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How to calculate power bank capacity

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First we should know two things:  voltage and current.

Use the formula:  power = voltage x current (Batteries of power banks are 3.7V output)


Every power bank has a conversion rate. Higher end power banks possess around 90% conversion rate. Power bank conversion rate is also related to batteries used in the production. Lithium-polymer rechargeable batteries have a work output conversion rate that is generally 85% -95%, and lithium-ion (18650) battery possesses an output conversion rate between 75% - 90%. 



To calculate conversion rate with the real capacity and charging times you do the following:

Smartphone charging is dynamic of around 500MA-1000MA with the voltage at 5V.



If the battery power is marked 4000mAh, then the power of a 4000mAh power bank should be 4 * 3.7 = 14.8W, calculate by our polymer batteries conversion rate of 80%, the real total power supply for charging can be 11.8W. Take a 1500mA smart phone for example, the demand power is 3.7*1.5 = 5.55W, then it can charge the phone two times. So if you want to charge your tablet, you need a larger capacity power bank, the power bank capacity should be more than 8000MA and can provide high current 2A output charging requirements.

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Hey guys - shameless plug but it might help some of you out there. If it's not in line with the rules take it straight off.


We built a super simple mAh calculator to help you decide what size charger to buy for your phone. The calculator takes the mAh of common devices and generates a likely amount of charges you might get from different mAh rated power banks.


Check it: Power Bank mAh Calculator

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Everybody is forgetting the most important fact. Just because a battery or power bank is labeled 10,000 mAH, doesn't mean it is 10,000 mAH. Is there a good way to test a battery/power bank to verity what it's real capacity really is? Calculating a battery's or power bank's capacity by using the printed on the label data is pure fantasy. Most Chinese devices data are wishful dreaming. Is there a circuit or other method that can drain a fully charge power storage device to it's half voltage point and then calculate it's storage capacity (my thought of how to do it)?

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