New Delhi, Dec. 30 -- This question doesn't come from a headline or a warning label. It usually comes from touch. You pick up your phone while it's charging and it feels warmer than expected. Not hot enough to panic. Just warm enough to notice. The case feels warm too. That's when the thought creeps in. Am I making this worse by leaving the case on?

Most people don't act on that thought. They put the phone back down and move on. But the question sticks around because batteries are the one part of a phone you can't see ageing until it's already too late.

Phones heat up when they charge. That's normal. Anyone who tells you otherwise is selling fear. Charging moves energy into the battery and that process creates warmth. Modern phones are ...