Above all, there’s one thing you want out of your phone’s battery: the ability to hold as much power for as long as possible. Nobody wants a phone that can only last half a day off a full charge.
There’s an incredibly pervasive myth that the best way to keep your battery healthy is to let it drain all the way to zero before recharging. Alas, like littering and the all-bread food pyramid, this is no longer considered a best practice. In fact, it’ll actually shorten the lifespan of your phone. Instead, to keep your battery healthy and ensure it’s able to maintain as much of a charge as possible, you want to give your phone regular charges — and avoid letting it get down to that 5 percent, everything all red, “Oh shit, how am I even gonna find the subway stop from here?” moment (easier said than done, I know).
Why? Let’s see if it’s possible to explain without getting too wonky about battery tech. All modern cell phones’ lithium-ion batteries consist of, essentially, two sections: an anode (where all your power is stored) and a cathode (where all your energy goes to). In between them is a layer called the electrolyte (yes, it’s the same name as the stuff Gatorade promises to replenish, but, trust me, these are very different). The anode is filled up with electrons. The cathode wants to suck up those electrons. The electrolyte makes it harder for those electrons to pass through, thus generating power. Think of it as a hydraulic power plant at the atomic level. When you plug your phone back in to charge it up, electrons flow back from the cathode into the anode.
So, why do lithium-ion batteries eventually lose their ability to hold as much of a charge? First off, if a lithium-ion battery ever falls to absolutely zero charge, it’ll actually never charge again. So when your phone is begging you to plug it in because it has 2 percent battery life left, it’s actually lying to you — it has more power than that, but if you were to use it all for one more Snapchat, you’d render the battery useless.
But more important, the ability of electrons to transfer smoothly between the anode and cathode degrades over time. Every time your phone battery goes to work, a layer of gunk (or, if you wanna get technical, “solid electrolyte interface”) builds up on the anode. The same happens to the cathode, with its gunk being called “electrolyte oxidation.” Both of these prevent the anode and cathode from being able to store as many electrons as before. Run enough cycles of depleting and then charging your phone, and eventually your battery stops being able to store many electrons at all, and your cell-phone battery starts to die after five hours, even after a 100 percent fill-up. This is why every lithium-ion battery is rated for a certain number of cycles. Nearly every consumer device, including your phone’s battery, comes in somewhere between 300 to 500 cycles.
So! What does all this have to do with why it’s better to give your phone regular top-offs instead of letting it drain to zero and then refilling to 100 percent? It all comes down to something called “depth of discharge” (which also is a very gross term if you think about it in the wrong way). Essentially, it means how much of your battery’s power you use before you start charging it again. What does this mean in real numbers? Over at Battery University, they broke down the number of cycles before you see degradation in a lithium-ion battery at different depths of discharge. When you consistently burn through 100 percent of your battery’s power, you’ll start to see your battery lose its ability to hold a charge after 300 to 500 cycles. But! If you use up only 50 percent of your battery before plugging it back into a charge, you can go 1,200 to 1,500 cycles before your battery starts to go into decline.
What’s more, if have the inclination to really micromanage your battery charging, it’s better to charge nearly, but not all the way, to 100 percent. Being charged at 100 percent produces a small amount of heat, and lithium-ion batteries hate heat.
Granted, this is a tremendous amount of work. It’s much easier to simply charge up overnight, not think about your phone during the day, and plug it back in at the end of the day. If you want to split the difference, aim for trying to not let your phone get below 50 percent as much as possible. Charging it to 100 percent will have a much more negligible effect on how long your battery will continue to hold enough power to get you through the day.