You have an original real capacity 10,000 mAh power bank, and that should charge your 1500mah phone about 6 times, right? Well, here is where the confusion begins.

What the advertised capacity actually tells you.

The rated capacity is what is actually inside the powerbank, the physical battery.


Inside a 10,000 mAh power bank.
Inside a 10,000 mAh power bank.

This powerbank has 4 3.7V 2,500mAh batteries, together that equals 10,000mAh and this is correct.

But USB is 5V!

Inside powerbanks are 3.7V batteries, but the USB standard is 5V.  Between the battery and the USB socket is a conversion circuit and this changes the 3.7 V into USB friendly 5V. When converting into a higher voltage, you must also convert the mAh into the new voltage.

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 experienced 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 loose 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;

  • Manufactures 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.

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 actual number of recharges, in reality, it is far from it.

You are now a powerbank expert and next time you are on the prowl for a new one have a look through our range.

  • sleeperjoe

    I’m glad someone finally wrote about this. Thank you.

  • Getipa

    Oh I didnt know that,thats a really good information to share!

  • Stu

    Gee, now that’s a taste of honesty. Few other retailers would dare make that info public. A very good explanation. Well done. Very similar to the wattage output of amplifiers. TMO Vs RMS etc. Only fostered by manufacturers trying to outdo their rivals and still used today.

  • User

    The USB socket needs 5V.
    If the voltage of the cell is 3.7V we need a circuit to change 3.7V to 5V: a step-up circuit
    So, there’s a law in physics that I know like “law of conservation of energy”.
    Nothing is destroyed but all changes.
    Let’s see another physics law: Electric Power
    We can say that P = V * I, where P is power, V is voltage and I is Current.
    if we have some cells and the total capacity is 10 Ah we can say that they can generate 10A in about 1 hour.
    The voltage of these cells is 3.7V.
    So P = 3.7V * 10A = 37W
    If 37W is the total power of all the power bank we can say that its real capacity of the cells isn’t 10 A but
    37W = 5V * X
    And X is 7.4A

    • Yeah that’s a great way to make the power universal, and should be something that responsible brands use. Some AC chargers use the W as the power rating but I have not seen many power banks using this term.
      One reason I see is phone battery’s are rated at mAh so its easier for people to understand as the average consumer without the knowledge we have would be confused with a W power rating but everyone knows mAh.

      • User

        Yes, you’re right.
        But like said in the article, the capacity on the cover of the power bank isn’t its real capacity. There’s always a loss due to the conversion from 3.7V to 5V. The usage of Wattage can resolve this misunderstanding.

  • DG

    While it’s great what you’re doing with this article, I saw that there’s one very important bit of information that you’ve missed out, and one that I was hoping you’d cover concerning the use of “mAh” to label a battery’s capacity. While the conversion equation and efficiency consideration you’ve pointed out definitely gives a much clearer picture of a power bank’s capacity, the labelled mAh capacity of a power bank is technically more “correct” before the conversion process you’ve provided. The reason for this is because a smartphone’s battery’s mAh rating is also done at 3.7V, even though it charges at 5V.

    What I’m proposing is that for a far more accurate picture of a battery’s capacity, get its capacity in Wh (Watt-Hours), which is a measure of actual energy, as opposed to mAh (Milliampere-Hours), which is a measure of “electrical current capacity” – something that’s completely dependant on voltage. The Watt-Hour rating remains the same no matter what voltage is used. It is, overall, a far clearer reading of stored capacity. There’s a very good reason you don’t pay your power bill based on the amount of mAh you’ve used up: mAh doesn’t measure energy; Wh does.

    Here’s a good example: Say, your ‘Average Joe’ (who isn’t very knowledgeable on the physics of electricity) purchases a UPS battery rated at 2600mAh that has 4 standard wall-style outlets that output 110V, along with 6 USB outlets that output 5V he can charge his Galaxy S4 phone with. He knows that his S4 phone has a 2600mAh battery. One day there’s a blackout in his area that will last 12 hours. He has to charge his phone and he uses the UPS to do so. He assumes that since the UPS battery is 2600mAh, and his phone battery is also 2600mAh, that he’d be able to charge his phone once. Therein lies the problem: “mAh” is incredibly misleading for the purpose of informing the public on a battery’s capacity. His UPS is rated 2600mAh, but only AT 110V, while his phone’s battery is rated 2600mAh at 3.7V. Considering his UPS is around 20x bigger and 30x heavier than his phone’s tiny battery, how can they possibly hold the same amount of power? They don’t. Theoretically, he’d be able to charge his phone at least 29 times over using that UPS. The reason for this is that the actual energy in his UPS (110V multiplied by 2600mAh divided by 1000) is 286Wh, while the actual energy in his phone (3.7V multiplied by 2600mAh divided by 1000) is only 9.62Wh. Definitely explains the massive difference in size and weight between the two batteries, doesn’t it?

    Concerning USB power bank batteries, every mAh rating for them is done at the standard Lithium Ion voltage: 3.7V. The mAh of laptop batteries, even though they’re also Lithium Ion, are done at whatever voltage is specified on them (generally around 15V).

    I should mention though, that the method in this article ALSO works:

    For a USB Power Bank rated at 10 000 mAh:

    10 000 mAh at 3.7V = 37Wh

    10 000 mAh at 5V = 50Wh (INCORRECT readout since the battery is rated at 3.7V, not 5V).

    Plug in the article’s equation:

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

    7400 mAh at 5V = 37Wh (CORRECT readout)

    This means that 7400mAh@5V = 10 000mAh@3.7V, and both are still 37Wh no matter what voltage is used.

    If Average Joe managed to use a giant transformer to power a 120-Volt, 2000-Watt air conditioner off his S4 smartphone’s 2600mAh (9.62Wh) battery, the battery would easily short-circuit and explode since it’s a much higher power draw than the battery is rated for… but ignoring that, theoretically, given the amount of energy in the battery, he could power the air conditioner (9.62 divided by 2000) for 0.00481 hours, which is 17.3 seconds. Just wanted to paint a clearer picture on how versatile the Watt-Hour capacity reading really is.

    • Yeah you are right in you analysis, the reason I didn’t include it is that it involves introducing a third unit variable, Wh.
      It is a much more simpler way of comparing power however it adds to the complexity of an average Joe as now they need to also understand another type of measurement. mAh is widely used in devices and a familiar term, Wh is rarer and would require deeper explaining (like you have done for us above).
      I will plan to create another article that explains why we should be using Watt hours as it is truly the universal way show capacity however it is not a common unit ad it should be.
      Thanks for spending the time to explain and cheers for contributing some great knowledge to the blog.

    • Nitin Chauhan

      Does it mean that the power bank of 10000 mah capacity at 3.7 V will actually charge a 2000 mah 3.7 V battery in phone, 5 times. Without any loss?

  • Delphi

    Why aren’t they required to state the exact Wh (not mah) supplied by USB port?

    One correction to author: It’s more probable that it’s a 7.2v arrangement and 5v is derived by a step-down converter because it’s much more efficient than step-up

    • True I will have to check the insides of another powerbank to check if it is wired as 2 pairs of 7.2 or just one big 14.8V or straight 3.7, from memory it was just one + and one – coming into the board with the connection in parallel. The smaller powerbanks 3000 use only one battery so will step up from 3.7/3.6 to 5V the same way your smartphone does.

  • sharkproductions

    So if you wanted to convert this to 19v, would it be correct to use this equation to get a better picture of the capacity, in comparison to a laptop battery:
    3.7v X 10000 / 19v = 1947mAH

    • Yeah that is correct, of course you would need to consider the efficiency of the conversion circuit.

  • nuliusz

    The calculation is wrong. If we take for example the 16000mah power bank, we first calculate the 20% loss (16000 – 20% = 12800mah)
    Then 12800 x 3.6 = 46080
    then 46080 x 5.1 = 9035mah

    Normally power banks true capacity is around 60% of what is stated.

    • That is going off the labeling Xiaomi have done, they do publish their actual on the label, they advertise the conversion loss is less than 10% so their calculation is inline if the 10% loss is accurate. They marketed this efficiency as one of their advantages when this range of powerbank was released back in 2015

    • Frederic Nagy

      even your calculations are wrong.
      last i checked 460580 x 5.1 isn’t 9035
      its divided by the 5 and not multiplied

      • nuliusz

        It is not a wrong calculation. It is obviously a typo. LOL

  • Given that the smartphone also has the same LiPo battery with the same voltage, there isn’t too much point converting the mAh rating of the bank, since you’ll use it for charging another LiPo device anyway…

    If a 10kmAh battery can only be used to charge a 2500mAh device 3 times, that’s not because of the missmathcing ratings. That’s because either the losses or the producer straight out lying.

    OTOT, it’s worth noting, that even if the powerbank itself has a very good efficiency, your phone can (and will) still lose energy while charging. If it warms up during charging, then that’s charge being ‘lost’ (used for heating instead of charging).

    • geamANDura

      kmAh, really? Kilomili? 😀

      • LOL, you’re right. Should have simplified/done the multiplication. But then it may have just caused more confusion :).

  • Bradley Berthold

    The only thing you need to consider is the conversion losses. The “mAh” taking 20% off thing isn’t needed since what you care about is Watt-Hour, which isn’t going to change with voltage/current level conversions (except for conversion loss).

    It’s Watt-Hours that matters that most and what should be compared. DC-DC converters are “constant power” not “constant current” – so mAh is useless to use for a comparison

  • geamANDura

    This is completely wrong, different voltage doesn’t impact the total capacity one single bit. mAh is just current over a period of time (current x time, it even says in the unit of measurement that you keep repeating — mAh = milliaps x hours, Jesus!). Regardless what voltage is used, the capacity (in mAh) stays the same. What you’re thinking of is power (W or Wh). Power is voltage x current. Your little article would have been valid if you talked about power (W or Wh) being advertised falsely due to a specific voltage that is unusable for the buyer.

    • Roootkit

      Well, the article is actually correct, note that we are taking a point of view in which “capacity” is measured as electric charge in mAh (1 mAh = 3.6 Coulomb). For example, you can’t expect a 10000mAh 3.7V battery inside a power bank to produce 10000mAh at 5V (or 9000mAh assuming 10% conversion loss) at the end of the step-up circuit, that would be violating the law of energy conservation. A 10000mAh 3.7V battery in a power bank can only produce up to an output of 7400mAh at 5V (or 6660mAh assuming 10% conversion loss) at the end of the step-up converter. I find this article very helpful in clearing up some possible confusions and misconceptions, others may wrongly think otherwise just because you don’t know what you are talking about.

      10000mAh @3.7V is equivalent to 37Wh or 133.2kJ
      10000mAh @5V is equivalent to 50Wh or 180kJ
      9000mAh @5V is equivalent to 45Wh or 162kJ
      7400mAh @5V is equivalent to 37Wh or 133.2kJ
      6660mAh @5V is equivalent to 33.3Wh or 119.88kJ

      ps: For better understanding perhaps you could learn about the basic principles of dc to dc step-up converter, and how ac to ac transformer works if you haven’t already.

      • geamANDura

        Thanks, it looks like I need to educate myself better.

  • Stephen Madeira

    Yeah, but smartphone’s battery aren’t 5 V, but rated to 3.8 (3.7 – 4.2) V. This is just 2,6% of difference, so, theorically, you have again almost your original mAh rating (except by losses on booster and monitor circuit inside the powerbank and charger circuit inside your device. Smaller losses doesn’t need to be said).