A power bank is a device consist of rechargeable batteries enclosed in a sturdy case with an electrical circuit. Power bank provides a source of power for recharging your electronic devices when in need. It is most useful in the situation when you do not have a phone charging facility nearby.
Top 5 Best 20000mAh Power Bank
1. Ambrane PP-20 20000mAh 
Product Dimensions : 16cm x 7.2cm x 2.5cm, Weight- 386 g
Battery: 20000mAh polymer battery.
High efficiency and charging conversion rates.
It is built with long-life batteries that are reliable and safer than non-branded Lithium-Ion Batteries.
Dual USB Output ports allow you to charge two devices simultaneously. The max. current output is 2.1A
Dual Input of Micro USB and Type C ports. You can charge the power bank with any of the micro or type c cables. Type c mobile users need not carry two separate cables for charging a power bank or your device. You can charge both the devices with the same type C cable.
Compatible with smartphones, tablets, Bluetooth speakers, and other USB devices
Rubberized Scratch Less Finish. The outer body of this power bank is coated with a special rubber material that is resistant to scratches and marks
It has 9 Layers of Protection. LED Charging Indicator
Charging time – 12 to 15 hours
Warranty 6 months
2. Coolnut 20000 mAh Power Bank 
Product Dimensions:16.5cm x 8cm x 2.2cm, Weight- 440 g
Battery: 20000 mAh Lithium-Ion.
Output – 2 USB Port. Maximum of 5V/3A,9V/2A,12V/1.5A output.
Advanced 2A fast recharging input technology.
Compatible with smartphones, tablets, blue tooth speakers, blue tooth headsets, etc. Apple and Android devices can also be charged.
It is built with an anti-slip silicon shell case with great elegance providing the feel of the perfect grip.
Intelligent IC protection and 7 layers multilevel safe charging-discharging mechanism.
Protected from overcharge, over-discharge, over-voltage, over current, overheating, and has an auto power-off feature.
Retains up to 70-80% of its capacity even after 500 charge cycles.
The digital LED display shows charge status. The LED flashlight is provided for light in emergencies.
One Year warranty
3. Intex IT-PBA 20K Poly 20000mAh 
Product Dimensions: 16cm x 7.5cm x 2.2cm, Weight- 395 g
20,000 mAh built-in Li-Polymer Battery which is durable, provides better charge retention and higher performance. It is Lightweight and slimmer than a standard Li-ion battery.
Provided with 2 USB Output ports for charging two devices. Output- 5V/1A and 5V/2.1A.
Fast Charging input of the power bank to get charge faster. Input: 5V/2A
Compatible with smartphones, Tabs, Digital Cameras, etc.
Elegant look and lively color combination.
High-quality internal circuits for higher safety of power bank.
Life cycle greater than 500 times. LED Indicator.
1-year warranty provided by the manufacturer
4. Mi 20000mAh Li-Polymer Power Bank 2i 
Product Dimensions: 15.1cm x 7.2cm x 2.6cm, Weight- 431 g
20000mAh Li-Polymer Battery which is a long life with optimized charging efficiency. It is capable to charge Redmi K20 3 times, Redmi Note 7 Pro 3 times, and iPhone 8 for 7.2 times.
Dual USB Output that supports charging outputs of 5V/2A, 9V/ 2A, and 12V/1.5A. This ensures the efficient and quick charging of devices.
Input Charging time of Approx 6.7 hours (18W charger, standard USB cable), Approx 10 hours ( 10W charger, standard USB cable
Compatible with smartphones, tablets, BT speakers, headsets, earphones, fitness bands, etc.
This Power bank uses PC + ABS material that provides a superior and comfortable hand feel.
It has a classic and attractive look.
Mi Power Bank features an advanced 9 layers of chipset protection to ensure protection against short-circuit, over-voltage, over-current, over-charge & discharge, etc
The new Mi Power Bank 2i features 18W Fast Charging.
6 months of domestic warranty
5. Portronics POR 695
Product Dimensions: 8.2cm x 16.6cm x 2.2cm, Weight- 417 g.
20000mAh Lithium Polymer battery
2 USB output ports, 5V/1.0A and 5V/2.1A. You can charge two devices simultaneously.
Input Micro USB port, Dc 5.0v/2.1A
Compatible with mobile phones, tablets, phablets, digital cameras, mp3 players.
The body is made of high-quality durable plastic. It is compact and can comfortably fit into your pockets.
8+ Inbuilt protection features. It gives protection from overcharge, voltage fluctuations, short circuits.
Charging Time 5 Hours.
LED Indicators to indicate charge status.
Warranty of 1-year.