For example if your device has a 2000mAh battery capacity then you must buy power bank with at least 2500mAh. If you buy a 4500mAh model it will charge your 2000mAh phone at least twice.

Depending upon the number of devices you carry, look for the number of ports and cables that your power bank should come with. This would let you charge more than one device at a time, saving you time.

Also check 'current draw' in your phone. Older smartphones do not draw more than 1 Ampere to charge their batteries while new handsets and tablets need a 2.1 Ampere input current. It's beneficial to a powerbank with two USB ports — 1A and 2.1A.

Always pay attention to the type of cells that are used in the power bank, Lithium-ion or Lithium-polymer. Li-ion is cheap and easily available. Li-poly cells, on the other hand, are more expensive but provide twice as much charge density per unit weight.

Always buy a reputed brand. Low-quality lithium-ion power banks will effect performance and are also a potential hazard. Cheap battery packs do not list the correct mAh capacity or the proper current draw. These batteries could also explode due to heating from over charging.

To ensure safety of your costly smartphones, choose a power bank that comes with a power-cut power in the event of a short circuit.

For better safety and performance, buy a model that comes with over voltage protection (OVP), over charge protection (OCP) and over-temperature protection (OTP) that ensure its longevity against spurts in current, and over heating during charging and discharging.

For a longer life of your power bank, look for the auto-cut feature in your bank. This would prevent it from getting overcharged.

Some popular power banks...

* Mi Power Bank (Rs 999): 10400mAh; L-ion; 2.1A (and 1A through single USB); OVP, OCP and OTP
* Adata PV110 (Rs 2,450): 10400mAh; Li-ion; 1A and 2.1A; OVP, OCP and OTP
* Portronics Charge 2 (Rs 2,900): 12000mAh; Li-ion; 1A and 2.1A; OVP, OCP and OTP
* Adata DashDrive Air AE400 (Rs 4,500): With 5000mAh battery and it can connect to existing wireless networks and create an internet bridge connection for up to 10 other devices. You also get a USB port that takes input from pen drives and hard drives, as well as a MicroSD card slot.
* JBL Charge (Rs 6,500): It's a Bluetooth speaker with 12-hour playback time and Its 6000mAh Li-ion battery can also charge your smartphones via its USB port.