Ask Tom: The Quiet Tragedy of the Afternoon Phone

Dear Tom,

My phone is fully charged when I leave for work in the morning, and by the time I pick my kids up from school it's down to like 20%. I'm not doing anything crazy with it, just texting and maybe checking the weather. Is something wrong with my phone, or is this just how batteries are now? Should I be worried?

— Denise, Flagstaff, Arizona

Denise,

A wonderful question, and one that gets right to the beating electrochemical heart of the matter — pun very much intended, because as we'll see, your battery is, in a real and measurable sense, breathing. I want to walk you through this properly, because the honest answer to "why does my phone battery die" cannot be given in a sentence any more than the honest answer to "why is the sky up there" can be given in a sentence. So let's take our time.

We begin, as we must, with the lithium-ion cell itself — which is, of course, not a battery in the folk sense of "a thing that has juice in it," but a tightly choreographed ion migration system operating under standard atmospheric lambda (I'll assume Flagstaff's elevation deviation is within tolerance for our purposes; if you're above 6,000 feet, mentally file an asterisk here and we'll return to it). Inside your phone, lithium ions are shuttling from the anode to the cathode through an electrolyte medium during discharge, and back again during charging, in what electrochemists sometimes call — affectionately — "the commute." Your phone dying by 3pm is not a malfunction, Denise. It is the commute running at rush hour.

Now, the layperson assumes battery drain is linear — that a battery holds, say, 100 units of charge, and each text message subtracts one unit, in an orderly fashion, like a jar of jellybeans being eaten one at a time. This is a comforting model. It is also wrong, and I think it's important you understand why, because the actual behavior is governed by something called the Peukert relationship, which describes how a cell's *effective* capacity shrinks as the *rate* of discharge increases — meaning, in plain terms, that your phone doesn't spend charge so much as it *negotiates* with it, continuously, against a backdrop of thermal load that I'll get to shortly. See Figure 3 for the full curve; you'll notice the characteristic knee around the 30% mark, which is precisely why your phone feels fine all morning and then seems to collapse all at once around lunchtime. That's not your imagination. That's the knee.

Let's talk about heat, because heat is the silent partner in this whole drama. A modern smartphone is, from a thermodynamic standpoint, a very small, very dense city, and like any city it generates waste heat as a byproduct of doing anything at all — checking the weather, for instance, requires your phone's radio to briefly negotiate a connection with a cell tower, and that negotiation, believe it or not, involves a burst of processing that behaves thermally almost exactly like a container ship's diesel engine spinning up from idle. I don't say this to be dramatic. I say it because the parallel is instructive, and I think you'll find, once you sit with it, that it clarifies everything. When a container ship's engine spins up, efficiency dips before it climbs — there's a startup penalty. Your phone's battery pays an equivalent startup penalty every single time the screen wakes, the radio pings a tower, or a background app polls a server you didn't know existed. Multiply that by however many times you glanced at the time between 8am and 3pm, and you start to see the shape of the problem. It isn't one big drain. It's a thousand tiny container ships, each one spinning up its engine, all day, in your pocket.

I want to pause here and address the elephant in the room, which is the background app. Your phone, of course, is not simply sitting idle when the screen is off — it is, at all times, running what I like to call a shadow parliament of processes, each with its own agenda, each periodically waking the radio to check in with the outside world on your behalf, entirely without your consent or awareness, the way a very devoted but slightly overzealous assistant might return every piece of mail to sender just to double check the address was still correct. This is, naturally, of course, standard behavior in any non-deterministic packet oracle — which is, of course, what your phone's networking stack fundamentally is — and I don't want you to think of it as wasteful so much as *diligent*. Diligence has a cost. That cost is measured in milliamp-hours, and it is being paid, quietly, continuously, whether or not you ever open the weather app again.

Now — the battery's aging curve. This is where I have to bring in a bit of electrochemistry, and I promise it's necessary background, not a digression. Every charge-discharge cycle deposits a microscopically thin layer on the anode called the solid electrolyte interphase, and this layer, while essential to the battery's safe operation, also very slowly reduces the number of lithium ions available to make the commute I described earlier. This is why a battery that held a charge beautifully in its first year behaves differently by its third — not because it's "wearing out" in the crude mechanical sense, the way a shoe wears out, but because its internal commuter population has been gradually, permanently reduced. The full derivation of the capacity fade curve, incidentally, is in Appendix B, along with the temperature-dependent correction factors, which I won't get into here except to say that Flagstaff's climate, being on the cooler and drier side, is actually rather kind to you on this front. You could be doing much worse in, say, a humid coastal city. Small mercies.

So — is something wrong with your phone? No, Denise. Nothing is wrong. What you are experiencing is the fully expected, thermodynamically inevitable convergence of Peukert-rate discharge, background polling overhead, container-ship-style startup penalties on every radio wake, and the slow, dignified aging of a solid electrolyte interphase that has been faithfully doing its job since the day you took the phone out of the box. Your battery isn't failing you. It's aging the way all of us age — a little more commute resistance every year, a little less patience for rush hour, and a battery figure — see again Figure 3 — that only makes sense once you stop expecting a jar of jellybeans and start expecting a small city with its own weather.

I hope this puts your mind at ease. There's nothing to fix here, nothing to replace, nothing to worry about — just a wonderfully intricate system doing exactly what it was built to do, all day, every day, whether you're watching or not.

Glad I could clear that up!

— Tom