**How can you calculate the number of charging cycles from a power bank?**

One of the most common misconception is that we can simply divide the capacity of a battery by the capacity of a smartphone/tablet to figure out how many cycles we can use this power bank to charge the smartphone/tablet. However, that is not the correct method!

The capacity is the amount of electric charge stored in a cell. The energy is the amount of work these charge will be able to achieve. Here we talk about powering your device type of work. To connect the energy and the capacity, you need the voltage! Simply put, energy = capacity x voltage. Now the power is the flow of energy. To understand the power imagine that you slowly walk for one hour. The amount of energy you spend might be exactly the same as if you were running for 30 minutes. The amount of work (the distance to simplify) is the same. But in the latter exercise you were using twice as much power to achieve the same work. The power is given by: power = energy / time.

The energy conservation principle dictates the amount of energy a power bank can transfer to a 5V phone or tablet. Take for example a 10000mAh capacity mobile charger. Its stored energy is 10000mAh x 3.7V = 37 Wh. Yes, the Li-ion battery voltage is 3.7V. It outputs 5V because of the internal potential conversion. Sp these electric charges need to be stored at 5V. We can calculate the capacity of this power bank at 5V given that the transferred energy remains constant (remember the energy conservation principle) as 37 Wh / 5V = 7.4 Wh/V = 7400mAh. Now the capacity of your phone's battery is listed in mAh at 5V. In other words, you need to apply a conversion to be able to figure out how many full charge you can apply. In our example, a power bank listed with 10000mAh at 3.7V corresponds to 7400mAh at 5V. The former voltage corresponds to your power bank capacity and the later voltage to your phone charging voltage. This lead us to our first, theoretical, conversion factor of 74% (74% of 10000mAh = 7400mAh). However, in both cases, the amount of energy is 37 Wh!

Certainly but to a lesser extent. With a performance battery pack you usually get an average loss of about 15% in the process of transferring the electrons from the charger to the phone (or other device). In the most efficient systems, like the VNT-PB021, the efficiency is superior to 90% and the loss is less than 10%. Where does this energy go? Mostly heat! Assuming 90% efficiency, this leads to a conversion factor of 90% x 74% = 66%.

Yes! The larger the current (in amper), the lesser the efficiency. Ideally, you want the chemical reactions that occur in the cells to revert the energy storage process to fully occur. High speed energy transfer also leaves nano crystals in place that will reduce the durability of your cells. You can notice also that external chargers with high current (more than 2.1A) that do not have a smart detection of the optimal and minimum current to apply, a lot of heat is generated. Lastly, the higher the speed (Amp) the lower the transferred capacity. This explains in part why some power bank do not deliver more than 85% efficiency.

So we are left with a capacity multiplier of 0.66 (or 66%). In other words, lets say we see a charger with 5500 mAh capacity at 3.7V, how many times can it charge a iPhone6? The iPhone6 has a 1800 mAh capacity at 5V (but this is accounted for in our conversion factor). So the charger transferred capacity is 0.66 x 5500 mAh = 3630 mAh. The iPhone6 can therefore be charged 3630 mAh / 1800 mAh = 2.02 times.

The formula: number-of-cycles = powerbank-listed-capacity(mAh) * 0.66 / device-listed-capacity(mAh)

Here is a very easy trick! Check the weight of the charger. The best cell technology used in portable chargers is Li-ion. Polymer Li-ion cells are based on the same principle, their main advantage over Li-ion is that they can be mold to fit different shapes. So if the technology is pretty flat and that 90% of the weight of a charger comes from the cells, one cannot expect a 150 g charger to contain 13000mAh! It is a lie! To give you an idea, our 13000mAh charger weight 350 g and our 2200mAh charger weight 66 g. This means 0.027 g/mAh (for the high capacity charger) and 0.030g/mAh for the ultra light charger. There is a fair amount of confusion coming from the over-rating of certain brands and the challenge of applying the right voltage conversion factor! Poor customer!

We can calculate an expected weight with 0.03 g/mAh x Capacity (mAh) and compare this to the charger listed weight value. Usually the weight is accurate because it is easier to verify. So any large deviation between the expected weight and the actual power bank weight (10% or more) should raise concerns. For instance a 5500mAh charger should weight around 0.03 g/mAh x 5500mAh = 165 g. The expected margins of 10% correspond to 0.1 * 165 g = 17 g. Hence, a 5500mAh should weight between 148 g and 182 g. Of course if there is a big flashlight, a cable on a small charger or if it is a rugged power bank, than the case might start to carry a significant weight.

true-estimated-capacity(mAh) = device-weight(g) / 0.027 (larger capacity ones like 10000mAh and above)

true-estimated-capacity(mAh) = device_weight(g)/ 0.030 (smaller capacity charger )

Let's do a case study with real life examples. Below are advertised 13000mAh products together with their listed weight. We calculate that 13000mAh should lead to a weight of about 0.027 g/mAh x 13000mAh = 350 g +/- 35 g (we use here the larger capacity metric to be more precise).

350g

308g http://www.amazon.com/RAVPower%C2%AE-13000mAh-External-Compatibility-Lightning/dp/B00MPIGPUY/

281g http://www.amazon.com/Nice-Texture-Built--Management-Lightning/dp/B00KT4E92M/

250g http://www.amazon.com/Portable-13000mAh-Aluminum-Smartphone-Bluetooth/dp/B00QUR0BR4/

181g http://www.amazon.com/13000mAh-Capacity-Bluetooth-headphones-Devices-Color/dp/B00N1WO7OY/

These five products have an estimated capacity of about 12960mAh (5 cells of 2600mAh), 11400mAh (5 cells of 2200mAh), 10400mAh (5 cells of 2000mAh), 9200mAh (4 cells of 2300 mAh), 6700mAh (3 cells of 2200mAh) respectively. Even including the 10% margins, some products are just so much over-rated it is not funny! Unfortunately, the price tag does not follow!

We find that using capacity only is misleading. This is why you'll see reputable brands listing the number of times a device can be charged by a given portable usb charger.

We list below the number of times a device can be fully charged from 0% to 100% by each of our products. The numbers under the PB021 and PB011 columns correspond to the number of times a given device can be fully charged. We also list the combined value.

**Capacity vs Energy vs Power**The capacity is the amount of electric charge stored in a cell. The energy is the amount of work these charge will be able to achieve. Here we talk about powering your device type of work. To connect the energy and the capacity, you need the voltage! Simply put, energy = capacity x voltage. Now the power is the flow of energy. To understand the power imagine that you slowly walk for one hour. The amount of energy you spend might be exactly the same as if you were running for 30 minutes. The amount of work (the distance to simplify) is the same. But in the latter exercise you were using twice as much power to achieve the same work. The power is given by: power = energy / time.

**A mobile charger's energy storage is more relevant than its capacity**The energy conservation principle dictates the amount of energy a power bank can transfer to a 5V phone or tablet. Take for example a 10000mAh capacity mobile charger. Its stored energy is 10000mAh x 3.7V = 37 Wh. Yes, the Li-ion battery voltage is 3.7V. It outputs 5V because of the internal potential conversion. Sp these electric charges need to be stored at 5V. We can calculate the capacity of this power bank at 5V given that the transferred energy remains constant (remember the energy conservation principle) as 37 Wh / 5V = 7.4 Wh/V = 7400mAh. Now the capacity of your phone's battery is listed in mAh at 5V. In other words, you need to apply a conversion to be able to figure out how many full charge you can apply. In our example, a power bank listed with 10000mAh at 3.7V corresponds to 7400mAh at 5V. The former voltage corresponds to your power bank capacity and the later voltage to your phone charging voltage. This lead us to our first, theoretical, conversion factor of 74% (74% of 10000mAh = 7400mAh). However, in both cases, the amount of energy is 37 Wh!

**Does the quality of the electronic board play a role?**Certainly but to a lesser extent. With a performance battery pack you usually get an average loss of about 15% in the process of transferring the electrons from the charger to the phone (or other device). In the most efficient systems, like the VNT-PB021, the efficiency is superior to 90% and the loss is less than 10%. Where does this energy go? Mostly heat! Assuming 90% efficiency, this leads to a conversion factor of 90% x 74% = 66%.

**Does the speed of charging matter?**Yes! The larger the current (in amper), the lesser the efficiency. Ideally, you want the chemical reactions that occur in the cells to revert the energy storage process to fully occur. High speed energy transfer also leaves nano crystals in place that will reduce the durability of your cells. You can notice also that external chargers with high current (more than 2.1A) that do not have a smart detection of the optimal and minimum current to apply, a lot of heat is generated. Lastly, the higher the speed (Amp) the lower the transferred capacity. This explains in part why some power bank do not deliver more than 85% efficiency.

**How to calculate the effective cell phone charger capacity?**So we are left with a capacity multiplier of 0.66 (or 66%). In other words, lets say we see a charger with 5500 mAh capacity at 3.7V, how many times can it charge a iPhone6? The iPhone6 has a 1800 mAh capacity at 5V (but this is accounted for in our conversion factor). So the charger transferred capacity is 0.66 x 5500 mAh = 3630 mAh. The iPhone6 can therefore be charged 3630 mAh / 1800 mAh = 2.02 times.

The formula: number-of-cycles = powerbank-listed-capacity(mAh) * 0.66 / device-listed-capacity(mAh)

**There are a lot of companies lying about their charger capacity, is there a trick to quickly identify this?**Here is a very easy trick! Check the weight of the charger. The best cell technology used in portable chargers is Li-ion. Polymer Li-ion cells are based on the same principle, their main advantage over Li-ion is that they can be mold to fit different shapes. So if the technology is pretty flat and that 90% of the weight of a charger comes from the cells, one cannot expect a 150 g charger to contain 13000mAh! It is a lie! To give you an idea, our 13000mAh charger weight 350 g and our 2200mAh charger weight 66 g. This means 0.027 g/mAh (for the high capacity charger) and 0.030g/mAh for the ultra light charger. There is a fair amount of confusion coming from the over-rating of certain brands and the challenge of applying the right voltage conversion factor! Poor customer!

We can calculate an expected weight with 0.03 g/mAh x Capacity (mAh) and compare this to the charger listed weight value. Usually the weight is accurate because it is easier to verify. So any large deviation between the expected weight and the actual power bank weight (10% or more) should raise concerns. For instance a 5500mAh charger should weight around 0.03 g/mAh x 5500mAh = 165 g. The expected margins of 10% correspond to 0.1 * 165 g = 17 g. Hence, a 5500mAh should weight between 148 g and 182 g. Of course if there is a big flashlight, a cable on a small charger or if it is a rugged power bank, than the case might start to carry a significant weight.

__The formula__true-estimated-capacity(mAh) = device-weight(g) / 0.027 (larger capacity ones like 10000mAh and above)

true-estimated-capacity(mAh) = device_weight(g)/ 0.030 (smaller capacity charger )

__Example__Let's do a case study with real life examples. Below are advertised 13000mAh products together with their listed weight. We calculate that 13000mAh should lead to a weight of about 0.027 g/mAh x 13000mAh = 350 g +/- 35 g (we use here the larger capacity metric to be more precise).

350g

**http://www.amazon.com/Viivant-External-Powerbank-Smartphone-Anywhere/dp/B00QR8X0GA/**308g http://www.amazon.com/RAVPower%C2%AE-13000mAh-External-Compatibility-Lightning/dp/B00MPIGPUY/

281g http://www.amazon.com/Nice-Texture-Built--Management-Lightning/dp/B00KT4E92M/

250g http://www.amazon.com/Portable-13000mAh-Aluminum-Smartphone-Bluetooth/dp/B00QUR0BR4/

181g http://www.amazon.com/13000mAh-Capacity-Bluetooth-headphones-Devices-Color/dp/B00N1WO7OY/

These five products have an estimated capacity of about 12960mAh (5 cells of 2600mAh), 11400mAh (5 cells of 2200mAh), 10400mAh (5 cells of 2000mAh), 9200mAh (4 cells of 2300 mAh), 6700mAh (3 cells of 2200mAh) respectively. Even including the 10% margins, some products are just so much over-rated it is not funny! Unfortunately, the price tag does not follow!

**What should we expect from the Viivant portable chargers?**We find that using capacity only is misleading. This is why you'll see reputable brands listing the number of times a device can be charged by a given portable usb charger.

We list below the number of times a device can be fully charged from 0% to 100% by each of our products. The numbers under the PB021 and PB011 columns correspond to the number of times a given device can be fully charged. We also list the combined value.

Device | Brand | Model | Capacity (mAh) | Capacity (Wh) | VNT-PB021 | VNT-PB011 | Combined | |

Phone | Apple | iPhone 6 Plus | 2915 | 14.6 | 3.0 | 0.5 | 3.5 | |

Phone | Apple | iPhone 6 | 1800 | 9.0 | 4.8 | 0.8 | 5.6 | |

Phone | Apple | iPhone 5S | 1570 | 7.9 | 5.5 | 0.9 | 6.4 | |

Phone | Apple | iPhone 5C | 1507 | 7.5 | 5.7 | 1.0 | 6.7 | |

Phone | Apple | iPhone 5 | 1440 | 7.2 | 6.0 | 1.0 | 7.0 | |

Phone | Apple | iPhone 4S | 1432 | 7.2 | 6.0 | 1.0 | 7.1 | |

Phone | Apple | iPhone 4 | 1420 | 7.1 | 6.1 | 1.0 | 7.1 | |

Phone | Apple | iPhone 3GS | 1219 | 6.1 | 7.1 | 1.2 | 8.3 | |

Phone | Apple | iPhone 3G | 1150 | 5.8 | 7.5 | 1.3 | 8.8 | |

Tablet | Apple | iPad 1 | 6613 | 33.1 | 1.3 | |||

Tablet | Apple | iPad 2 | 6944 | 34.7 | 1.2 | |||

Tablet | Apple | iPad 3 | 11560 | 57.8 | 0.7 | |||

Tablet | Apple | iPad 4 | 11560 | 57.8 | 0.7 | |||

Tablet | Apple | iPad Air | 8820 | 44.1 | 1.0 | |||

Tablet | Apple | iPad Air 2 | 7340 | 36.7 | 1.2 | |||

Tablet | Apple | iPad Mini 1st | 4440 | 22.2 | 2.0 | |||

Tablet | Apple | iPad Mini 2nd | 6471 | 32.4 | 1.3 | |||

Tablet | Apple | iPad Mini 3rd | 6471 | 32.4 | 1.3 | |||

Phone | Samsung | Galaxy S2 | 1650 | 8.3 | 5.2 | 0.9 | 6.1 | |

Phone | Samsung | Galaxy S3 | 2100 | 10.5 | 4.1 | 0.7 | 4.8 | |

Phone | Samsung | Galaxy S4 | 2600 | 13.0 | 3.3 | 0.6 | 3.9 | |

Phone | Samsung | Galaxy S5 | 2800 | 14.0 | 3.1 | 0.5 | 3.6 | |

Phone | Nexus 6 | 3220 | 16.1 | 2.7 | 0.5 | 3.1 | ||

Phone | Nexus 5 | 2300 | 11.5 | 3.8 | 0.6 | 4.4 | ||

Phone | Nexus One | 1400 | 7.0 | 6.2 | 1.0 | 7.2 | ||

Phone | Nexus S | 1500 | 7.5 | 5.8 | 1.0 | 6.7 | ||

Phone | Nexus 4 | 2100 | 10.5 | 4.1 | 0.7 | 4.8 | ||

Phone | Galaxy Nexus | 1750 | 8.8 | 4.9 | 0.8 | 5.8 | ||

Phone | Galaxy Nexus LTE | 1850 | 9.3 | 4.7 | 0.8 | 5.5 | ||

Tablet | Nexus 7 (tablet, 2012) | 4325 | 21.6 | 2.0 | ||||

Tablet | Nexus 7 (tablet, 2013) | 3950 | 19.8 | 2.2 | ||||

Tablet | Nexus 10 (tablet) | 9000 | 45.0 | 1.0 | ||||

Tablet | Nexus 9 (tablet) | 6700 | 33.5 | 1.3 | ||||

Phone | HTC | HTC One | 2300 | 11.5 | 3.8 | 0.6 | 4.4 | |

Phone | HTC | HTC One (M8) | 2600 | 13.0 | 3.3 | 0.6 | 3.9 | |

Phone | HTC | HTC One (M7) | 2300 | 11.5 | 3.8 | 0.6 | 4.4 | |

Phone | HTC | HTC One mini | 1800 | 9.0 | 4.8 | 0.8 | 5.6 | |

Phone | HTC | HTC Desire 320 | 2100 | 10.5 | 4.1 | 0.7 | 4.8 | |

Phone | HTC | HTC Desire 510 | 2100 | 10.5 | 4.1 | 0.7 | 4.8 | |

Phone | HTC | HTC Desire | 2100 | 10.5 | 4.1 | 0.7 | 4.8 | |

Phone | HTC | HTC Desire C | 1230 | 6.2 | 7.0 | 1.2 | 8.2 | |

Phone | HTC | HTC One X | 2100 | 10.5 | 4.1 | 0.7 | 4.8 | |

Phone | HTC | HTC One V | 1500 | 7.5 | 5.8 | 1.0 | 6.7 | |

Phone | HTC | HTC One S | 1650 | 8.3 | 5.2 | 0.9 | 6.1 | |

Phone | HTC | HTC One X+ | 2100 | 10.5 | 4.1 | 0.7 | 4.8 | |

Phone | HTC | Windows Phone 8X by HTC | 1800 | 9.0 | 4.8 | 0.8 | 5.6 | |

Phone | HTC | Windows Phone 8S by HTC | 1700 | 8.5 | 5.1 | 0.9 | 6.0 | |

Phone | Sony | Xperia Z3 | 3100 | 15.5 | 2.8 | 0.5 | 3.3 | |

Phone | Sony | Xperia Z3 Compact | 2600 | 13.0 | 3.3 | 0.6 | 3.9 | |

Phone | Sony | Xperia Z3 Dual | 3100 | 15.5 | 2.8 | 0.5 | 3.3 | |

Phone | Sony | Xperia Z3v | 3200 | 16.0 | 2.7 | 0.5 | 3.2 | |

Phone | Sony | Xperia Z | 2330 | 11.7 | 3.7 | 0.6 | 4.3 | |

Phone | Sony | Xperia Z Ultra | 3050 | 15.3 | 2.8 | 0.5 | 3.3 | |

Phone | Sony | Xperia Z2 | 3200 | 16.0 | 2.7 | 0.5 | 3.2 | |

Phone | Sony | Xperia Z2a | 3000 | 15.0 | 2.9 | 0.5 | 3.4 | |

Phone | Sony | Xperia T3 | 2500 | 12.5 | 3.5 | 0.6 | 4.0 | |

Phone | Sony | Xperia T3 LTE | 2500 | 12.5 | 3.5 | 0.6 | 4.0 | |

Phone | Sony | Xperia T2 Ultra | 3000 | 15.0 | 2.9 | 0.5 | 3.4 | |

Phone | Sony | Xperia M Dual | 1750 | 8.8 | 4.9 | 0.8 | 5.8 | |

Phone | Sony | Xperia M2 LTE | 2300 | 11.5 | 3.8 | 0.6 | 4.4 | |

Phone | Sony | Xperia M2 Dual | 2300 | 11.5 | 3.8 | 0.6 | 4.4 | |

Phone | Sony | Xperia Z1 | 3000 | 15.0 | 2.9 | 0.5 | 3.4 | |

Phone | Sony | Xperia Z1 Compact | 2300 | 11.5 | 3.8 | 0.6 | 4.4 | |

Phone | Sony | Xperia Z1s | 3000 | 15.0 | 2.9 | 0.5 | 3.4 | |

Phone | Sony | Xperia C | 2390 | 12.0 | 3.6 | 0.6 | 4.2 | |

Phone | Sony | Xperia C3 | 2500 | 12.5 | 3.5 | 0.6 | 4.0 | |

Phone | Sony | Xperia SP | 2370 | 11.9 | 3.7 | 0.6 | 4.3 | |

Tablet | Sony | Xperia Z2 Tablet | 6000 | 30.0 | 1.4 | |||

Tablet | Sony | Xperia Z3 Tablet Compact | 4500 | 22.5 | 1.9 | |||

Tablet | Sony | Xperia Z Tablet LTE | 6000 | 30.0 | 1.4 | |||