You have to consider some salient factors such as:
1. Power Output:
Your power bank capacity doesn’t matter as much as the power output of a power bank .The most prime reason for buying is to charge your device and if fails to do that right, it just as good as a plastic box.All devices which people use needs at least 2 Ampere to charge it .So before buying a power bank check, at least, one of the two output ports has a 2.1 Ampere output .
Many people complain that while charging the phone the battery percentage still decreases.The answer is simple if your rate of charging is lower than the rate of discharging the battery percentage will decrease and may also reach zero and the phone might switch off.
Advantages of high-power output power banks are:-
Phone charges quickly
Usage of phone and charging can be done together
Rate of charging is always greater than rate of discharging of your phone.
2.Type of battery cells
Li-ion and Li-poly are nothing but two variations of the battery cells which are popularly used to constitute the best power banks. Most of the battery packs go for the Li-ion as they are cheaper but portable chargers being constituted with Li-ion tends to become much bulkier in comparison to the Li-Poly.
Not only Li-poly, being sleeker, can provide the provision to the manufacturers to manipulate the size and shape of the power banks further, they are safer in terms of usage as well. A Li-Po Battery can also retain its charge longer.
The next important thing you should consider is power capacity after the above two.This depends solely depends on what devices you are using.The power bank’s capacity should be at least 2.5 times your phone’s capacity to get 2 full charges out of it.Only 70% of a power bank’s capacity is usable .
For example, If i buy a 9,000MAH power bank to charge a phone with 3000MAH battery .The usable power I can use is just 6300MAAH which will only give 2 full charges, not 3 as per expectation.
4.Weight and Form Factor
The power bank is also called a portable charger so you would want to buy a portable and not a bulky one.Not all power banks are really portable to carry around.If you always carry your power bank to where ever you go you should consider a light power bank with a sleek design. Everyone wants a power bank that you can carry about at all time.
5. The number of USB ports
Number of ports on a Power Bank plays the vital role if you are going to have a need to charge multiple devices simultaneously. Usually most of the portable chargers below 10000 MAH capacity (smaller capacity) do not come up with multiple output port option. The reason is obvious; they are simply not large enough to charge up more than one device at a time.
By the way, it is important to know the number of output ports of a power bank affect output current when charging multiple devices at the same time. To get more clear about this, let’s take the example of the Mi 16000MAH power bank, which comes with two USB output ports (whereas the 10400MAH variant has only one output port) and claims to charge devices (Smartphone or tablet) at 2.1 A at 5V.
However, this claim is only true when charging only one device at a time. When you use both ports at the same time, this particular power bank is capable of providing at max 3 A at 5 V (i.e. 1.5 A per port) instead of otherwise presumed 4.2 A. So, before picking one battery pack, especially if fast charging is a priority to you, then this aspect should be taken care of scrupulously.
6. Buy from respected brands
Never become excited at power banks selling extremely cheap especially when they are unbranded, where those power banks not only failed to charge the phone, but also damaged the devices severely.
So be careful while balancing between quality and price, when particularly it comes to a power bank. Some good brands of power bank include jakery giant, lumsing power bank, romoss, veeda etc.