iPhone Battery Health, Explained

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Open Settings on an iPhone and you'll find a number: Maximum Capacity, shown as a percentage. Most people know it should be high. Far fewer know what it actually measures, what a low number means day to day, or at what point it's worth doing something about.

It matters most when you're buying a used iPhone, where that number is one of the few hard facts you have about a device's history. This guide explains what it means and how to judge it.

What the percentage actually measures

Battery health is your battery's current maximum capacity as a percentage of what it held when it was new.

At 100%, the battery holds exactly what it did on day one. At 85%, it holds 85% of that. The phone works identically either way. What changes is how long it runs between charges.

You'll find it under Settings › Battery › Battery Health & Charging.

One thing this number does not measure is whether the battery is faulty. A battery at 82% is a normal, working battery that has been used. A battery at 100% on a three-year-old phone would be more suspicious than one at 85%.

Why batteries wear out

Lithium-ion batteries age chemically. They lose capacity through use, and they lose it through time even when idle. Two things drive it.

Charge cycles. A cycle is 100% of your battery's capacity used, not a single charge. Use 60% today and recharge, then 40% tomorrow, and that's one cycle between them. This trips people up constantly. Charging twice a day doesn't mean two cycles if you're only using half the battery each time.

Apple designs iPhone batteries to retain up to 80% of original capacity at 500 complete cycles for most models. Newer iPhones, from the 15 onward, are rated to 80% at 1,000 cycles, which is a substantial improvement.

Heat. This is the bigger factor for most people, and it's the one you can control. High temperatures accelerate chemical ageing permanently. A battery that spends its life getting hot will degrade faster than one that doesn't, regardless of cycles.

What the numbers mean in practice

95–100%. Effectively new. A phone that's been lightly used or recently had its battery replaced.

85–95%. Normal for a phone a year or two old. You may notice slightly shorter runtime than new, but most people wouldn't without measuring.

80–85%. Noticeably shorter days. Heavy users will want a charger by evening. Still a perfectly functional phone.

Below 80%. iOS displays a "Service Recommended" message. Runtime is meaningfully reduced, and you'll likely be charging daily regardless of how you use it.

What "Service Recommended" actually means

This message causes more alarm than it should. It doesn't mean the battery is broken, unsafe, or about to fail. It means the battery has dropped below 80% of original capacity, which is Apple's threshold for suggesting replacement.

The phone continues working normally. Many people run phones below 80% for years without replacing anything, and simply charge more often.

Where it matters more is if the phone has also experienced an unexpected shutdown. In that case iOS may apply performance management, which smooths out peak power demands to prevent it happening again. This can make the phone feel slower under load. You can turn it off in the same Battery Health screen, though it reappears if another unexpected shutdown occurs.

The other message: "Unable to verify this iPhone has a genuine Apple battery"

This one is different, and it's the message that matters most when buying used.

If a battery has been replaced with a non-genuine cell, or fitted without Apple's servicing process, iOS displays a notice saying it can't verify the battery is a genuine Apple part. You may also see the battery listed as "Unknown Part" under Settings › General › About, in the Parts and Service History section.

The important consequence is not the message itself. It's that battery health readings may stop working entirely. With an unverified battery, Maximum Capacity and Peak Performance Capability can show no data at all. The phone works, the battery charges, but the one objective measure you'd use to judge it is gone.

That matters when buying. A seller showing you a Battery Health screen with no percentage isn't necessarily hiding anything, but you have lost your main way of assessing the battery, and you're relying entirely on their account of what was fitted.

Worth knowing that the message doesn't always mean a cheap battery. A genuine Apple cell installed outside Apple's servicing process can trigger it too, because verification depends on the part being paired to the phone rather than on its quality. So it isn't proof of a bad battery. It is proof that you can't verify what's in there, which is a different problem but still a problem.

If you see it, ask what was fitted and when, and price the phone accordingly.

Looking after a battery in Zambia

Most battery advice online is written for temperate climates and reliable electricity. Neither describes conditions here, and two local realities are worth planning around.

Heat is the main threat. Apple's guidance is to avoid ambient temperatures above 35°C, which Lusaka comfortably exceeds for much of October. The practical rules:

  • Never leave a phone on a car dashboard or seat in direct sun. A parked car in the middle of the day is far hotter than the air outside, and this does permanent damage in a single afternoon.
  • Take the case off when charging if the phone gets warm. Charging generates heat, and a thick case traps it.
  • Avoid heavy use like navigation or gaming while charging in hot conditions, since you're generating heat from two sources at once.
  • Don't leave it in direct sun on a table.

Charging around power cuts. Load shedding pushes people into habits that are hard on batteries: topping up whenever power is available, running down to zero waiting for it to return, and charging from car sockets or power banks of uncertain quality.

Some of this is unavoidable. A few things help:

  • Partial charges are fine. Battery health is not damaged by charging to 70% and stopping. Consistently running to 0% and charging to 100% is worse than charging in the middle of the range.
  • Avoid charging in a hot car if you can. It combines both problems.
  • Use decent cables and adapters. Cheap counterfeit chargers are common and can deliver unstable power, which generates heat.
  • A good power bank is gentler on a battery than repeatedly draining it flat.

Optimised Battery Charging, on by default, learns your routine and holds the charge at 80% until shortly before you normally unplug. It reduces time spent at full charge, which is when batteries age fastest. Leave it on. If your charging routine is irregular because of power cuts it may not learn well, but it does no harm.

Newer iPhones also let you cap charging at 80% permanently under Battery Health & Charging. If you're rarely far from a charger, this genuinely slows ageing.

When is it worth replacing?

Replacement makes sense when the phone no longer lasts as long as you need it to, and the phone is otherwise fine.

Points in favour: a battery replacement is far cheaper than a new phone, it restores capacity to full, and a phone that's otherwise working can gain years from it.

Points against: on an older model already at the end of its software support, that money may be better spent toward a newer device.

If you're getting it done locally, use a technician you trust and ask what battery they're fitting. Poor-quality replacement cells are common and a bad one degrades quickly, leaving you back where you started.

Buying a used iPhone: what to check

Battery health is one of the few objective facts you can get about a used phone's history, so use it.

Ask for the actual number, and ask for a photo of the Battery Health screen rather than taking someone's word for it.

Be suspicious of 100% on an old phone. Either the battery was recently replaced, which is fine if disclosed, or the figure isn't genuine. Some third-party replacement batteries report inaccurate health readings.

Check for the genuine battery warning. If Battery Health shows no percentage at all, or you see the "Unable to verify" notice, the battery has been replaced with an unverified part. Ask what was fitted, and treat the missing reading as a gap in what you know rather than assuming the worst.

Check whether Service Recommended is showing, and factor it into the price rather than treating it as a dealbreaker.

Ask whether performance management is active, which indicates a history of unexpected shutdowns.

A seller who won't show you the battery screen is telling you something.

What about MacBooks?

MacBooks work differently. macOS reports a condition status, either Normal or Service Recommended, and a cycle count, under Battery Health in System Settings. Newer Apple Silicon models also show a Maximum Capacity percentage; older Intel machines generally don't.

Cycle count matters less than people assume. What you actually want to know is the condition status and how long the machine runs unplugged in real use. A high cycle count on a machine still reporting Normal is not a problem.

Common questions

Does charging overnight damage the battery? Not meaningfully, especially with Optimised Battery Charging on. Time spent sitting at 100% in the heat is worse than the charging itself.

Should I let it run to 0% occasionally? No. That advice comes from older nickel-based batteries and doesn't apply to lithium-ion. Deep discharges are harder on the battery than partial ones.

Does fast charging damage the battery? It generates more heat, which over years has some effect. It's not something to avoid, but don't fast charge a hot phone in a hot room if you can wait.

Why did my battery health drop suddenly? The reading is an estimate that recalibrates periodically, so it can move in steps rather than smoothly. A sudden drop of several percent is usually recalibration rather than sudden damage.

Can battery health go back up? No, other than through recalibration correcting an inaccurate reading. Capacity loss is chemical and permanent.

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