Smartphone batteries are consumables — they degrade with every charge cycle. Most lithium-ion batteries retain around 80% of their original capacity after 500 charge cycles, but real-world usage means most phones start showing battery problems after 2–3 years. The good news? A battery replacement is one of the cheapest and most impactful repairs you can do — it can make your phone feel brand new.
Here are the seven signs that your battery has had it and needs replacing.
The 7 Warning Signs
Your phone doesn't last the day
This is the most obvious sign. If your phone used to comfortably get through a full day and now you're scrambling for a charger by early afternoon — despite similar usage — your battery capacity has dropped significantly. A healthy battery holds 100% of its design capacity; a degraded one might only hold 70% or less, cutting your screen-on time by hours.
Your phone shuts down unexpectedly
If your phone powers off when it still shows 20%, 30%, or even 50% charge, your battery can no longer supply enough voltage under load. This is a classic sign of a worn-out battery that can't sustain power spikes when you're actively using demanding apps or making calls. It'll also randomly restart or throttle itself.
Your phone is noticeably slower than it used to be
Apple's battery management throttles CPU performance on iPhones with degraded batteries to prevent unexpected shutdowns. Samsung and other Android phones have similar behaviour. If your phone feels sluggish — apps taking longer to open, animations stuttering — and it's a few years old, the battery is often the culprit. On iPhone, you can check this under Settings → Battery → Battery Health.
The battery percentage jumps around erratically
A degraded battery gives inaccurate readings to the phone's battery management system. You might be at 50%, then suddenly drop to 10% without explanation — or jump back up to 30% when you plug in for a few seconds. This erratic behaviour is a clear sign the battery cells are failing and can no longer report accurate state-of-charge data.
Your phone gets unusually hot
All phones generate some heat during charging and intensive use, but if your phone regularly gets hot — especially warm to the touch during normal tasks like browsing or texting — the battery could be the cause. A failing battery works inefficiently, generating excess heat as it struggles to manage charge and discharge cycles properly.
Your phone takes ages to charge
If charging times have increased significantly — particularly if you're using the same charger you always have — the battery's internal resistance may have risen as it's aged. A related sign is the battery charging quickly but draining just as fast; this often indicates the battery is swollen or the cells are unevenly degraded.
The back of your phone is bulging or the screen is lifting
This is a serious warning sign. Lithium-ion batteries can swell (known as "battery bloat") as they degrade, causing the phone's back to bow outward or the screen to lift from the frame. A swollen battery is a safety risk — it can rupture, leak, or in rare cases catch fire. If you notice any bulging, stop charging the phone and bring it in immediately.
How to Check Your Battery Health
On iPhone
Go to Settings → Battery → Battery Health & Charging. Apple recommends replacing the battery when capacity falls below 80%. If you see a message about "Peak Performance Capability" being affected, your battery is already throttling performance.
On Samsung
Samsung includes a battery diagnostic in its built-in device health checker. Open the Phone app and dial *#0228# to see raw battery stats including voltage and temperature. For a more detailed health check, Samsung Members app (pre-installed on most Galaxy devices) includes a diagnostic tool.
On Other Android Phones
Most Android phones don't have a built-in battery health readout. Third-party apps like AccuBattery can estimate capacity degradation based on charge cycle monitoring over time. Alternatively, your repair technician can test the battery with specialist diagnostic equipment.
🔋 At Mobex, we offer free battery diagnostics. Pop in to our North Finchley shop at 824 High Road and we'll test your battery and tell you honestly whether it needs replacing — no charge for the check.
Is a Battery Replacement Worth It?
Almost always, yes. A battery replacement typically costs a fraction of a new phone and can extend the life of your device by 2–3 years. If your phone is otherwise working well — the screen is fine, storage is sufficient, the camera is good enough — replacing the battery is far better value than upgrading.
A battery replacement at Mobex takes around 30–60 minutes for most iPhone and Samsung models. We use quality replacement cells and back every battery repair with a 1-year warranty.
Battery Replacement in North Finchley
Most battery swaps take under an hour. Competitive pricing, quality cells, 1-year warranty — and we're open 6 days a week.