How many times will a Power Bank charge my device?

This question lacks a lot of information. However, basic assumptions can be made to give it life.
This depends on what you want to use it for. Most people want to know how many times they can recharge their phones with a fully charged power bank, before deciding on the one to buy.

How long can a power bank last?

Smartphones typically come with 1500 - 2500 mAh batteries on average. It is important that you check your phone to find out the capacity of your battery.  Assuming it is 2500 mAh, in a perfect world, divide the capacity of the power bank by that of your phone. You will find that

1. A fully charged 4000 mAh power bank will charge a smartphone 1.6 times on average, subject to the health of your gadget and battery.
2. A fully charged 10000 mAh power bank will charge up a smartphone up to 4 times on average; also subject to the health of your gadget and battery.

The outcome should be the same for all smartphones. However we know that all things are not always equal because it is not a perfect world after all.

Power bank capacity

It is therefore pertinent to note some of the factors that can affect the result, such as:

1. Your phone/battery being of good quality and optimal performance. 
2. Climatic conditions affect power banks and batteries generally. Typically you end up with about 80% power conversion efficiency rate. 
3. Quality of the USB charger cord.
4. Efficiency of the Internal Control Circuit. 
5. Quality of the device being charged, etc.

It is best to err on the side of caution and expect battery loss of 20% due to general climatic conditions. A more accurate calculation must account for this viz:

•  power bank capacity (mAh) × 80% efficiency (due to estimated battery loss of 20%) ÷ device capacity (mAh).

As a result, for the above two examples, a fully charged 4000 power bank should charge your device at least 1.3 times. And a fully charged 10,000 mAh power bank should charge your device at least 3.2 times.

For more information about power banks See Solar Power Bank Quality Assurance: Power Bank phone Chargers

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