformed laptops and mobile devices. They allowed for a greater density of stored energy compared to earlier generations of nickel-metal hydride batteries. This newer material loses charge much more gradually when not plugged in and not in use. And they shed the “memory effect,” which required careful charging to avoid permanently reducing the capacity of your battery. Modern Li-ion batteries can be shaped to fit into any space, further increasing device run time.
But, like all components that carry electricity, they do age. A Li-ion battery has a specific maximum safe charge that’s built in by design. (In fact, it’s below the actual maximum, as there’s a safety margin built in to avoid overcharging the battery.) As you recharge the battery over and over again, its maximum capacity reduces.
Read more at MacWorld.com