Why your Mac’s battery ages (and why 100% isn’t always best)

Ask about battery health and the conversation goes straight to one number: the cycle count. It’s the number macOS keeps, so it’s the number people watch. It’s also the smaller half of the story. What ages a battery isn’t mainly how often you charge it. It’s the chemistry running inside it every day, whether you’re working or the lid is shut.

So here’s what’s actually happening in there, and what it means for the way you charge.

A battery is a chemical system

Your MacBook runs on a lithium-ion cell. Three parts do the work: a graphite anode, a cathode made of a lithium metal oxide, and an electrolyte that lets lithium ions travel between the two.

Charge the Mac and those ions leave the cathode for the graphite. Use it, and they travel back. Over the life of the battery that trip happens thousands of times.

It’s never perfectly reversible. Every charge and every discharge leaves behind small chemical changes that don’t undo. On any given day it amounts to nothing at all. Over months and years it adds up, and that accumulation is what battery aging really is.

Why sitting at 100% costs you

The usual shorthand is that charging to 100% is bad for your battery. That’s not quite right. Reaching 100% now and then isn’t the problem. Staying there is.

A full cell sits at its highest voltage, and voltage is what drives the unwanted side reactions. Research on lithium-ion aging keeps coming back to the same handful of mechanisms, and they all run faster the longer a cell is held near full:

• The protective SEI layer on the anode keeps thickening, and every bit of growth locks lithium away for good.
• The electrolyte slowly decomposes.
• Active lithium goes missing, so there’s less of it left to carry charge.
• Internal resistance creeps up, which means more heat for the same work.

None of this shows up on a Tuesday. Your Mac behaves exactly as it did yesterday. The capacity just quietly stops being what it was.

Empty isn’t kind either

The other extreme has a cost of its own. Near the bottom of the range, cell voltage drops very low, and deep discharges repeated often wear on the electrode materials too.

macOS won’t let you anywhere near real danger. When your Mac reads 0%, there’s a hidden reserve underneath it, and the machine shuts down long before the cell does something it can’t recover from. Even so, running from nearly empty to completely full over and over asks more of a battery than living somewhere in the middle.

Heat makes all of it faster

If there’s one thing a lithium-ion cell likes less than high voltage, it’s heat. Chemistry speeds up as temperature rises, and that goes for the reactions you want as much as the ones you don’t.

Exporting video, compiling, a game, a couple of VMs, fast charging: all of it warms the machine, and the battery is sitting right in the middle of it. Warm and full at the same time is the fastest way to age a cell there is. It’s why Apple’s own advice keeps circling back to temperature.

Why people land on 20–80%

You’ve seen the range recommended everywhere. There’s a real reason behind it, and it’s the shape of the curve rather than anything magic about those two numbers.

In the middle of the range, voltage is moderate, the side reactions are slow, and the battery is simply doing less damage to itself while it waits for you. Toward either end the curve turns up, and it turns up hardest at the top.

Look at where the curve actually flattens out and you can be a little more precise than 20 to 80. The flat part sits roughly between 60% and 80%, and that’s the band worth aiming for. Below 60 you’re giving up runtime for very little extra benefit, because the curve is already flat down there. Above 80 it starts climbing.

None of which puts 100% off limits. If you’re flying tomorrow, charge it full and enjoy the runtime. What costs you is unnecessary time at full charge, day after day, when nothing was going to use it.

Doesn’t macOS handle this already?

Partly, and what’s there is good. Optimized Battery Charging learns your routine and holds off on the last stretch until you’re about to need it. Apple Silicon Macs also got a proper 80% limit.

For plenty of people that’s enough. It runs out in two places: when your machine has no routine to learn, and when 80% isn’t the number you’d have picked. A Mac that lives on a desk wired to a display spends its whole life in the part of the curve you’d rather avoid, and the built-in limit won’t go any lower than 80%.

Where Cooked comes in

Cooked isn’t magic. It doesn’t repair a battery, and it can’t hand you back capacity that’s already gone. Nothing can.

What it does is keep your Mac out of the conditions that cost capacity in the first place. You set the limit yourself, one percent at a time, and the battery spends its days in the calm middle of that curve instead of pinned at the top. Full charge becomes something you ask for the night before a trip, not the default state of your machine.

So here’s the setting we’d actually recommend, and it’s the one we run ourselves:

If you never unplug, that second setting does nothing at all, and that’s fine. A battery sitting on the charger with charging stopped barely moves: it loses a percent or two a month, not a day. It just rests at 80% in the flat part of the curve, which is exactly where you want it.

The range earns its keep the moment your Mac comes off the desk. Without it, every reconnection is a small charge to top the last few percent back up, and those add up over a year of meetings and cafés. With 20 points underneath, you get one charge when the battery has genuinely come down, and nothing the rest of the time.

Then lift the limit to 100% the night before a flight, and put it back when you’re home. A handful of full charges a year costs you almost nothing. It’s the other 360 days that decide how your battery ends up.

Put plainly: you can’t stop a battery from aging. You can stop giving it reasons to age faster.

The bottom line

Every lithium-ion battery wears out, and that part isn’t up for negotiation. The goal was never to prevent it. It’s to stop paying for stress that bought you nothing.

Small differences, repeated every day for three or four years, are what decide whether the health figure reads 90% or 78% when you finally look. Understanding why is most of the work. The rest is one setting.

The free version of Cooked is the limit itself: anywhere from 20% to 100%, one percent at a time, with active discharge to walk an already-full battery back down, and cell temperature in plain sight. Free forever, no account. The hot-cell pause, the history and the automatic pilot are in Pro, $19 once.