Battery health, explained
What the percentage in your settings actually means, when it matters, and when to replace.
Battery health is one of the most useful diagnostic numbers on a modern phone, and one of the most misunderstood. Here's what it tells you and what to do about it.
What the number means
Every lithium-ion battery has a maximum capacity it can hold. New, that capacity matches what the manufacturer specified. As the battery ages, that capacity slowly drops. Battery health is a percentage of original capacity:
- 100 percent means the battery still holds as much charge as the day you bought it.
- 80 percent means it now holds 80 percent of original capacity.
On iPhone, you find this in Settings > Battery > Battery Health. On most Android devices, you'll need a third-party app like AccuBattery, or a service centre check.
When to worry
Apple sets 80 percent as the threshold for a "service" recommendation. That's a sensible benchmark, but it isn't a hard cliff. Some patterns matter more:
- Sudden shutdowns at 30 to 50 percent battery, often in cold weather, are a stronger signal than the percentage alone.
- Performance throttling messages indicate the battery can no longer deliver peak current.
- Visible swelling anywhere on the device is a safety issue. Stop using and seek replacement immediately. This is non-negotiable.
If you have any of those, replace the battery regardless of what the percentage says.
What replacement actually costs
In London, an iPhone battery replacement at an independent shop runs roughly £35 to £80 depending on model and parts source. Authorised service through Apple is closer to £70 to £150. The difference is mostly genuine versus aftermarket cells.
For most users on devices three years old or newer, going authorised is worth it. The cell quality and warranty matter. For older devices, an independent shop is usually fine if the shop has a track record.
Battery longevity tips that are actually true
A lot of advice online is folklore. The actual evidence-backed practices:
- Avoid extreme heat. Sitting in a hot car is the single biggest accelerator of battery degradation.
- Avoid charging from 0 to 100 daily. Keeping the battery between 20 and 80 percent extends total cycles meaningfully. iOS Optimised Charging does this automatically once it learns your routine.
- Slow charging is fine for longevity. Fast charging adds heat, which is the real culprit. Modern devices manage this well, so don't overthink it.
- Update your software. Battery management algorithms genuinely improve over time.
What doesn't matter much: cable brand, charging frequency, leaving the device plugged in overnight (with optimised charging on).
When in doubt
Run a diagnosis on FixIt Quick. If the answers point to a battery issue, you'll get an estimate and a fixer near you in under two minutes.