Good explanation/analogy:
Here’s iFixit’s resident battery expert, Arthur Shi:
“In general, li-ion batteries don’t like to operate near max capacity. My go-to analogy is a sponge and a cup of water. The sponge represents the graphite anode and the water represents the lithium ions.
“When you charge a li-ion battery, the lithium ions have to embed into the graphite (intercalation). When a sponge is bone dry and you pour water onto it, it readily absorbs water. As it saturates, you have to slow down the water input, or water will start puddling on top of the sponge.”
But this last part is not only slower, it’s also the part that causes the most damage to the battery.
“Imagine that the water that puddles on top solidifies irreversibly. This would reduce the mating surface area of the sponge and reduce the amount of water that can be recovered from the sponge,” says Shi. “For a li-ion battery, this is known as ‘lithium plating,’ which irreversibly reduces the battery capacity.”
Limiting the maximum charge gives a substantial increase to the overall life of a battery. If we measure the life of a battery not in cycles, but in the number of electrons it can deliver to your phone over its lifetime, then we’re looking at a possible increase of 400%. That is, your battery could deliver 4X the number of electrons. And yes, that means it could last up to four times as long, according to some simple testing:
“[C]harging to 80% instead of 100% multiplies by 4 the amount of energy the battery will have transferred to you over its life—the only tradeoff being to compromise on how much energy you can get out of a full charge (big slices, small cake VS small slices, large cake). This also means you can use your battery for 4 times longer before it gets to the end of its rated life,” “space mechatronics engineer” Mister Mystère wrote on Stack Exchange last year.