Definition
Battery health management is a set of habits that slow the aging of the lithium-ion battery in a smartphone or laptop, so it keeps its original capacity for longer. It works by adjusting how you charge and the temperatures you use it at, so the parts inside take as little physical and chemical damage as possible.
A battery wears out with use, like a rubber band
Wear a new pair of shoes for long enough and the soles wear down. A rubber band goes loose with use. A smartphone battery ages a little each day the same way. Inside the battery are tiny particles called lithium ions. When you charge, these particles cross over to one side (the anode). When you use the phone, they come back to the other side (the cathode), and that movement makes electricity.
Every time the particles travel between the two sides, the walls and passages inside stretch and shrink by a tiny amount. After hundreds of rounds, the passages warp. Particles get trapped halfway, or the walls get damaged.
In time, the battery can no longer hold a full charge the way it did when new. To slow this natural aging, you need habits that reduce the physical stress of charging and discharging.
The trick to staying between 20% and 80%
People get sick when they overeat until they're stuffed or go too hungry. A battery has the hardest time at the two extremes, too. One is full discharge, when the energy hits 0%. The other is full charge, packed to 100%.
Actually, even when the screen says 0%, a protection circuit keeps a small reserve, so the battery isn't completely empty. But if you leave it that way for a long time and it really drains to the bottom, the electrodes can be damaged in ways that are hard to undo. On the other side, if you keep a phone plugged in at 100%, the high voltage squeezes the chemicals inside the battery and slowly wears them down.
That's why a battery is happiest, and lasts longest, when you keep it between 20% and 80%. Many recent smartphones and laptops have a "battery protection mode" or an "80% charge limit." Turn it on, and the charger avoids the extra strain for you, even while it stays plugged in.
To be a bit more precise: heat is a battery's biggest enemy
To be a bit more precise, one of the biggest things that eats away at battery life is hot heat, more than the number of charges itself. Every chemical reaction gets more intense as the temperature rises. So when a battery gets hot, the protective layer inside it grows abnormally thick and performance drops.
Fast charging with a thick case on, or charging while running a heavy game, can push the temperature inside the device above 40°C (104°F). That heat speeds up the breakdown of the electrolyte (the liquid the ions swim in) and damages the electrodes. A phone that shuts off for a while in winter cold comes back once it warms up. But chemical wear that happens at high temperatures causes permanent damage that can't be reversed.
So small habits are the secret to getting the most from a battery. Don't charge your phone under your pillow while you sleep, and don't leave a device in a hot car in summer.
Common misconceptions
To make a battery last longer, you should use it all the way down to 0% and then charge it fully to 100%.
That was common wisdom for old nickel batteries. With today's lithium-ion batteries, the longer one sits near 0% or packed at 100%, the faster damage builds up. There's no need to drain it completely and then fill it up.
Leaving a charger plugged in overnight overcharges the battery and makes it explode.
Modern electronics have a protection circuit that stops charging once the battery is full, so leaving it plugged in overnight doesn't overcharge it. But sitting at 100% high voltage for hours is itself bad for battery life.
Where you meet it
It's turning on the "battery protection (charge up to 80% or 85%)" feature in your smartphone settings.
While charging, avoid tasks that make a lot of heat, like a high-end 3D game or recording video.
In one sentence
To protect battery life, charge between 20% and 80%, avoiding 0% and 100%, and keep it away from hot heat.