A power bank, also known as a portable charger or external battery pack, is a portable device that stores electrical energy and can be used to recharge electronic devices such as smartphones, tablets, laptops, and other gadgets when they run out of battery power. Power banks typically contain lithium-ion batteries and come in various capacities, sizes, and shapes to suit different needs. They are convenient for charging devices on the go, especially when access to electrical outlets is limited.

The capacity of your power bank depends on the devices you need to charge and their battery sizes. If you're charging just a smartphone, a power bank with 5000mAh to 10000mAh should suffice. However, if you're charging multiple devices or larger ones like tablets or laptops, you might need a power bank with 20000mAh or more.

To choose a good quality power bank, consider the following factors:

1. Capacity: Select a power bank with sufficient capacity to charge your devices multiple times. Generally, higher capacity means more charging cycles.  Common power bank capacities include 5000mAh, 10000mAh, and 20000mAh.

2. Output Power: Ensure the power bank has adequate output power to support fast charging of your devices. Higher output power typically results in faster charging speeds.

3. Charging Ports: Choose a power bank with suitable charging ports for your devices, such as USB-A, USB-C, etc.

4. Safety Features: Opt for a power bank with safety features like overcharge protection, over-discharge protection, and short circuit protection to ensure safe charging of your devices.