Power banks and overnight phone charging misconceptions explained

Power banks are marketed by their milliampere‑hour (mAh) rating, but that figure reflects the internal cell voltage (around 3.7 V) rather than the output voltage used to charge phones. Converting mAh to watt‑hours (Wh) provides a realistic energy estimate; a 10 000 mAh pack (3.7 V) holds about 37 Wh and typically delivers only around one and a half full charges to a 5 000 mAh smartphone because of voltage conversion losses and efficiency factors.

Modern smartphones use lithium‑ion batteries with integrated charge‑management systems that stop charging at 100 % and switch to direct power draw, so plugging a device in overnight does not cause immediate damage. However, keeping a battery at full charge for extended periods creates small “topping‑off” cycles that generate heat, which accelerates chemical aging. Elevated temperatures (≈40 °C) can cut capacity by up to 35 % in a year, and higher heat increases fire risk. Experts therefore recommend maintaining charge levels between 20 % and 80 % for longevity and charging on hard, ventilated surfaces to avoid overheating.