We are typically exposed to somewhere between 4000 to 10,000 advertisements per day. Many of these come in the form of packaging – bold claims by manufacturers of the ability and performance specs of their products. For example, your cereal will now often proclaim that it has all your daily essential vitamins and nutrients. In the world of power banks, however, the most often cited indicator used is that of **charge capacity – how many times will your power bank charge an external device?** But how are these numbers created? Today, we will explore the formula behind charge performance in power banks so that you’re armed with the best possible knowledge base when buying a new product.

*The Math*

** **While math may not be the most exciting subject in school (rocket ship designers might disagree), it’s important to understand how exactly a manufacturer calculates the number of times their power bank will charge your cellphone. Here’s the formula they rely on:

### (Labeled capacity of the power bank x 3.7 / output voltage of power bank) x 0.85 / battery capacity of device = Total number of recharges

With this in mind, let’s look at this in a couple of examples. First up, the iPhone 8, which has a battery size of 1,821mAh, a little bit smaller than the iPhone 7. Using the above formula we can expect 7.7 recharges of the iPhone 8 from a power bank with a 16000mAh capacity.

### (16000 x 3.7 / 5) x 0.85 / 1821 = 5.5 charges

For the Samsung Galaxy S8, we can do the same calculation for its 3000mAh battery:

**(16000 x 3.7 / 5) x 0.85 / 3000 = 3.3 charges**

*The Details*

Okay, so we’ve given you a formula, but what does it all mean? Let’s start with the **first multiplication number – 3.7**. This is the average voltage that you find in a battery cell. Normally you calculate this number by finding the mid-point between a fully charged cell and cell cut off (when it’s empty), and it’s specific but not universal to all lithium-ion batteries. Li-ion batteries featuring cobalt average at about 3.6 while those with manganese hover at 3.7.

**How about the next one – 5?** We use this here as the average voltage output of a typical power bank. They can often range depending on output type (USB, fast charge, lightning) and internal electronics. Again, you can normally find these details in the product specifications of whatever it is you’re buying. If using the above formula, be sure to swap out the 5 for whatever that devices voltage output is.

After that **we have 0.85**, which is multiplied by the average output voltage above. 0.85 is the average efficiency of a power bank, meaning that you’ll never get 100% of the total expected output as advertised. This is due to two things. First, because power banks boost output voltage from the average of 3.7 to 5, energy is lost when moving through its circuitry. Adding to that energy is lost as heat during a transfer and within the smartphone itself when moving to the battery; you can expect about 85% efficiency due to wasted and lost voltage. Please note that 0.85% conversion rate varies between chargers.

## Conclusion

Ta da! Just like that, you’re equipped with the knowledge and tools needed to see past those attractive marketing lines and find out for yourself the effectiveness of your power bank. The rechargeable battery world is now your oyster. Go out and explore!

I have a Jetsun that’s 16750mah, and by your formula should be able to charge my iPhone 7 plus 5 times, but in fact it can only do 2.

The 0.85 conversion rate varies depending on the charger. Maybe our power banks are more efficient 😀

When I have my Google Pixel 2 XL plugged into my RAVPower bank, it pulls at 9V == 2A.

Following the equation above, this would indicate that higher voltage into the device being charged (Pixel 2 XL) means you’ll get less recharges. Is this actually true, if so why?

This power bank battle I am having has been driving me crazy.

I’m afraid that you cannot use this formula if you are charging with the USB-C PD port of our 26800mah power bank. This is because the output changes according to the phone’s input needs. It’s not set like other power banks.

However, you can use 5V as reference.

The part “3.7/5” is suspicious:

the voltage of smartphone battery is also 3.7, just like the voltage of the powerbank, so you don’t need this coefficient.

USB output is at 5 V, so the power bank has to step up from 3.7 V to 5 V (which wastes energy).

You done a great job about this category, I got the best and useful information and suggestions from this category. You made a good site and it’s very interesting one. Thanks for sharing the best information. Regards.

Help !!!! My charger won’t go into the piece I charged it once and used it a couple of times and now nothing will go into the place that is supose to go into the piece to go into the wall ????? What happened ????? I’m very sad I can’t use this after getting it ??? Please help

Hi Micki

We suggest you contact support@ravpower.com detailing the model of your charger and someone should be able to help you with this issue!

can i power a device off the input on the ravpower?

Can you email us at support@ravpower.com and tell us which model you are using. Thanks!

I just bought a tenfly 16800mah power bank and my phone’s battery capacity is 5000. the output of the power bank is reading 2.1A, by using your formula it is expected to charge my phone 6 times but that’s not the case. I am doing something wrong or should I just return the power bank and get a refund. Please I would appreciate a quick answer because I have just 6days left to refund.

Hi there, we suggest reaching Tenfly as they will be better to provide assistance for their specific power bank. Thanks.

16800 x 3,7 / 5 x 0,85 = 10567,2 / 5000 = 2,1134 times

I am sorry, this is completely incorrect information. Even if the USB port functions at 5V, the battery inside the phone is rated the same (as with all Li-ion batteries) i.e., 3.7V. All transformations to higher voltage and then re-conversion would incur some losses for sure, but based on the numbers shown here (assuming the end result is what you take into account as the charging capacity of the power bank, the losses come up to 37% (based on the 0.85x 3.7/5), which is insane and frankly, if you are selling products with this kind of performance, you should totally fire your R&D team, or else market the power banks accordingly.

Hi there, this is a normal behavior for all power banks on the market — not only RAVPower. The standard efficient rate is 80%, however ours is higher than normal at 85%.

Totally agree with Arup. This information is just trying to hide the real efficiency of the power bank.

The power capacity of the above product is 16000mAh * 3.7 = 59.2Wh.

The power capacity of the iPhone 8 is 1821mAh * 3.7 = 6.7Wh.

So if the efficiency is 80%, the number of full charges is 59.2*0.8/6.7 = 7 charges, not 5.5.

“swap out the 5 for whatever that devices voltage output is” is totally improper, what if a PD device charges at 20V?

(16000 x 3.7 / 20) x 0.85 / 1821 = 1.38 charges, who wanna buy this product?

Hi there, the calculation method is correct, however the energy loss from the mobile to the battery hasn’t been considered.

How many times can I charge my phone with a power bank 4000amp

Hi there which RAVPower bank are you using?