Phone battery life gets shorter over time because lithium-ion batteries gradually lose capacity with every charge cycle, heat exposure and month of age. Battery health is the measure of how much charge your battery can hold now compared with when it was new. On many phones you can check it in settings; Apple, for example, designs iPhone batteries to retain a stated percentage of their capacity when new after a set number of cycles. When health drops to around 80% or below, or your phone shuts down unexpectedly, slows under load or the battery swells, replacement is usually worth considering. For most phones that are otherwise in good condition, a battery replacement costs far less than a new phone and restores day-long use. This guide explains how battery health works, how to check it, what damages batteries, the warning signs, and how to decide between repair and replacement.
What is battery health?
Every lithium-ion battery has a design capacity, measured in milliamp-hours (mAh). As the battery ages, chemical changes inside the cell reduce how much charge it can store. Battery health, often shown as a percentage, compares the current maximum capacity with the capacity when new. A battery at 85% health holds roughly 85% of the charge it held when new, so a full charge lasts noticeably less time.
Battery health is different from your charge level. Your charge level shows how full the battery is right now, from 0 to 100%. Battery health shows how big the "tank" has become. A phone at 100% charge with 75% health has less usable energy than the same phone did when new.
Charge cycles
A charge cycle is using the equivalent of 100% of the battery's capacity, not necessarily in one go. Using 50% one day, recharging, and using another 50% the next day counts as one cycle. Apple explains on its iPhone battery and performance page that its batteries are designed to retain a stated share of their capacity when new after a set number of complete cycles, with newer models rated for more cycles than older ones. Other manufacturers publish their own figures.
How to check phone battery health
iPhone
Go to Settings > Battery > Battery Health (on some versions, Battery Health & Charging). You will see Maximum Capacity as a percentage, and a message if the battery is significantly degraded or iOS cannot verify it. On newer models, the same screen may show cycle count and manufacture date. Apple's support page explains what each message means, including peak performance capability and when service is recommended.
Samsung Galaxy
On many Galaxy phones, you can check battery status through the Samsung Members app, under the diagnostics or phone care section, where a battery test reports the battery's condition. Some newer software versions also show battery information in Settings under Battery or Device care. Menu names vary by model and One UI version.
Google Pixel and other Android phones
Recent Pixel models include a battery health section in Settings > Battery, while older ones do not. Other Android brands vary: some show health in settings, some in a diagnostics app, and some not at all. If your phone does not show battery health, the warning signs below are the best guide.
Warning signs your battery needs replacing
- Battery health at or below about 80%, or a service message in settings.
- Unexpected shutdowns, especially in cold weather or at 20 to 30% charge.
- Slower performance under load, such as when gaming or using the camera. Some phones manage performance to prevent shutdowns on worn batteries.
- Rapid drops in charge, for example from 40% to 10% in minutes.
- The phone only works when plugged in.
- The phone gets unusually hot during normal use or charging.
- A swollen battery: the screen lifting, the back panel bulging, or the phone rocking on a flat surface. This needs immediate attention.
Battery swelling: what to do
A swollen battery is a safety concern. Lithium-ion cells can swell when gas builds up inside, which can happen with age, physical damage, overheating or faulty charging equipment. If you notice swelling:
- Stop using and charging the phone.
- Do not press on, puncture or try to flatten the battery.
- Keep the phone away from flammable materials and somewhere cool.
- Arrange a battery replacement promptly, or take it to a repairer.
- Dispose of the old battery at a proper battery recycling point, never in household waste.
London Fire Brigade publishes practical lithium battery safety advice, including charging and disposal tips that apply to phones.
What shortens phone battery life?
Battery ageing is unavoidable, but some habits speed it up. Independent battery guidance, such as Battery University's article on prolonging lithium batteries, consistently points to heat and time spent at very high or very low charge as key factors.
Heat
Heat is the biggest enemy of lithium-ion batteries. Leaving your phone in a hot car, on a sunny windowsill or under a pillow while charging accelerates wear. Gaming or navigating while charging also generates heat.
Living at 100% or 0%
Keeping a battery at full charge for long periods, or regularly running it completely flat, adds stress. Many phones now offer optimised or adaptive charging features that hold the charge around 80% overnight and finish charging just before you usually unplug. Turning these on helps.
Poor-quality chargers
Uncertified or damaged chargers and cables may not regulate power properly. Use reputable chargers that meet safety standards.
Wireless charging
Wireless charging can generate more heat than wired charging, particularly with poorly aligned phones or thick cases. It is not harmful in itself, but heat matters. We look at this in detail in our article on whether wireless charging is bad for your battery.
Simple habits that extend battery life
- Turn on optimised, adaptive or protective charging in settings.
- Avoid heat: do not charge under pillows or in direct sun.
- Remove thick cases if the phone gets hot while charging.
- Top up between roughly 20% and 80% when convenient, rather than draining fully.
- Store a spare phone at around half charge, not full or empty.
- Keep software updated, as updates often include battery management improvements.
- Check which apps use the most battery in settings and restrict background activity for heavy users.
When is battery replacement worth it?
Replacing a battery is worth it when the rest of the phone is in good condition and still receives software updates. A new battery restores day-long use and can return performance to normal if the phone had been throttling to protect a worn battery.
Replace the battery if:
- The phone is otherwise working well, with no major screen or board faults.
- It still receives security updates from the manufacturer.
- You are happy with its speed, camera and storage.
- The cost of a battery is a small fraction of the price of a comparable phone.
Consider a new or refurbished phone if:
- The phone no longer receives security updates.
- It has several faults, such as a cracked screen, failing charging port and worn battery together.
- It is too slow for the apps you need even with a good battery.
- You need features it cannot offer, such as more storage or 5G.
Our guide to battery replacement vs a new phone walks through the cost comparison in more detail.
DIY or professional battery replacement?
Battery replacement ranges from relatively simple on some older phones to difficult on modern sealed designs, where batteries are glued in and the back glass or screen must be removed. If you are comfortable with careful repair work, have the right tools and follow a good guide, DIY replacement is possible. You will need precision screwdrivers, opening picks, a suction handle, gentle heat to soften adhesive, and new adhesive strips. Disconnect the battery connector as early as possible and never puncture or bend the old battery.
If your phone is water resistant, a DIY repair may reduce that resistance unless new seals are fitted correctly. If the battery is swollen, extra care is needed; if in doubt, use a professional repairer.
You can find tools and adhesives in our repair tools and adhesive collection, and parts for popular models in our iPhone parts, Samsung parts and Google Pixel parts collections.
Choosing a replacement battery
Replacement batteries vary in quality. Look for batteries described as OEM standard, meaning they are made to the manufacturer's specification for capacity and connector fit, from a supplier that states capacity honestly and offers a warranty. Be wary of batteries advertising capacities much higher than the phone's factory specification, as these claims are often unrealistic. On iPhones, a non-Apple battery may show a message in Battery Health settings that the battery cannot be verified; the battery can still work normally, but the health percentage may not display. Apple explains these messages on its support pages.
A worked example: is replacement worth it?
Here is a hypothetical example to show how to weigh the decision. Imagine a three-year-old phone that still receives security updates, has an undamaged screen and enough storage, but now lasts only until mid-afternoon and shows battery health in the high 70s. The owner is happy with its camera and speed. In this situation, a battery replacement is almost always the sensible choice: it restores full-day use for a small fraction of the cost of a comparable new or refurbished phone, and it avoids moving data and accessories to a new device.
Now imagine a six-year-old phone that has stopped receiving security updates, has a cracked screen and a loose charging port as well as a tired battery. Fixing all three would cost a significant amount, and the phone would still be unsupported. Here, putting the money towards a newer phone is usually better value. Most real cases sit between these two, so list every fault, check update status and compare the total repair cost with replacement.
Disposing of old batteries safely
Never put lithium-ion batteries in household bins, where they can be crushed and start fires in bin lorries and recycling centres. Take them to a battery recycling point; many supermarkets and household waste recycling centres have collection bins. Retailers and repairers selling batteries in the UK have obligations under the battery regulations described on GOV.UK's batteries and waste batteries guidance. Tape over the terminals of loose batteries before recycling.
Frequently asked questions
How do I check my phone battery health?
On iPhone, go to Settings > Battery > Battery Health. On Samsung, use the Samsung Members app diagnostics or the battery section in Settings, depending on your model. Recent Pixels show battery health in Settings > Battery.
Can your phone battery go down without using it?
Yes. Batteries slowly self-discharge, and phones use power in standby for background tasks, notifications and network connections. A healthy phone should only lose a small amount overnight; rapid standby drain can indicate a worn battery or an app running in the background.
Can your phone battery explode?
It is rare, but lithium-ion batteries can overheat and catch fire if damaged, punctured, swollen, or charged with faulty equipment. Stop using any phone with a swollen or very hot battery and have it checked.
Can your phone battery go over 100%?
No. The charge percentage cannot exceed 100%. A new replacement battery can show 100% health, and some calibration quirks may briefly show unusual readings, but capacity does not exceed its design.
Can your phone battery expand?
Yes. Ageing, heat, physical damage and faulty charging can cause gas to build up and the battery to swell. A swollen battery can push the screen or back panel outwards and should be replaced promptly.
At what battery health should I replace my battery?
Around 80% is a common point to consider replacement, especially if you also notice shutdowns or short battery life. If the phone still meets your needs, replace sooner rather than later for a better daily experience.
The bottom line
Battery health falls with age, cycles and heat. Check it in settings, watch for shutdowns, rapid drain and swelling, and treat swelling as urgent. If the phone is otherwise healthy and still updated, a battery replacement is usually the best-value fix. Browse the parts collections for your phone, pick up the tools you need from our repair tools range, or use a professional repairer if you would rather not open the phone yourself.
