How to calculate theoretical USB output capacity

A simple equation can be used to convert the 3.7V into 5V.

ACTUAL 5V mAh = 3.7 X Advertised Capacity / 5

For a 10,000mAh powerbank – 3.7 X 10,000 / 5 = 7,400 mAh

So a 3.7V 10,000 mAh powerbank really only supplies 7,400 mAh at the 5V USB connection. So straight out of the box is a 23% reduction in the stated mAh.  This is not the actual level as there is also conversion loss.

What is conversion loss?

As you use your powerbank the circuit inside that converts 3.7V to 5V USB uses some energy and also creates heat. During this conversion, you lose some extra mAh.  There is a wide range in conversion efficiency and most brands don’t state the losses, Xiaomi has prized themselves on their conversion efficiency chips which are up to 98% efficient, meaning you only lose 2% off your battery power in the conversion. Some others can consume as much as 10% during conversion.

Why don’t they just state the actual output?

They don’t need to as technically that is what’s inside the box and most people have no idea. By not giving the actual 5V output brands can reap these benefits;

  • Manufacturers can have a higher number of mAh for their powerbank and sound more powerful.
  • It avoids the talk or testing of conversion loss and brands with low efficiency conversion chips can still market them in the same league as other efficient brands.

Some manufacturers will state in the manual or in small print on the device but most will not state anything other than the 3.7V mAh. As a general rule I would recommend taking 25% – 30% off the advertised capacity straight away and then you have a more realistic indication of performance.

Powerbank2
This power bank actually states both capacity’s, you wouldn’t notice it tho unless you are one of the people who actually read product labels. This one has 16,000 mAh but only has 10,200mAh at 5.1V.

It is a very gray area and not common knowledge but I hope this has shed a bit of light on how powerbanks are advertised and busted the classic assumption that dividing the rated capacity by your phone battery capacity is an accurate measurement of the actual number of recharges, in reality, it is far from it.

 

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