Choosing a Portable Solar Power Bank

HOW MUCH POWER DO I NEED?!

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If you are thinking of choosing a portable power bank for your mobile devices, but just aren’t sure what to buy, or if you have one and it isn't performing as you thought it might, you are in the right place. So many spec. So many details. It can be confusing. Let’s break it all down. This conversation is exclusive to lithium ion batteries, primarily used in portable power banks and portable electronic devices.

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).

Real-World Application

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.

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Meantime, stay tuned for our future blogs, where we will discuss solar panels in relation to charging your portable devices, power banks, and batteries, both small and large.

Here is a table of common portable devices and their associated battery capacity expressed in mAh and watt hours.

Device	Brand	Model	Capacity (mAh)	Capacity (Wh)
Phone	Apple	iPhone 6 Plus	2915	14.6
Phone	Apple	iPhone 6	1800	9
Phone	Apple	iPhone 5S	1570	7.9
Phone	Apple	iPhone 5C	1507	7.5
Phone	Apple	iPhone 5	1440	7.2
Phone	Apple	iPhone 4S	1432	7.2
Phone	Apple	iPhone 4	1420	7.1
Phone	Apple	iPhone 3GS	1219	6.1
Phone	Apple	iPhone 3G	1150	5.8
Tablet	Apple	iPad 1	6613	33.1
Tablet	Apple	iPad 2	6944	34.7
Tablet	Apple	iPad 3	11560	57.8
Tablet	Apple	iPad 4	11560	57.8
Tablet	Apple	iPad Air	8820	44.1
Tablet	Apple	iPad Air 2	7340	36.7
Tablet	Apple	iPad Mini 1st	4440	22.2
Tablet	Apple	iPad Mini 2nd	6471	32.4
Tablet	Apple	iPad Mini 3rd	6471	32.4
Phone	Samsung	Galaxy S2	1650	8.3
Phone	Samsung	Galaxy S3	2100	10.5
Phone	Samsung	Galaxy S4	2600	13
Phone	Samsung	Galaxy S5	2800	14
Phone	Google	Nexus 6	3220	16.1
Phone	Google	Nexus 5	2300	11.5
Phone	Google	Nexus One	1400	7
Phone	Google	Nexus S	1500	7.5
Phone	Google	Nexus 4	2100	10.5
Phone	Google	Galaxy Nexus	1750	8.8
Phone	Google	Galaxy Nexus LTE	1850	9.3
Tablet	Google	Nexus 7 (tablet, 2012)	4325	21.6
Tablet	Google	Nexus 7 (tablet, 2013)	3950	19.8
Tablet	Google	Nexus 10 (tablet)	9000	45
Tablet	Google	Nexus 9 (tablet)	6700	33.5
Phone	HTC	HTC One	2300	11.5
Phone	HTC	HTC One (M8)	2600	13
Phone	HTC	HTC One (M7)	2300	11.5
Phone	HTC	HTC One mini	1800	9
Phone	HTC	HTC Desire 320	2100	10.5
Phone	HTC	HTC Desire 510	2100	10.5
Phone	HTC	HTC Desire	2100	10.5
Phone	HTC	HTC Desire C	1230	6.2
Phone	HTC	HTC One X	2100	10.5
Phone	HTC	HTC One V	1500	7.5
Phone	HTC	HTC One S	1650	8.3
Phone	HTC	HTC One X+	2100	10.5
Phone	HTC	Windows Phone 8X by HTC	1800	9
Phone	HTC	Windows Phone 8S by HTC	1700	8.5
Phone	Sony	Xperia Z3	3100	15.5
Phone	Sony	Xperia Z3 Compact	2600	13
Phone	Sony	Xperia Z3 Dual	3100	15.5
Phone	Sony	Xperia Z3v	3200	16
Phone	Sony	Xperia Z	2330	11.7
Phone	Sony	Xperia Z Ultra	3050	15.3
Phone	Sony	Xperia Z2	3200	16
Phone	Sony	Xperia Z2a	3000	15
Phone	Sony	Xperia T3	2500	12.5
Phone	Sony	Xperia T3 LTE	2500	12.5
Phone	Sony	Xperia T2 Ultra	3000	15
Phone	Sony	Xperia M Dual	1750	8.8
Phone	Sony	Xperia M2 LTE	2300	11.5
Phone	Sony	Xperia M2 Dual	2300	11.5
Phone	Sony	Xperia Z1	3000	15
Phone	Sony	Xperia Z1 Compact	2300	11.5
Phone	Sony	Xperia Z1s	3000	15
Phone	Sony	Xperia C	2390	12
Phone	Sony	Xperia C3	2500	12.5
Phone	Sony	Xperia SP	2370	11.9
Tablet	Sony	Xperia Z2 Tablet	6000	30
Tablet	Sony	Xperia Z3 Tablet Compact	4500	22.5
Tablet	Sony	Xperia Z Tablet LTE	6000	30