Power Bank

A power bank is a lithium cell pack with a charging circuit on one side and a boost converter on the other. The cells sit near 3.6 or 3.7 volts. The printed mAh figure only becomes useful when converted: multiply by 3.7 and divide by 1,000 for watt hours. That makes a 10,000 mAh bank about 37 Wh and a 20,000 mAh bank about 74 Wh. That conversion is also what airport security reads, so a bank without a watt hour figure on the case invites an argument at the gate.

Output wattage matters as much as capacity, and gets far less attention. A 5 W bank refills a phone in four hours, a 20 W USB Power Delivery bank in ninety minutes, and a 100 W bank will run a laptop while you work. Input speed decides how long the bank itself takes: 74 watt hours through an 18 W port needs about five hours. A 65 W input port cuts that under two. Look for pass-through support if you want to charge the bank and the phone from one outlet overnight.

Sizes map to trips. Around 5,000 mAh is a pocket top-up worth roughly one phone charge; 10,000 mAh covers a long day and stays near 200 g; 20,000 mAh handles a weekend or two people at 350 to 450 g; anything above 27,000 mAh crosses the 100 watt hour airline threshold. A Magnetic Wireless Power Bank trades roughly a third of that delivered energy for losing the cable, and a Car Charger removes the need for a bank on any journey where the engine is running.

Weight is the best test of a capacity claim, because lithium cells have a fixed energy density and no amount of engineering hides it. A 20,000 mAh pack containing real cells cannot weigh 200 g. Beyond that, check for a safety listing number, a display that shows percentage rather than four vague dots, and whether the bank powers down when a low-draw device like a hearing aid or earbud case is attached.

Keep it near 50 to 60 percent in storage and cycle it every three months, since a pack left flat can fall below its restart voltage permanently. Expect 300 to 500 full cycles before you notice the shortfall. Never leave one charging on a bed or in a hot car, and take any swollen pack to a battery recycling point the same day.