Used iPhone Battery Health: What to Check Before You Buy
Updated
What Maximum Capacity actually measures
Every iPhone battery wears down with use. Maximum Capacity is Apple's own measure of that wear: the full charge the battery can hold today, compared to when it was new. A phone at 87 % holds 87 % of the charge it held on day one — it runs exactly the same, just fewer hours between charges.
The number says nothing about the rest of the phone. Screen, cameras, Face ID and the charging port are separate checks — that is what a function test and a condition grade cover.
How to check it in 30 seconds
On the phone itself, open Settings → Battery → Battery Health & Charging and read the Maximum Capacity percentage. No apps needed.
- On iPhone 15 and newer, Settings → General → About also shows the battery's cycle count — how many full charges it has been through.
- If the screen shows a service note saying the battery's health is significantly degraded, the battery is due for replacement.
- An « Unknown Part » message means the battery was replaced outside Apple's process — ask who replaced it, and with what part.
What's a good number for a used iPhone?
Batteries are consumable, so a used phone will almost never read 100 % — and that's fine. As a buyer's rule of thumb:
- 90 % and up — excellent; barely worn.
- 85–89 % — good for everyday use; most well-kept used iPhones land here.
- 80–84 % — usable, but budget for a battery replacement at some point.
- Below 80 % — the battery is due. Apple itself treats 80 % as the service threshold, and the price should reflect it.
Apple designs iPhone batteries to keep about 80 % of their capacity after 500 complete charge cycles — 1,000 cycles on iPhone 15 and newer — which is why the cycle count and the capacity number tell the same story two ways.
When the number looks too good
A reading at or near 100 % on an older iPhone usually means the battery was replaced. That can be good news — if it was a quality part. Check Battery Health for an « Unknown Part » note: it means the battery is not verified as genuine, so ask about the part before you pay.
And the number only moves one way. Battery health never recovers on its own — it declines slowly, cycle after cycle. A phone bought at 88 % today won't stay at 88 % forever; that's normal wear, not a defect.
How it works when you buy from SubTech
Every used phone we sell is function-tested, given a condition grade of A+, A, B or C, and its battery health is shown before you buy. Nothing to take on faith: you can buy in person at 757 rue Beaubien Est in Montréal — open Settings on the phone at the counter and read the number yourself before you pay.
Every phone is also carrier-unlocked and covered by a 30-day warranty, so a battery surprise after purchase is our problem, not yours.
Frequently asked questions
›Is 85 % battery health good for a used iPhone?
Yes. At 85 % the phone runs exactly the same as new, with somewhat less time between charges. Anything from 85 % up is a good buy at the right price; from 90 % up it's excellent.
›What battery health is too low on a used iPhone?
Below 80 %. That's the point Apple itself treats as worn enough to service, and where iOS can show a degraded-battery notice. It doesn't make the phone a bad deal — it makes a battery replacement part of the real price.
›Can battery health go back up?
No. Maximum Capacity only goes down as the battery ages through charge cycles. The only thing that resets it is a battery replacement — which is why an older phone reading near 100 % usually has a new battery in it.
›How do I check the cycle count?
On iPhone 15 and newer, open Settings → General → About and look for the Battery section. One cycle equals one full battery's worth of charge, even spread across several partial charges.
›Does SubTech show battery health before I buy?
Yes — every phone is function-tested, graded A+ to C, and its battery health is shown before you buy. Buying in person at our Montréal counter? Open Settings on the phone and read it yourself before paying.