A power bank may drain fast because rated mAh is measured at the internal battery voltage, not phone output voltage. Fast charging, large phones, tablets, heat, old cells, poor cables, standby drain, and low-quality battery cells can also reduce usable runtime.

Your power bank may charge slowly because the wall charger is weak, the cable is not rated for enough current, the power bank has low input wattage, the battery is large, the USB-C PD profile is not negotiated, or temperature protection is limiting charging speed.

You can leave a good-quality power bank charging overnight occasionally if it has proper protection circuits and uses the correct adapter. It is still better to charge it on a hard surface, avoid heat, and unplug it after full charge when practical.

You need a 240W USB-C cable only if your charger and device can use USB Power Delivery Extended Power Range. For most phones, earbuds, and basic tablets, a 60W or 100W cable is usually enough. For high-power laptops, docks, and premium bundles, 240W can be worth it.

Yes, a power bank can catch fire if its lithium-ion battery enters thermal runaway, is physically damaged, overheats, is poorly manufactured, or is charged with unsafe electronics. Stop using any power bank that swells, leaks, smells strange, becomes unusually hot, or makes popping sounds.