When considering the choice of a power bank, this is usually the most important issue. No wonder – everyone wants to know for how long their energy will last on a trip. Not many people know, however, how to calculate the number of full charges you can get from a power bank.

The short answer i – give or take this much (using the Green Cell GC PowerPlay10):

Where do these figures come from? The number of times you can charge a phone using a specific power bank is influenced by several factors. Let’s discuss them one by one:

1. The telephone’s battery capacity

The differences in battery size can be very large. The smallest batteries used in older phones can have less than 1000 mAh, while the largest ones – 4000-5000 mAh. Current smartphones most often feature batteries in a range of 2000-3000 mAh. It’s important to remember, however, that the battery’s total capacity decreases with use. Depending on the number of cycles performed, after one year a battery’s capacity can drop by more than ten percent compared to its initial value. Getting back to the point, the correlation here is simple – the smaller the battery, the more charges a power bank will provide.

2. The power bank’s capacity

Much like with the capacity of telephone batteries, the power bank’s capacity is expressed in milliampere  hours (mAh). Therefore, if your battery has 3000 mAh, then a 10000 mAh capacity power bank should easily charge your phone 3 times, right?

As you might have figured out, the matter is much more complicated than this.
First of all, you need to keep in mind that the nominal capacity of cells comprising a power bank is not equal to the actually usable output capacity. This is due to the fact that the voltage of a cell in a range of 4,2V – 3,0V is converted to the voltage required for USB charging, which is 5V, and in the case of Quick Charge – also 9V and 12V. This conversion always takes place with certain losses. For an estimate calculation, it can be assumed that brand name products have an efficiency rating at a level of at least 90%. How to accurately calculate it, then?

Equation for calculating the number of possible power bank phone charges:

Assuming that you have new equipment and don’t use your phone too often while charging, you can use the following equation:

where: PBC is the power bank capacity
TBC is the telephone battery capacity

The telephone battery capacity is multiplied by a factor of 1,1 in order to account for the energy usage of a phone in operation, estimated at about 10% of the battery capacity.
How many times, then, can you charge a phone with a 3000 mAh battery using a 10000 mAh power bank?

About 2,7 times 🙂 

When charging a phone while it’s turned on, you can simplify the equation to the following form:

Keep in mind, that the above equations assume charging a completely depleted phone battery, but you rarely ever charge devices at 0% battery power. When charging a phone at X percent battery power:

This raises yet another issue – not every seller and manufacturer provides the true device capacity in the product information. It’s easy to encounter power banks with a declared 50000 mAh capacity and the size of another, 15000 mAh model, which is absolutely impossible. This is, unfortunately, a common practice and you need to be very careful. Not to worry, because in our store all power banks have a declared actual capacity.

The final issue is cell degradation. Just like telephone batteries, cells in a power bank also decrease in capacity with use. You need to remember, that after a year a power bank will most certainly have a lower capacity than when it was brand new. How much the power bank’s capacity will decrease depends primarily on how often we use it an in what conditions. Of course, this is also impacted by the quality of cells. Using high class cells can greatly extend a power bank’s longevity.

3. Current telephone energy usage

If you can calculate more or less how many times a power bank will charge a phone, another thing that has to be accounted for is the congoing energy usage. While the equation we presented earlier takes into account minor telephone energy usage while charging, when in intense use, energy consumption may be significantly higher, which needs to be considered. Current smartphone capabilities give you a lot of options when it comes to battery usage. Browsing the Internet, listening to music, Bluetooth use, playing games… the differences in battery discharge rates when idle and during intense use are really large.

How large of a power bank do I need, then?

Unfortunately, there is no clear answer to that question. It all depends on the intended use. If you’re not planning longer trips, then a 5000-10000 mAh capacity model will surely be sufficient. However, a power bank with this capacity might not handle a longer campout. I believe the best answer to that question is “buy two – one for travel and one for your pocket” – and with that thought, and a link to our store, I bid you farewell.