Let’s start with mAh: It stands for milliampere-hour, or in other words, it is the amount of energy available that could possibly be stored in a given battery. When selecting a portable power bank, start by comparing the mAh rating of your mobile device to that of your portable power bank.
Now, it is tempting to do some “easy” math. If you have a 5,000mAh power bank, and your cell phone internal battery rating is 3,200mAh, you might think that 5,000 / 3,200 would give you about 1.5 charges. But you would be wrong. Transferring stored energy from one battery to another is not 100% efficient.
There are two things to consider when determining how much energy can be transferred from your power bank to your battery.
#1 – Power Bank Efficiency
Performance power banks are only about 85-90% efficient. When a power bank is charging a phone, typically 10-15% of the charge is lost in the transfer of power in the form of heat due to the resistance of the battery and USB connection. If you drain the power bank at low power for a long time (e.g. 1 Amp), you get better efficiency and less heat than if you drain it quickly at high power (e.g., 2-3 Amps).
#2 – Difference Between Storage/Output Voltage
More importantly, the voltage of a power bank’s stored energy is different from its output. Take, for example, a 5,000mAh capacity power bank. Its stored energy is 5,000mAh x 3.7V = 18.5 watt-hours (Wh). Yes, although the Li-ion battery outputs 5V, the storage voltage is 3.7V.
Since the output capacity of your phone's battery is listed in mAh at 5V, you need to apply a conversion to be able to figure out how many full charges you can get. Using the Conservation of Energy principle (remember that from high school?), we can calculate the capacity of this power bank at 5V given that the transferred energy remains constant as 18.5 Wh /5V = 3.7 Wh/V = 3,700mAh.
In our example, a power bank listed with 5,000mAh at 3.7V corresponds to 3,700mAh at 5V. The former voltage corresponds to your power bank capacity and the later voltage to your phone charging voltage. This leads us to a conversion factor of 74% (74% of 5,000mAh = 3,700mAh).
If you multiple the efficiency of the power bank (90%) times the 74% conversion factor based on the difference between storage/output voltage, you obtain the actual charger capacity:
90% x 74% = 66.7%
In the real world, a 5,000mAh power bank would provide approximately 3,335mAH (5,000mAh x 66.7%), which would barely be enough to charge your 3,200mAh cell phone battery once. To determine which portable solar bank best suits your needs, use this formula:
(mAh capacity of your cell phone / 66.7%) x desired number of charges
For example, the iPhone6 Plus has a 2,915mAh capacity battery. If you wish to charge it twice, you would need a power bank with a capacity of at least 8,744mAh.
(2,916mAh/66.7%) x 2 = 8,744mAh
A Nexus 10 (tablet) has a 9,000mAh capacity battery. If you charge it twice, you would need a power bank with a capacity of at least 28,616mAH. (I am using 62.9% efficiency in the formula below because the high Amp charging is less efficient.)
(9,000mAH/62.9%) x 2 = 28,616mAH
Keep in mind, now, that this math represents best case scenarios for charging your devices. Other factors, such as age and degradation of lithium ion battery cells over time, and what you are doing with your device during a charge, will further reduce the number of times a portable power bank can charge your device.
Alas! Don’t feel defeated! Shop our most popular portable solar power sources to keep your mobile devices as charged and on the go as possible.