By the time a battery visibly swells and lifts the back panel, it's been degrading for a while. The earlier signs are quieter: the phone gets noticeably warm during normal use, the battery percentage jumps suddenly instead of draining smoothly, or it shuts down at 15–20% instead of running to zero.
Battery health readouts (in Settings on most phones) are a rough guide, but they don't catch everything — a cell can be physically swelling under a reading that still says 85%. That's why we test capacity and internal resistance directly rather than trusting the on-screen number alone.
If you see the back panel or screen starting to lift at the edges, stop charging it and bring it in the same day. A swollen cell is a fire risk, not just a performance one.
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