Open Settings, tap Battery, then tap Battery Health, and read the cycle count before you spend anything. On an iPhone 15 that number decides whether you need a battery or whether something else is draining it, and the two problems have completely different fixes.
Most people replace the battery because the phone dies by mid-afternoon. Roughly half the time on our bench the cell is fine and the drain is software. Checking first costs you nothing.
Read the cycle count before you spend anything
On iPhone 15 and later running iOS 17.4 or newer, the Battery Health screen shows maximum capacity, cycle count, the date the battery was manufactured and the date it was first used [1]. That is far more than earlier models exposed, and it settles the question quickly.
Two numbers matter. Maximum capacity is the charge the cell holds now against the charge it held new. Cycle count is how many full charges it has been through. Read them together: capacity alone tells you the state, cycle count tells you whether that state is reasonable for the phone's age.
What the number actually means on an iPhone 15
This is where most advice online is out of date. Batteries in iPhone 15 models and later are designed to retain 80 per cent of their original capacity at 1,000 complete charge cycles, where iPhone 14 models and earlier are rated to 500 cycles under the same ideal conditions [1].
The practical consequence: an iPhone 15 at 400 cycles is not near the end of anything. If it is already showing poor capacity at that count, the cell has aged badly or the phone has a fault, and replacing the battery without finding out why can leave you in the same position months later.
Use these thresholds as a working rule:
- Capacity above 85 per cent with a low cycle count: the battery is not your problem. Look at software.
- Capacity 80 to 85 per cent: borderline. Replacement will help, but confirm nothing else is draining it first.
- Capacity below 80 per cent: replace it. Performance throttling and sudden shutdowns follow.
- Capacity below 80 per cent at under 300 cycles: something is wrong beyond normal wear. Get it diagnosed, not just swapped.
When it is not the battery at all
Four faults imitate a failing battery, and all four are cheaper to fix.
A rogue background app is the most common. If drain started after an update or a new install, that is your suspect, not the cell. Check Settings, Battery, and read the per-app breakdown for the last ten days. An app consuming a large share while you were not using it is the answer.
A failing charging port is the second. If the phone charges slowly, charges only at certain cable angles, or stops overnight, the port may be worn or packed with pocket lint rather than the battery being dead. That is a smaller job than a battery replacement.
Poor signal is the third and the least obvious. A phone held in weak reception raises transmit power to stay connected, and that burns charge quickly. If drain is worse in one building than everywhere else, the battery is not the variable.
Finally, a botched previous repair. A third-party cell without proper calibration reports capacity inaccurately, so the figure on screen may be fiction. If someone else has been inside the phone, treat the Battery Health reading with suspicion.
If the fault is software rather than hardware, our software fixing service will find it faster than trial and error will.
What we do on the bench
We run the free diagnostic first and confirm the fault before quoting, and the price is fixed before any work starts. For a battery we check capacity, cycle count and charge behaviour under load, because a cell can read acceptable at rest and collapse the moment the phone works hard.
If the battery is the fault, we replace it with a premium OEM-quality part, then retest charge behaviour rather than handing it straight back. Before the phone returns to you it goes through a full quality check covering touch sensitivity, display brightness, colour accuracy and the surrounding functions, because a battery replacement on an iPhone 15 means separating a glued display and everything that passes behind it.
Every screen replacement carries a 90-day warranty on parts and labour. If we find the fault is not the battery, we tell you that instead of selling you one. Our battery replacement page covers the other devices we handle.
Do not do these three things first
Do not run a full discharge to "recalibrate" the cell. Lithium-ion batteries do not have the memory effect that made this useful decades ago, and deep discharges add wear.
Do not fit a cheap unbranded battery to save money. An uncalibrated cell reports capacity wrongly, which removes the one diagnostic signal you had.
Do not ignore physical warning signs while you troubleshoot software. If the display has lifted at an edge, the phone rocks when laid flat on glass, or the back has separated, stop charging it and bring it in the same day.
What actually ages the cell, and what does not
Apple frames battery lifespan as chemical age, which takes in the number of charge cycles and how the battery was cared for rather than the calendar alone [2]. Two habits do most of the damage, and both are easy to change.
Heat is the first. A phone charging inside a case on a warm windowsill, clamped to a car dashboard in traffic, or running sat-nav while charging will age faster than the cycle count predicts. If the phone is hot to hold while charging, take the case off and move it off the charger until it cools.
Sustained full charge is the second. A phone held at 100 per cent for eight hours every night spends most of that time under load it does not need. Optimised Battery Charging exists precisely to avoid this, and leaving it enabled costs you nothing.
What does not matter: closing background apps by swiping them away, which iOS reloads from cold and which costs more energy than it saves, and charging in short bursts through the day, which is fine for lithium-ion cells and is not a partial cycle in any harmful sense.
Bring it in
Check the cycle count tonight. If capacity is under 80 per cent, or the number looks wrong for the phone's age, walk into Best Tech at Unit 4, Piccadilly Circus Underground Station. We are open 10am to 10pm, seven days. The diagnostic is free and takes minutes, and most battery replacements are done the same day.
Call 020 3417 7090 if you want to check we have the part for your model before you travel.
Sources
- About the battery and performance of iPhone 11 and later — support.apple.com
- iPhone battery and performance — support.apple.com
