Battery health explained: when to replace, when to wait

What "battery health" actually means

Every rechargeable lithium-ion battery degrades over time. Each charge cycle (drain to ~20%, charge to ~80%, repeat) causes a tiny, irreversible chemical change inside the cell. After enough cycles, the battery can't hold as much charge as it could when it was new.

Battery health is a percentage of original capacity. When your iPhone says "Maximum Capacity: 87%", it means the battery can now hold 87% of the charge it held on day one. A phone that lasted 12 hours new now lasts roughly 10.5 hours under the same usage. Samsung shows this in Settings → Battery → Battery Health (or via third-party apps like AccuBattery on older models).

This number only goes down. It never goes back up. There is no software update, no calibration trick, and no "battery conditioning" technique that reverses chemical degradation. Anyone claiming otherwise is selling something.

The degradation curve — it's not linear

This is the part most people get wrong. Battery degradation isn't a straight line from 100% to 0%. It follows a curve:

Year 1 (100% → ~92%): The steepest single-year drop. New batteries lose capacity fastest in their first year because the initial chemical reactions are the most aggressive. This is normal and not a sign of a defective battery.

Year 2 (92% → ~85%): The decline slows down. Most phones are still perfectly usable at 85% — you'll notice slightly shorter battery life but it's not disruptive. This is where most 2-year-old phones sit.

Year 3 (85% → ~78%): This is the decision zone. The difference between 85% and 78% is where most people start actually feeling the degradation — charging mid-afternoon instead of making it to bedtime, or the phone dying during a long call.

Year 4+ (below 75%): Below 75%, the phone becomes actively annoying. Unexpected shutdowns at 15–20% indicated charge (the battery can't deliver the voltage the processor demands under load), aggressive throttling on iPhones (Apple's performance management kicks in), and "all-day battery" becoming "half-day battery."

The decision framework — honest and simple

Above 85%: don't replace

Your battery is fine. It's degraded — every battery is, from day one — but not enough to justify the cost or risk of a replacement. If you're worried about battery life at this level, the issue is more likely a rogue app, background refresh settings, or screen brightness than the battery itself. Check Settings → Battery → Battery Usage by App before spending money on a replacement.

80–85%: watch it, don't panic

You're in the monitoring zone. The battery is measurably weaker than new but still functional for most people. If you're a light user (calls, messaging, light browsing), you're fine for another 6–12 months. If you're a heavy user (camera, GPS, gaming, always-on display), you're probably already feeling it — and a replacement now is reasonable. At an independent shop, battery replacement runs $69–$169 AUD depending on the model (see our calculator for your specific phone).

75–80%: replace it

This is the sweet spot for replacement. The battery is degraded enough that you're genuinely losing usable hours every day, but the phone itself is still worth keeping. A $79–$129 battery replacement on a 3-year-old Samsung or a $89–$149 replacement on an iPhone buys you another 18–24 months of full-day battery life. That's dramatically better value than a $1,500 new phone.

Below 75%: replace it or replace the phone

Below 75%, the battery is failing. Unexpected shutdowns, aggressive throttling, and half-day battery life are now normal. A battery replacement still makes sense if the rest of the phone is in good shape — screens, cameras, speakers, charging port all working. If the phone has multiple issues (cracked screen AND degraded battery AND slow charging), the total repair cost may approach the price of a good refurbished replacement, and that changes the equation. See our repair-vs-replace guide for that calculation.

iPhone vs Samsung: the same chemistry, different transparency

iPhones show battery health clearly: Settings → Battery → Battery Health & Charging → Maximum Capacity. Apple also shows whether performance management (throttling) has been activated, and gives you the option to disable it (at the risk of unexpected shutdowns). iPhones display a "Service" recommendation when health drops below 80%, but this is a suggestion, not a command — the phone still works.

Samsung phones added a native battery health indicator in One UI 5 (2023): Settings → Battery → Battery Health. Older Samsung phones need a third-party app like AccuBattery or the hidden diagnostic dialer code (*#0228#). Samsung doesn't throttle as aggressively as Apple — they let the phone run at full speed and accept the risk of shutdowns instead. This means a degraded Samsung battery is more likely to give you unexpected shutdowns than slow performance.

Google Pixel phones show battery health in Settings → Battery → Battery Health. The Pixel approach is similar to Samsung — less throttling, more shutdown risk.

What actually kills batteries faster

Some usage patterns accelerate degradation meaningfully. None of these are catastrophic on their own, but they compound:

Heat is the biggest enemy. Charging in direct sunlight, using GPS navigation while charging in a hot car mount, or gaming while plugged in — all of these push the battery temperature above 35°C, which accelerates the chemical degradation. A phone that's regularly charged hot will hit 80% health 6–12 months earlier than one charged at room temperature.

Keeping the battery at 100% for long periods. Lithium-ion cells are most stressed at very high and very low charge states. Overnight charging to 100% and leaving it there for 8 hours isn't ideal. Most modern phones (iPhone 13+, Samsung S21+, Pixel 6+) have "optimised charging" that learns your schedule and holds at 80% until just before you wake up — leave this feature on.

Fast charging isn't as bad as people think. Modern fast-charging protocols (USB-PD, Samsung Adaptive Fast Charging, Apple's 20W+) manage heat and voltage carefully. The measurable difference in battery degradation between fast charging and slow charging is about 2–3% over two years — real but not worth worrying about unless you're keeping the phone for 4+ years.

Deep discharges (below 10%) are worse than fast charging. Letting the battery drain to 0% regularly puts more stress on the cell than fast charging does. If you're the type who runs your phone dead every night, that habit is costing you more battery life than the charging speed.

The real cost of waiting too long

Here's the thing people miss: a battery replacement at 78% health costs the same as a battery replacement at 65% health. But the phone you get back is dramatically different. Replace at 78% and you get a phone that feels new again — full-day battery, no throttling, no shutdowns. Replace at 65% and you get the same result, but you've spent 6–12 extra months suffering with a dying phone for no reason.

The sweet spot is 75–80%. Below that, every week you wait is a week of worse battery life that you'll never get back. The replacement cost doesn't go down by waiting — but your quality of life does.

What a battery replacement actually costs

At an independent repair shop in Australia (2026 pricing):

iPhones: $89–$149 AUD depending on model. iPhone SE and iPhone 12 sit at the low end; iPhone 15 Pro Max at the high end. Apple's authorised service runs $129–$179 (battery is one of the few repairs where authorised pricing is close to independent). For specific pricing, see our model pages: iPhone 15 Pro Max, 14 Pro Max, iPhone 13, iPhone 12, iPhone SE.

Samsung Galaxy S-series: $69–$159 AUD. The standard S23/S25 sit at the low end (~$69–$109); the S25 Ultra and S26 Ultra at the high end (~$99–$159). Samsung authorised service runs $109–$219. See: S25 Ultra, S25, S23, S26 Ultra.

Independent shop turnaround: 20–45 minutes if the part is in stock. It's one of the fastest, cheapest repairs available — and one of the highest-value in terms of how much usable life it adds to your phone.

The bottom line

Battery health is a number, not a crisis. Above 85%, you're fine. Between 80–85%, start paying attention. Below 80%, a $69–$149 battery replacement is one of the best investments you can make in a phone you're otherwise happy with. Below 75%, stop waiting — you're suffering for no reason.

Use the cost calculator to see what a battery replacement costs for your specific model, or browse our brand pages for detailed pricing.

Battery health explained: when to replace, when to wait.