Fundamentally, batteries work by converting potential, chemical energy into electrical energy, and vice versa. Inside the battery is an anode and diode. Together, they create a chemical reaction that moves electrons from to the other. As electrons flow from one to the other, electricity is created. Running a current into the battery (charging it) reverses this reaction and moves the electrons back. This reversal process isn’t perfectly efficient; every time it happens, some electrons are left behind and stay there until the battery is rebuilt. Once enough electrons are stuck this way, the battery’s capacity is diminished. How quickly this happens and the circumstances that accelerate it depend on the materials used to build the battery. When it comes to battery materials, there are many options for the anode/diode combination and each have their strengths and weaknesses.

Car batteries use lead and acid, which is cheap and powerful, but also bulky, heavy, and prefers to be charged near to full capacity or it runs the risk of the cells not being able to reverse their reaction. Basically, once they “die” they sometimes don’t charge back up.

Phones, laptops, and other modern electronics use lithium-ion batteries, which are lighter and more compact, but also weaker and behave differently. Lithium-ion batteries prefer to be nearly dead and operating at low capacity.

Basically, lead-acid batteries create a chemical reaction that prefers to move as many electrons as possible for maximum power, while lithium-ion batteries create a chemical reaction that prefers to move as few as possible for maximum longevity. Like anything else, you get the most out of them by playing to their strengths and avoiding their weaknesses.

Now, as to the nitty gritty why do lithium-ion batteries prefer to be charged that way, and what causes those electrons to become stuck more frequently when charged to max capacity? I dunno. That’s a good question for a chemist or a physicist; I’m just a person who reads a lot.